Hi,

I'm trying to figure out a dead monitor power supply that uses a UC3842B
controller. I'd like to hook my scope to the chip and observe any signals
during startup, etc. Since this is connected to rectified line voltage on
the ground plane and a large power resistor on the vcc line, I'm unsure
where to hook my ground for the scope probe. I'm using an isolation
transformer, variac and series light bulb in addition to the scope.

I'm just a tinkerer used to scoping clock signals on TTL circuits and
hesitant to do something that will toast my scope.

Any suggestions? Oh yeah, no schematics for the monitor either. I'm
looking at a similar schematic using the same control chip. I see the
startup voltage of +16vdc and no reference voltage or output strobe signals.

Help!

Kirk S.

"Kirk S." wrote in message
...
Hi,

I'm trying to figure out a dead monitor power supply that uses a UC3842B
controller. I'd like to hook my scope to the chip and observe any signals
during startup, etc. Since this is connected to rectified line voltage on
the ground plane and a large power resistor on the vcc line, I'm unsure
where to hook my ground for the scope probe. I'm using an isolation
transformer, variac and series light bulb in addition to the scope.

I'm just a tinkerer used to scoping clock signals on TTL circuits and
hesitant to do something that will toast my scope.

Any suggestions? Oh yeah, no schematics for the monitor either. I'm
looking at a similar schematic using the same control chip. I see the
startup voltage of +16vdc and no reference voltage or output strobe
signals.

Help!

Kirk S.



The best place to connect the ground of your 'scope for primary side
monitoring, is the negative terminal of the main primary side smoothing cap.
But please, please be very careful if you're not an experienced engineer. I
know you say that you're using an isolation transformer, but the belief that
this makes you totally safe against electrocution, can lead to complacency.
If you manage to get the mains across you, even if it's being supplied by an
isolation transformer, this can still at the very least give you a very
nasty shock, and under extreme conditions, result in death. Also be aware
that under certain circumstances, with SMPS's, the mains earth on your scope
can be an issue.

If you have startup voltage, but no chopper drive output from the '3842,
suspect first any small electrolytic caps associated with the chip,
particularly the one connected to the supply pin. It's also not at all
uncommon for the chip itself to fail.

BE CAREFUL - Keep one hand in your pocket, whilst taking readings.

Arfa

Look here for the datasheet on the ic. The first shows ac
waveforms. The last one is a 138 page manual on PWM
switch mