Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Default Repairing/understanding PNP in charging circuit

I have a small circuit and I think I may have crossed something and
wrecked a transistor. I'm pretty much self teaching and want to see if
I have this figured out properly

I am fixing a small board that charges a battery and powers a device.
When the board is plugged in the output power works fine and the
device will function. The battery has charging voltage going to it
while plugged in.
When the board is not plugged in it will not power the device from
battery. I have traced the circuit. Between the battery and where the
power supply meet up there is only a resistor and a pnp transistor. If
I jump the collector and emitter on the PNP everything works as
expected and the device will receive power.

The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?

Doesn't electric charge flow on C and E if there is no flow on base ?

Did this transistor go bad ? if there is flow between C and E why
place this in a circuit ?

Just trying to figure out how this works and why only C and E of this
transistor are used ?

Thanks if anyone can shed some light on this for me.
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Default Repairing/understanding PNP in charging circuit


"Charon" wrote in message
...
I have a small circuit and I think I may have crossed something and
wrecked a transistor. I'm pretty much self teaching and want to see if
I have this figured out properly

I am fixing a small board that charges a battery and powers a device.
When the board is plugged in the output power works fine and the
device will function. The battery has charging voltage going to it
while plugged in.
When the board is not plugged in it will not power the device from
battery. I have traced the circuit. Between the battery and where the
power supply meet up there is only a resistor and a pnp transistor. If
I jump the collector and emitter on the PNP everything works as
expected and the device will receive power.

The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?

Doesn't electric charge flow on C and E if there is no flow on base ?

Did this transistor go bad ? if there is flow between C and E why
place this in a circuit ?

Just trying to figure out how this works and why only C and E of this
transistor are used ?

Thanks if anyone can shed some light on this for me.


Looks like they're using the transistor as a diode, silly but it works.


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Default Repairing/understanding PNP in charging circuit

"Charon" wrote in message
...
I have a small circuit and I think I may have crossed something and
wrecked a transistor. I'm pretty much self teaching and want to see if
I have this figured out properly

I am fixing a small board that charges a battery and powers a device.
When the board is plugged in the output power works fine and the
device will function. The battery has charging voltage going to it
while plugged in.
When the board is not plugged in it will not power the device from
battery. I have traced the circuit. Between the battery and where the
power supply meet up there is only a resistor and a pnp transistor. If
I jump the collector and emitter on the PNP everything works as
expected and the device will receive power.

The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?

Doesn't electric charge flow on C and E if there is no flow on base ?

Did this transistor go bad ? if there is flow between C and E why
place this in a circuit ?

Just trying to figure out how this works and why only C and E of this
transistor are used ?

Thanks if anyone can shed some light on this for me.


If you don't have a schematic, trace it out and draw one. Often you have to
re arrange what you get into a more recognizable form. Research the
devices. This is called "reverse engineering". The cost of this for a
small item you describe might be $200-$500 worth of labor depending on
research obstacles. This can be a fun thing to do if you are into
challenges. Whole lot clearer when we are looking at the same page.

I have heard of transistors being used as Zeners, Varactors or other Diodes.
Some devices use the same package as transistors.

http://infotech.awardspace.com/

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Default Repairing/understanding PNP in charging circuit


"Charon" wrote in message
...
I have a small circuit and I think I may have crossed something and
wrecked a transistor. I'm pretty much self teaching and want to see if
I have this figured out properly

I am fixing a small board that charges a battery and powers a device.
When the board is plugged in the output power works fine and the
device will function. The battery has charging voltage going to it
while plugged in.
When the board is not plugged in it will not power the device from
battery. I have traced the circuit. Between the battery and where the
power supply meet up there is only a resistor and a pnp transistor. If
I jump the collector and emitter on the PNP everything works as
expected and the device will receive power.

The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?

Doesn't electric charge flow on C and E if there is no flow on base ?

Did this transistor go bad ? if there is flow between C and E why
place this in a circuit ?

Just trying to figure out how this works and why only C and E of this
transistor are used ?

Thanks if anyone can shed some light on this for me.



What makes you think that the device in question is a PNP transistor ? Does
it actually *have* a third leg that isn't connected anywhere ? The reason I
ask this is that there is a range of wire-ended fuses which are in a TO92
package, just like a transistor, but they physically have only two legs at
the two 'corners' or the package. These devices are typically marked "Nx"
such as "N10" for instance. What is the descriptor silk screened on the PCB
for this device ? If it really is a transistor, it is likely to be something
like "Qx" or "Trx", but if it is a fuse of this type, it will likely be
"ICPx"

Arfa


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Default Repairing/understanding PNP in charging circuit

On Sun, 19 Apr 2009 00:37:39 -0700 (PDT), Charon
wrote:

I have a small circuit and I think I may have crossed something and
wrecked a transistor. I'm pretty much self teaching and want to see if
I have this figured out properly


Welcome to "Learn By Destroying(tm)". You're off to a great start.

The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?


Because it's probably a power FET and not a PNP transistor. Are there
any identifying marks on the device? A photograph perhaps?

Doesn't electric charge flow on C and E if there is no flow on base ?


Electrons flow from negative to positive.
Holes flow from positive to negative.
PNP transistors have the collector more negative than the emitter.
Therefore, electrons will flow from collector to emitter in a PNP.
It's the same if you think of "electric charge flow" as the flow of
electrons. If you're confused, you have plenty of company. This
might help:
http://www.tpub.com/neets/book7/25b.htm
(or make things worse).

Did this transistor go bad ?


I don't think the transistor went anywhere. It's still in front of
you. Where did you expect it to go?

--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558


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Default Repairing/understanding PNP in charging circuit

On Apr 19, 10:16*am, "Arfa Daily" wrote:
"Charon" wrote in message

...



I have a small circuit and I think I may have crossed something and
wrecked a transistor. I'm pretty much self teaching and want to see if
I have this figured out properly


I am fixing a small board that charges a battery and powers a device.
When the board is plugged in the output power works fine and the
device will function. The battery has charging voltage going to it
while plugged in.
When the board is not plugged in it will not power the device from
battery. I have traced the circuit. Between the battery and where the
power supply meet up there is only a resistor and a pnp transistor. If
I jump the collector and emitter on the PNP everything works as
expected and the device will receive power.


The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?


Doesn't electric charge flow on C and E if there is no flow on base ?


Did this transistor go bad ? if there is flow between C and E why
place this in a circuit ?


Just trying to figure out how this works and why only C and E of this
transistor are used ?


Thanks if anyone can shed some light on this for me.


What makes you think that the device in question is a PNP transistor ?

I tracked the part down to
http://www.diodes.com/datasheets/FZT789A.pdf
Silk screen on the part is FZT 789A

Does
it actually *have* a third leg that isn't connected anywhere ? The reason I
ask this is that there is a range of wire-ended fuses which are in a TO92
package, just like a transistor, but they physically have only two legs at
the two 'corners' or the package. These devices are typically marked "Nx"
such as "N10" for instance. What is the descriptor silk screened on the PCB
for this device ? If it really is a transistor, it is likely to be something
like "Qx" or "Trx", but if it is a fuse of this type, it will likely be
"ICPx"

Arfa


The transistor is only connected on the E and the C (tab) the other
two are not connected to anything.
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Default Repairing/understanding PNP in charging circuit

On Apr 19, 1:56*pm, Jeff Liebermann wrote:
On Sun, 19 Apr 2009 00:37:39 -0700 (PDT), Charon
wrote:

I have a small circuit and I think I may have crossed something and
wrecked a transistor. I'm pretty much self teaching and want to see if
I have this figured out properly


Welcome to "Learn By Destroying(tm)". *You're off to a great start.

The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?


Because it's probably a power FET and not a PNP transistor. *Are there
any identifying marks on the device? *A photograph perhaps?

Doesn't electric charge flow on C and E if there is no flow on base ?


Electrons flow from negative to positive.
Holes flow from positive to negative.
PNP transistors have the collector more negative than the emitter.
Therefore, electrons will flow from collector to emitter in a PNP.
It's the same if you think of "electric charge flow" as the flow of
electrons. *If you're confused, you have plenty of company. *This
might help:
http://www.tpub.com/neets/book7/25b.htm
(or make things worse).

Did this transistor go bad ?


I don't think the transistor went anywhere. *It's still in front of
you. *Where did you expect it to go?

--
Jeff Liebermann * *
150 Felker St #D * *http://www.LearnByDestroying.com
Santa Cruz CA 95060http://802.11junk.com
Skype: JeffLiebermann * * AE6KS * *831-336-2558


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Default Repairing/understanding PNP in charging circuit


"Charon" wrote in message
...
On Apr 19, 10:16 am, "Arfa Daily" wrote:
"Charon" wrote in message

...



I have a small circuit and I think I may have crossed something and
wrecked a transistor. I'm pretty much self teaching and want to see if
I have this figured out properly


I am fixing a small board that charges a battery and powers a device.
When the board is plugged in the output power works fine and the
device will function. The battery has charging voltage going to it
while plugged in.
When the board is not plugged in it will not power the device from
battery. I have traced the circuit. Between the battery and where the
power supply meet up there is only a resistor and a pnp transistor. If
I jump the collector and emitter on the PNP everything works as
expected and the device will receive power.


The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?


Doesn't electric charge flow on C and E if there is no flow on base ?


Did this transistor go bad ? if there is flow between C and E why
place this in a circuit ?


Just trying to figure out how this works and why only C and E of this
transistor are used ?


Thanks if anyone can shed some light on this for me.


What makes you think that the device in question is a PNP transistor ?

I tracked the part down to
http://www.diodes.com/datasheets/FZT789A.pdf
Silk screen on the part is FZT 789A

Does
it actually *have* a third leg that isn't connected anywhere ? The reason
I
ask this is that there is a range of wire-ended fuses which are in a TO92
package, just like a transistor, but they physically have only two legs at
the two 'corners' or the package. These devices are typically marked "Nx"
such as "N10" for instance. What is the descriptor silk screened on the
PCB
for this device ? If it really is a transistor, it is likely to be
something
like "Qx" or "Trx", but if it is a fuse of this type, it will likely be
"ICPx"

Arfa


The transistor is only connected on the E and the C (tab) the other
two are not connected to anything.


The device having only collector and emitter connected, makes no sense at
all. A transistor connected in this configuration, would represent
essentially an open circuit (not a diode as someone else suggested) and no
current would (or even *could*) flow between these terminals.

I see from the data sheet that it is in fact a surface mount device, so
presumably all three terminals and the tab are at least soldered down to
pads ? Could it be that the base connection is actually underneath the
device - maybe even via a thru' plated hole ?

Arfa


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Default Repairing/understanding PNP in charging circuit

On Apr 19, 5:55*pm, "Arfa Daily" wrote:
"Charon" wrote in message

...
On Apr 19, 10:16 am, "Arfa Daily" wrote:

"Charon" wrote in message


....


I have a small circuit and I think I may have crossed something and
wrecked a transistor. I'm pretty much self teaching and want to see if
I have this figured out properly


I am fixing a small board that charges a battery and powers a device.
When the board is plugged in the output power works fine and the
device will function. The battery has charging voltage going to it
while plugged in.
When the board is not plugged in it will not power the device from
battery. I have traced the circuit. Between the battery and where the
power supply meet up there is only a resistor and a pnp transistor. If
I jump the collector and emitter on the PNP everything works as
expected and the device will receive power.


The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?


Doesn't electric charge flow on C and E if there is no flow on base ?


Did this transistor go bad ? if there is flow between C and E why
place this in a circuit ?


Just trying to figure out how this works and why only C and E of this
transistor are used ?


Thanks if anyone can shed some light on this for me.


What makes you think that the device in question is a PNP transistor ?


I tracked the part down tohttp://www.diodes.com/datasheets/FZT789A.pdf
Silk screen on the part is FZT 789A



*Does
it actually *have* a third leg that isn't connected anywhere ? The reason
I
ask this is that there is a range of wire-ended fuses which are in a TO92
package, just like a transistor, but they physically have only two legs at
the two 'corners' or the package. These devices are typically marked "Nx"
such as "N10" for instance. What is the descriptor silk screened on the
PCB
for this device ? If it really is a transistor, it is likely to be
something
like "Qx" or "Trx", but if it is a fuse of this type, it will likely be
"ICPx"


Arfa
The transistor is only connected on the E and the C (tab) the other
two are not connected to anything.


The device having only collector and emitter connected, makes no sense at
all. A transistor connected in this configuration, would represent
essentially an open circuit (not a diode as someone else suggested) and no
current would (or even *could*) flow between these terminals.

I see from the data sheet that it is in fact a surface mount device, so
presumably all three terminals and the tab are at least soldered down to
pads ? Could it be that the base connection is actually underneath the
device - maybe even via a thru' plated hole ?

Arfa


I started to trace this out, and found what you had said. There was a
trace under the transistor that was painted over. And was very hard to
see.

I made this trace diagram of the circuit.

http://bauld.com/~eric/pics/trace.jpeg

By jumping the C and E on the transistor it works as expected when not
plugged into the dc power(12v) which is expected as then it is
connected just as the DC 12v power is. The only thing it appears
stopping the battery power from reaching the device load is the
transistor and the base resistor.
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Default Repairing/understanding PNP in charging circuit

On Apr 19, 3:52*pm, Charon wrote:

The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?


Just trying to figure out how this works and why only C and E of this
transistor are used ?



I tracked the part down tohttp://www.diodes.com/datasheets/FZT789A.pdf


The transistor is only connected on the E and the C (tab) the other
two are not connected to anything.


Aha! It's a temperature-compensated Zener diode, made by using the
reverse breakdown of the BE junction and the forward CB diode. It
should have about 7 V at breakdown for lowest temperature
coefficient; datasheet suggests you will get 9V or so from this part..

Either that, or there IS some kind of connection to the base, and
it evaporated or is hiding from you...

Purpose-build compensated diodes (like 1N821) are expensive by


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Default Repairing/understanding PNP in charging circuit


"Charon" wrote in message
...
On Apr 19, 5:55 pm, "Arfa Daily" wrote:
"Charon" wrote in message

...
On Apr 19, 10:16 am, "Arfa Daily" wrote:

"Charon" wrote in message


...


I have a small circuit and I think I may have crossed something and
wrecked a transistor. I'm pretty much self teaching and want to see if
I have this figured out properly


I am fixing a small board that charges a battery and powers a device.
When the board is plugged in the output power works fine and the
device will function. The battery has charging voltage going to it
while plugged in.
When the board is not plugged in it will not power the device from
battery. I have traced the circuit. Between the battery and where the
power supply meet up there is only a resistor and a pnp transistor. If
I jump the collector and emitter on the PNP everything works as
expected and the device will receive power.


The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?


Doesn't electric charge flow on C and E if there is no flow on base ?


Did this transistor go bad ? if there is flow between C and E why
place this in a circuit ?


Just trying to figure out how this works and why only C and E of this
transistor are used ?


Thanks if anyone can shed some light on this for me.


What makes you think that the device in question is a PNP transistor ?


I tracked the part down tohttp://www.diodes.com/datasheets/FZT789A.pdf
Silk screen on the part is FZT 789A



Does
it actually *have* a third leg that isn't connected anywhere ? The
reason
I
ask this is that there is a range of wire-ended fuses which are in a
TO92
package, just like a transistor, but they physically have only two legs
at
the two 'corners' or the package. These devices are typically marked
"Nx"
such as "N10" for instance. What is the descriptor silk screened on the
PCB
for this device ? If it really is a transistor, it is likely to be
something
like "Qx" or "Trx", but if it is a fuse of this type, it will likely be
"ICPx"


Arfa
The transistor is only connected on the E and the C (tab) the other
two are not connected to anything.


The device having only collector and emitter connected, makes no sense at
all. A transistor connected in this configuration, would represent
essentially an open circuit (not a diode as someone else suggested) and no
current would (or even *could*) flow between these terminals.

I see from the data sheet that it is in fact a surface mount device, so
presumably all three terminals and the tab are at least soldered down to
pads ? Could it be that the base connection is actually underneath the
device - maybe even via a thru' plated hole ?

Arfa


I started to trace this out, and found what you had said. There was a
trace under the transistor that was painted over. And was very hard to
see.

I made this trace diagram of the circuit.

http://bauld.com/~eric/pics/trace.jpeg

By jumping the C and E on the transistor it works as expected when not
plugged into the dc power(12v) which is expected as then it is
connected just as the DC 12v power is. The only thing it appears
stopping the battery power from reaching the device load is the
transistor and the base resistor.

Looking at your schematic, which maybe doesn't look *quite* right, then if
R3 is good, the transistor should be on. You could try measuring it
in-circuit with the battery disconnected, and you should see sensible
readings. Failing that, remove it and read it. I don't know how much
experience you have of getting devices like this off a board, but if you
only have access to 'conventional' soldering equipment rather than hot air
rework equipment, you need to be careful that you don't 'lose' any traces or
pads.

Use a good quality desolder braid with a good sized iron - preferably a
temperature controlled one of perhaps 50 watts max - to remove as much
solder as you can with the braid, from both the pins and the tab. Then heat
the tab alone with a small scalpel under its edge, until the solder flows
enough to be able to slightly twist the blade to lift the tab no more than
1mm off the board. Then heat the pins, all at once if possible, and repeat
the blade twisting under the device body, to lift them up from the board a
little. This can be difficult to achieve, if the manufacturer has kindly
glued the device to the board, before flowing it ...

You should now be able to use a fresh piece of desolder braid to get the
remaining solder under the pins and tab, and the device should come off the
board cleanly, and with no damage. It's not easy, but if you can solder ok,
and understand about not overheating solder pads and causing them to
delaminate from the board, then it's not overly difficult, either.

Arfa


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Default Repairing/understanding PNP in charging circuit

Arfa Daily wrote:
"Charon" wrote in message
...
On Apr 19, 5:55 pm, "Arfa Daily" wrote:
"Charon" wrote in message

...
On Apr 19, 10:16 am, "Arfa Daily" wrote:

"Charon" wrote in message
...
I have a small circuit and I think I may have crossed something and
wrecked a transistor. I'm pretty much self teaching and want to see if
I have this figured out properly
I am fixing a small board that charges a battery and powers a device.
When the board is plugged in the output power works fine and the
device will function. The battery has charging voltage going to it
while plugged in.
When the board is not plugged in it will not power the device from
battery. I have traced the circuit. Between the battery and where the
power supply meet up there is only a resistor and a pnp transistor. If
I jump the collector and emitter on the PNP everything works as
expected and the device will receive power.
The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?
Doesn't electric charge flow on C and E if there is no flow on base ?
Did this transistor go bad ? if there is flow between C and E why
place this in a circuit ?
Just trying to figure out how this works and why only C and E of this
transistor are used ?
Thanks if anyone can shed some light on this for me.
What makes you think that the device in question is a PNP transistor ?

I tracked the part down tohttp://www.diodes.com/datasheets/FZT789A.pdf
Silk screen on the part is FZT 789A



Does
it actually *have* a third leg that isn't connected anywhere ? The
reason
I
ask this is that there is a range of wire-ended fuses which are in a
TO92
package, just like a transistor, but they physically have only two legs
at
the two 'corners' or the package. These devices are typically marked
"Nx"
such as "N10" for instance. What is the descriptor silk screened on the
PCB
for this device ? If it really is a transistor, it is likely to be
something
like "Qx" or "Trx", but if it is a fuse of this type, it will likely be
"ICPx"
Arfa
The transistor is only connected on the E and the C (tab) the other
two are not connected to anything.

The device having only collector and emitter connected, makes no sense at
all. A transistor connected in this configuration, would represent
essentially an open circuit (not a diode as someone else suggested) and no
current would (or even *could*) flow between these terminals.

I see from the data sheet that it is in fact a surface mount device, so
presumably all three terminals and the tab are at least soldered down to
pads ? Could it be that the base connection is actually underneath the
device - maybe even via a thru' plated hole ?

Arfa


I started to trace this out, and found what you had said. There was a
trace under the transistor that was painted over. And was very hard to
see.

I made this trace diagram of the circuit.

http://bauld.com/~eric/pics/trace.jpeg

By jumping the C and E on the transistor it works as expected when not
plugged into the dc power(12v) which is expected as then it is
connected just as the DC 12v power is. The only thing it appears
stopping the battery power from reaching the device load is the
transistor and the base resistor.

Looking at your schematic, which maybe doesn't look *quite* right, then if
R3 is good, the transistor should be on. You could try measuring it
in-circuit with the battery disconnected, and you should see sensible
readings. Failing that, remove it and read it. I don't know how much
experience you have of getting devices like this off a board, but if you
only have access to 'conventional' soldering equipment rather than hot air
rework equipment, you need to be careful that you don't 'lose' any traces or
pads.

Use a good quality desolder braid with a good sized iron - preferably a
temperature controlled one of perhaps 50 watts max - to remove as much
solder as you can with the braid, from both the pins and the tab. Then heat
the tab alone with a small scalpel under its edge, until the solder flows
enough to be able to slightly twist the blade to lift the tab no more than
1mm off the board. Then heat the pins, all at once if possible, and repeat
the blade twisting under the device body, to lift them up from the board a
little. This can be difficult to achieve, if the manufacturer has kindly
glued the device to the board, before flowing it ...

You should now be able to use a fresh piece of desolder braid to get the
remaining solder under the pins and tab, and the device should come off the
board cleanly, and with no damage. It's not easy, but if you can solder ok,
and understand about not overheating solder pads and causing them to
delaminate from the board, then it's not overly difficult, either.

Arfa


It is entirely possible that I missed something, the traces are hard to
see and hide under a few components. I spent quite a bit of time double
checking with a multi meter for no resistance to help find beginning and
end of traces. only if they made sense and would not have crossed
another trace.

Testing the resistance on the PNP in circuit no battery it seems to
change every time I test it.

To get a proper reading I will have to pull it out.

With battery in circuit, there is no resistance reading between C and E
I get 10.5 volts to B
11.9 volts to E
2.4 volts on C (not sure where the 2.4 volts is coming from)

This should be letting the 11.9 volts through to C but its not

When this was working "properly" there was a parasitic load on the
battery that would drain it within a a few days. which was the original
reason for opening this up. Was to put a switch on the battery
connection to disable the battery when not in use.

So there would be two switches.
Original to turn device on.
New switch to disconnect battery.

If the transistor problem eludes me too long, just removing it and using
the switches to control it manually would suffice.
Although if the device is on while charging the battery will not be
disconnected from the circuit via the pnp.
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Default Repairing/understanding PNP in charging circuit


"Eric Bauld" wrote in message
...
Arfa Daily wrote:
"Charon" wrote in message
...
On Apr 19, 5:55 pm, "Arfa Daily" wrote:
"Charon" wrote in message

...
On Apr 19, 10:16 am, "Arfa Daily" wrote:

"Charon" wrote in message
...
I have a small circuit and I think I may have crossed something and
wrecked a transistor. I'm pretty much self teaching and want to see if
I have this figured out properly
I am fixing a small board that charges a battery and powers a device.
When the board is plugged in the output power works fine and the
device will function. The battery has charging voltage going to it
while plugged in.
When the board is not plugged in it will not power the device from
battery. I have traced the circuit. Between the battery and where the
power supply meet up there is only a resistor and a pnp transistor. If
I jump the collector and emitter on the PNP everything works as
expected and the device will receive power.
The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?
Doesn't electric charge flow on C and E if there is no flow on base ?
Did this transistor go bad ? if there is flow between C and E why
place this in a circuit ?
Just trying to figure out how this works and why only C and E of this
transistor are used ?
Thanks if anyone can shed some light on this for me.
What makes you think that the device in question is a PNP transistor ?
I tracked the part down tohttp://www.diodes.com/datasheets/FZT789A.pdf
Silk screen on the part is FZT 789A



Does
it actually *have* a third leg that isn't connected anywhere ? The
reason
I
ask this is that there is a range of wire-ended fuses which are in a
TO92
package, just like a transistor, but they physically have only two legs
at
the two 'corners' or the package. These devices are typically marked
"Nx"
such as "N10" for instance. What is the descriptor silk screened on the
PCB
for this device ? If it really is a transistor, it is likely to be
something
like "Qx" or "Trx", but if it is a fuse of this type, it will likely be
"ICPx"
Arfa
The transistor is only connected on the E and the C (tab) the other
two are not connected to anything.
The device having only collector and emitter connected, makes no sense
at
all. A transistor connected in this configuration, would represent
essentially an open circuit (not a diode as someone else suggested) and
no
current would (or even *could*) flow between these terminals.

I see from the data sheet that it is in fact a surface mount device, so
presumably all three terminals and the tab are at least soldered down to
pads ? Could it be that the base connection is actually underneath the
device - maybe even via a thru' plated hole ?

Arfa


I started to trace this out, and found what you had said. There was a
trace under the transistor that was painted over. And was very hard to
see.

I made this trace diagram of the circuit.

http://bauld.com/~eric/pics/trace.jpeg

By jumping the C and E on the transistor it works as expected when not
plugged into the dc power(12v) which is expected as then it is
connected just as the DC 12v power is. The only thing it appears
stopping the battery power from reaching the device load is the
transistor and the base resistor.

Looking at your schematic, which maybe doesn't look *quite* right, then
if R3 is good, the transistor should be on. You could try measuring it
in-circuit with the battery disconnected, and you should see sensible
readings. Failing that, remove it and read it. I don't know how much
experience you have of getting devices like this off a board, but if you
only have access to 'conventional' soldering equipment rather than hot
air rework equipment, you need to be careful that you don't 'lose' any
traces or pads.

Use a good quality desolder braid with a good sized iron - preferably a
temperature controlled one of perhaps 50 watts max - to remove as much
solder as you can with the braid, from both the pins and the tab. Then
heat the tab alone with a small scalpel under its edge, until the solder
flows enough to be able to slightly twist the blade to lift the tab no
more than 1mm off the board. Then heat the pins, all at once if possible,
and repeat the blade twisting under the device body, to lift them up from
the board a little. This can be difficult to achieve, if the manufacturer
has kindly glued the device to the board, before flowing it ...

You should now be able to use a fresh piece of desolder braid to get the
remaining solder under the pins and tab, and the device should come off
the board cleanly, and with no damage. It's not easy, but if you can
solder ok, and understand about not overheating solder pads and causing
them to delaminate from the board, then it's not overly difficult,
either.

Arfa

It is entirely possible that I missed something, the traces are hard to
see and hide under a few components. I spent quite a bit of time double
checking with a multi meter for no resistance to help find beginning and
end of traces. only if they made sense and would not have crossed another
trace.

Testing the resistance on the PNP in circuit no battery it seems to change
every time I test it.

To get a proper reading I will have to pull it out.

With battery in circuit, there is no resistance reading between C and E
I get 10.5 volts to B
11.9 volts to E
2.4 volts on C (not sure where the 2.4 volts is coming from)

This should be letting the 11.9 volts through to C but its not

When this was working "properly" there was a parasitic load on the battery
that would drain it within a a few days. which was the original reason for
opening this up. Was to put a switch on the battery connection to disable
the battery when not in use.

So there would be two switches.
Original to turn device on.
New switch to disconnect battery.

If the transistor problem eludes me too long, just removing it and using
the switches to control it manually would suffice.
Although if the device is on while charging the battery will not be
disconnected from the circuit via the pnp.




OK. You should not read any resistance between collector and emitter i.e. it
should be open circuit. For measuring junction resistances on bipolar semis,
I have always preferred an 'old fashioned' 20k ohms per volt analogue
multimeter - in my case an AVO 8 MkIV. Digital meters can give very
misleading resistance readings across transistor junctions. With an analogue
ohm meter, with the red probe to the base pin, you should read around 700 or
800 ohms to the collector and emitter, with the black probe, for a silicon
PNP transistor. All other combinations of test probe polarity and pins,
should read open circuit.

As far as the voltage readings that you are getting, they would seem to be
wrong. With 11.9v to the collector, the base voltage should be about 0.6 to
0.7v below that - i.e. around 11.3v, not 10.5v. Assuming that R3 is in good
order, this voltage difference is a fixed function of a silicon PN junction,
so any deviation from that figure would tend to indicate a faulty device.
The 2.4v 'output' that you are seeing on the collector, is probably just a
voltage that is 'battering its way through' the device from the emitter
input voltage. My next move would be to get some proper resistance readings
for the device junctions, removing it if necessary. From the information you
have given, I think that you are going to find that the transistor is indeed
faulty.

Arfa


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Posts: 6
Default Repairing/understanding PNP in charging circuit

Arfa Daily wrote:
"Eric Bauld" wrote in message
...
Arfa Daily wrote:
"Charon" wrote in message
...
On Apr 19, 5:55 pm, "Arfa Daily" wrote:
"Charon" wrote in message

...
On Apr 19, 10:16 am, "Arfa Daily" wrote:

"Charon" wrote in message
...
I have a small circuit and I think I may have crossed something and
wrecked a transistor. I'm pretty much self teaching and want to see if
I have this figured out properly
I am fixing a small board that charges a battery and powers a device.
When the board is plugged in the output power works fine and the
device will function. The battery has charging voltage going to it
while plugged in.
When the board is not plugged in it will not power the device from
battery. I have traced the circuit. Between the battery and where the
power supply meet up there is only a resistor and a pnp transistor. If
I jump the collector and emitter on the PNP everything works as
expected and the device will receive power.
The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?
Doesn't electric charge flow on C and E if there is no flow on base ?
Did this transistor go bad ? if there is flow between C and E why
place this in a circuit ?
Just trying to figure out how this works and why only C and E of this
transistor are used ?
Thanks if anyone can shed some light on this for me.
What makes you think that the device in question is a PNP transistor ?
I tracked the part down tohttp://www.diodes.com/datasheets/FZT789A.pdf
Silk screen on the part is FZT 789A



Does
it actually *have* a third leg that isn't connected anywhere ? The
reason
I
ask this is that there is a range of wire-ended fuses which are in a
TO92
package, just like a transistor, but they physically have only two legs
at
the two 'corners' or the package. These devices are typically marked
"Nx"
such as "N10" for instance. What is the descriptor silk screened on the
PCB
for this device ? If it really is a transistor, it is likely to be
something
like "Qx" or "Trx", but if it is a fuse of this type, it will likely be
"ICPx"
Arfa
The transistor is only connected on the E and the C (tab) the other
two are not connected to anything.
The device having only collector and emitter connected, makes no sense
at
all. A transistor connected in this configuration, would represent
essentially an open circuit (not a diode as someone else suggested) and
no
current would (or even *could*) flow between these terminals.

I see from the data sheet that it is in fact a surface mount device, so
presumably all three terminals and the tab are at least soldered down to
pads ? Could it be that the base connection is actually underneath the
device - maybe even via a thru' plated hole ?

Arfa
I started to trace this out, and found what you had said. There was a
trace under the transistor that was painted over. And was very hard to
see.

I made this trace diagram of the circuit.

http://bauld.com/~eric/pics/trace.jpeg

By jumping the C and E on the transistor it works as expected when not
plugged into the dc power(12v) which is expected as then it is
connected just as the DC 12v power is. The only thing it appears
stopping the battery power from reaching the device load is the
transistor and the base resistor.

Looking at your schematic, which maybe doesn't look *quite* right, then
if R3 is good, the transistor should be on. You could try measuring it
in-circuit with the battery disconnected, and you should see sensible
readings. Failing that, remove it and read it. I don't know how much
experience you have of getting devices like this off a board, but if you
only have access to 'conventional' soldering equipment rather than hot
air rework equipment, you need to be careful that you don't 'lose' any
traces or pads.

Use a good quality desolder braid with a good sized iron - preferably a
temperature controlled one of perhaps 50 watts max - to remove as much
solder as you can with the braid, from both the pins and the tab. Then
heat the tab alone with a small scalpel under its edge, until the solder
flows enough to be able to slightly twist the blade to lift the tab no
more than 1mm off the board. Then heat the pins, all at once if possible,
and repeat the blade twisting under the device body, to lift them up from
the board a little. This can be difficult to achieve, if the manufacturer
has kindly glued the device to the board, before flowing it ...

You should now be able to use a fresh piece of desolder braid to get the
remaining solder under the pins and tab, and the device should come off
the board cleanly, and with no damage. It's not easy, but if you can
solder ok, and understand about not overheating solder pads and causing
them to delaminate from the board, then it's not overly difficult,
either.

Arfa

It is entirely possible that I missed something, the traces are hard to
see and hide under a few components. I spent quite a bit of time double
checking with a multi meter for no resistance to help find beginning and
end of traces. only if they made sense and would not have crossed another
trace.

Testing the resistance on the PNP in circuit no battery it seems to change
every time I test it.

To get a proper reading I will have to pull it out.

With battery in circuit, there is no resistance reading between C and E
I get 10.5 volts to B
11.9 volts to E
2.4 volts on C (not sure where the 2.4 volts is coming from)

This should be letting the 11.9 volts through to C but its not

When this was working "properly" there was a parasitic load on the battery
that would drain it within a a few days. which was the original reason for
opening this up. Was to put a switch on the battery connection to disable
the battery when not in use.

So there would be two switches.
Original to turn device on.
New switch to disconnect battery.

If the transistor problem eludes me too long, just removing it and using
the switches to control it manually would suffice.
Although if the device is on while charging the battery will not be
disconnected from the circuit via the pnp.




OK. You should not read any resistance between collector and emitter i.e. it
should be open circuit.


I should have clarified my post better. By no resistance reading between
collector and emitter I meant it was closed circuit. I was expecting
open circuit. But then everything would be working.

For measuring junction resistances on bipolar semis,
I have always preferred an 'old fashioned' 20k ohms per volt analogue
multimeter - in my case an AVO 8 MkIV. Digital meters can give very
misleading resistance readings across transistor junctions. With an analogue
ohm meter, with the red probe to the base pin, you should read around 700 or
800 ohms to the collector and emitter, with the black probe, for a silicon
PNP transistor. All other combinations of test probe polarity and pins,
should read open circuit.

As far as the voltage readings that you are getting, they would seem to be
wrong. With 11.9v to the collector, the base voltage should be about 0.6 to
0.7v below that - i.e. around 11.3v, not 10.5v. Assuming that R3 is in good
order, this voltage difference is a fixed function of a silicon PN junction,
so any deviation from that figure would tend to indicate a faulty device.
The 2.4v 'output' that you are seeing on the collector, is probably just a
voltage that is 'battering its way through' the device from the emitter
input voltage. My next move would be to get some proper resistance readings
for the device junctions, removing it if necessary. From the information you
have given, I think that you are going to find that the transistor is indeed
faulty.

Arfa



Thanks for the assistance on this. And I can relate to the advantages of
using a analog meter. I once trouble shot a wiring harness in a car.
And was getting a rogue voltage reading on a DMM when there should have
been none. An anolog meter drained the residual charge it was getting
from surrounding wires and then it would read as expected(no voltage).
But until this was pointed out to me I was confounded. As it was just
enough to light a dash led when the harness was unplugged.

Ill track down an analog and try to pull this out this week.
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Default Repairing/understanding PNP in charging circuit

Arfa Daily wrote:
I have always preferred an 'old fashioned' 20k ohms per volt analogue
multimeter - in my case an AVO 8 MkIV. Digital meters can give very
misleading resistance readings across transistor junctions.

That's why even the chepest digital multimeters have a "diode test"
mode that measures the forward voltage at 1 to 4 mA (depending on
the multimeter, check the manual). Low-power-transistors should read
about 600mV B-E and B-C, and open circuit C-E.

Regards,
Michael Karcher


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Default Repairing/understanding PNP in charging circuit


"Michael Karcher" wrote in message
...
Arfa Daily wrote:
I have always preferred an 'old fashioned' 20k ohms per volt analogue
multimeter - in my case an AVO 8 MkIV. Digital meters can give very
misleading resistance readings across transistor junctions.

That's why even the chepest digital multimeters have a "diode test"
mode that measures the forward voltage at 1 to 4 mA (depending on
the multimeter, check the manual). Low-power-transistors should read
about 600mV B-E and B-C, and open circuit C-E.

Regards,
Michael Karcher


Yes, Michael, I've been in the service business long enough to understand
the differences between a digital DMM and its diode test facility. However,
it's not at all uncommon to get unstable readings, particularly if you are
measuring 'in-circuit', and significant (in terms of actual digits)
differences between two equally good devices. Even the temperature of your
fingers can cause the digits to shift, as can the long cooling down period
if the device has just been unsoldered from a board. And of course,
differences between different categories of device.

When training others, I have found that this can give rise to confusion in
students, who expect to see 'absolutes' on meters with digital readouts. An
analogue meter used for this purpose gives a simple go / no go on a junction
that in my experience, is a better than 99% indicator of a good or faulty
device. The OP said at the start that he was a self-trainer in electronics
service. He actually stated in one post that measuring the junction
resistances in-circuit, resulted in a different reading every time. This is
typically what you find when using a digital meter. Often, in these cases, a
good old analogue meter will give a stable and consistent reading, which
will allow you to make a sensible evaluation. This was the point that I was
trying to make to the OP.

Arfa


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Posts: 102
Default Repairing/understanding PNP in charging circuit


"Charon" wrote in message
...
On Apr 19, 5:55 pm, "Arfa Daily" wrote:
"Charon" wrote in message

...
On Apr 19, 10:16 am, "Arfa Daily" wrote:

"Charon" wrote in message


...


I have a small circuit and I think I may have crossed something and
wrecked a transistor. I'm pretty much self teaching and want to see if
I have this figured out properly


I am fixing a small board that charges a battery and powers a device.
When the board is plugged in the output power works fine and the
device will function. The battery has charging voltage going to it
while plugged in.
When the board is not plugged in it will not power the device from
battery. I have traced the circuit. Between the battery and where the
power supply meet up there is only a resistor and a pnp transistor. If
I jump the collector and emitter on the PNP everything works as
expected and the device will receive power.


The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?


Doesn't electric charge flow on C and E if there is no flow on base ?


Did this transistor go bad ? if there is flow between C and E why
place this in a circuit ?


Just trying to figure out how this works and why only C and E of this
transistor are used ?


Thanks if anyone can shed some light on this for me.


What makes you think that the device in question is a PNP transistor ?


I tracked the part down tohttp://www.diodes.com/datasheets/FZT789A.pdf
Silk screen on the part is FZT 789A



Does
it actually *have* a third leg that isn't connected anywhere ? The

reason
I
ask this is that there is a range of wire-ended fuses which are in a

TO92
package, just like a transistor, but they physically have only two legs

at
the two 'corners' or the package. These devices are typically marked

"Nx"
such as "N10" for instance. What is the descriptor silk screened on the
PCB
for this device ? If it really is a transistor, it is likely to be
something
like "Qx" or "Trx", but if it is a fuse of this type, it will likely be
"ICPx"


Arfa
The transistor is only connected on the E and the C (tab) the other
two are not connected to anything.


The device having only collector and emitter connected, makes no sense at
all. A transistor connected in this configuration, would represent
essentially an open circuit (not a diode as someone else suggested) and no
current would (or even *could*) flow between these terminals.

I see from the data sheet that it is in fact a surface mount device, so
presumably all three terminals and the tab are at least soldered down to
pads ? Could it be that the base connection is actually underneath the
device - maybe even via a thru' plated hole ?

Arfa


I started to trace this out, and found what you had said. There was a
trace under the transistor that was painted over. And was very hard to
see.

I made this trace diagram of the circuit.

http://bauld.com/~eric/pics/trace.jpeg

By jumping the C and E on the transistor it works as expected when not
plugged into the dc power(12v) which is expected as then it is
connected just as the DC 12v power is. The only thing it appears
stopping the battery power from reaching the device load is the
transistor and the base resistor.

It would appear that with no DC1, the battery would forward bias U1 through
R3 and power the device through SW1 unless the EB juction was wacked. With
DC1 active, D1 keeps the base Hi and U1 is cut off. Battery is charged
through D3 and device is powered by D2. D4 protects the device from spikes.

  #18   Report Post  
Posted to sci.electronics.repair
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Posts: 6
Default Repairing/understanding PNP in charging circuit

Eric Bauld wrote:
Arfa Daily wrote:
"Eric Bauld" wrote in message
...
Arfa Daily wrote:
"Charon" wrote in message
...

On Apr 19, 5:55 pm, "Arfa Daily" wrote:
"Charon" wrote in message

...

On Apr 19, 10:16 am, "Arfa Daily" wrote:

"Charon" wrote in message
...

I have a small circuit and I think I may have crossed something and
wrecked a transistor. I'm pretty much self teaching and want to
see if
I have this figured out properly
I am fixing a small board that charges a battery and powers a
device.
When the board is plugged in the output power works fine and the
device will function. The battery has charging voltage going to it
while plugged in.
When the board is not plugged in it will not power the device from
battery. I have traced the circuit. Between the battery and where
the
power supply meet up there is only a resistor and a pnp
transistor. If
I jump the collector and emitter on the PNP everything works as
expected and the device will receive power.
The PNP, only the C and E are soldered to anything, the Base is not
connected to anything. Why would this be ?
Doesn't electric charge flow on C and E if there is no flow on
base ?
Did this transistor go bad ? if there is flow between C and E why
place this in a circuit ?
Just trying to figure out how this works and why only C and E of
this
transistor are used ?
Thanks if anyone can shed some light on this for me.
What makes you think that the device in question is a PNP
transistor ?
I tracked the part down tohttp://www.diodes.com/datasheets/FZT789A.pdf
Silk screen on the part is FZT 789A



Does
it actually *have* a third leg that isn't connected anywhere ? The
reason
I
ask this is that there is a range of wire-ended fuses which are in
a TO92
package, just like a transistor, but they physically have only two
legs at
the two 'corners' or the package. These devices are typically
marked "Nx"
such as "N10" for instance. What is the descriptor silk screened
on the
PCB
for this device ? If it really is a transistor, it is likely to be
something
like "Qx" or "Trx", but if it is a fuse of this type, it will
likely be
"ICPx"
Arfa
The transistor is only connected on the E and the C (tab) the other
two are not connected to anything.
The device having only collector and emitter connected, makes no
sense at
all. A transistor connected in this configuration, would represent
essentially an open circuit (not a diode as someone else suggested)
and no
current would (or even *could*) flow between these terminals.

I see from the data sheet that it is in fact a surface mount
device, so
presumably all three terminals and the tab are at least soldered
down to
pads ? Could it be that the base connection is actually underneath the
device - maybe even via a thru' plated hole ?

Arfa
I started to trace this out, and found what you had said. There was a
trace under the transistor that was painted over. And was very hard to
see.

I made this trace diagram of the circuit.

http://bauld.com/~eric/pics/trace.jpeg

By jumping the C and E on the transistor it works as expected when not
plugged into the dc power(12v) which is expected as then it is
connected just as the DC 12v power is. The only thing it appears
stopping the battery power from reaching the device load is the
transistor and the base resistor.

Looking at your schematic, which maybe doesn't look *quite* right,
then if R3 is good, the transistor should be on. You could try
measuring it in-circuit with the battery disconnected, and you
should see sensible readings. Failing that, remove it and read it. I
don't know how much experience you have of getting devices like this
off a board, but if you only have access to 'conventional' soldering
equipment rather than hot air rework equipment, you need to be
careful that you don't 'lose' any traces or pads.

Use a good quality desolder braid with a good sized iron -
preferably a temperature controlled one of perhaps 50 watts max - to
remove as much solder as you can with the braid, from both the pins
and the tab. Then heat the tab alone with a small scalpel under its
edge, until the solder flows enough to be able to slightly twist the
blade to lift the tab no more than 1mm off the board. Then heat the
pins, all at once if possible, and repeat the blade twisting under
the device body, to lift them up from the board a little. This can
be difficult to achieve, if the manufacturer has kindly glued the
device to the board, before flowing it ...

You should now be able to use a fresh piece of desolder braid to get
the remaining solder under the pins and tab, and the device should
come off the board cleanly, and with no damage. It's not easy, but
if you can solder ok, and understand about not overheating solder
pads and causing them to delaminate from the board, then it's not
overly difficult, either.

Arfa
It is entirely possible that I missed something, the traces are hard
to see and hide under a few components. I spent quite a bit of time
double checking with a multi meter for no resistance to help find
beginning and end of traces. only if they made sense and would not
have crossed another trace.

Testing the resistance on the PNP in circuit no battery it seems to
change every time I test it.

To get a proper reading I will have to pull it out.

With battery in circuit, there is no resistance reading between C and E
I get 10.5 volts to B
11.9 volts to E
2.4 volts on C (not sure where the 2.4 volts is coming from)

This should be letting the 11.9 volts through to C but its not

When this was working "properly" there was a parasitic load on the
battery that would drain it within a a few days. which was the
original reason for opening this up. Was to put a switch on the
battery connection to disable the battery when not in use.

So there would be two switches.
Original to turn device on.
New switch to disconnect battery.

If the transistor problem eludes me too long, just removing it and
using the switches to control it manually would suffice.
Although if the device is on while charging the battery will not be
disconnected from the circuit via the pnp.




OK. You should not read any resistance between collector and emitter
i.e. it should be open circuit.


I should have clarified my post better. By no resistance reading between
collector and emitter I meant it was closed circuit. I was expecting
open circuit. But then everything would be working.

For measuring junction resistances on bipolar semis, I have always
preferred an 'old fashioned' 20k ohms per volt analogue multimeter -
in my case an AVO 8 MkIV. Digital meters can give very misleading
resistance readings across transistor junctions. With an analogue ohm
meter, with the red probe to the base pin, you should read around 700
or 800 ohms to the collector and emitter, with the black probe, for a
silicon PNP transistor. All other combinations of test probe polarity
and pins, should read open circuit.

As far as the voltage readings that you are getting, they would seem
to be wrong. With 11.9v to the collector, the base voltage should be
about 0.6 to 0.7v below that - i.e. around 11.3v, not 10.5v. Assuming
that R3 is in good order, this voltage difference is a fixed function
of a silicon PN junction, so any deviation from that figure would tend
to indicate a faulty device. The 2.4v 'output' that you are seeing on
the collector, is probably just a voltage that is 'battering its way
through' the device from the emitter input voltage. My next move would
be to get some proper resistance readings for the device junctions,
removing it if necessary. From the information you have given, I think
that you are going to find that the transistor is indeed faulty.

Arfa


Thanks for the assistance on this. And I can relate to the advantages of
using a analog meter. I once trouble shot a wiring harness in a car.
And was getting a rogue voltage reading on a DMM when there should have
been none. An anolog meter drained the residual charge it was getting
from surrounding wires and then it would read as expected(no voltage).
But until this was pointed out to me I was confounded. As it was just
enough to light a dash led when the harness was unplugged.

Ill track down an analog and try to pull this out this week.


I pulled the transistor from the pcb and tested it. Using a diode test
there was no V reading between the B and E using the leads either way.
It was open both ways. A replacement PNP that I had provided a .6 v
reading on the diode test(only one way as expected). Once soldered into
the circuit everything is working now as it once was.

Thanks for the help everyone. esp Arfa
I learn so much more working on something then reading about it.

- Eric
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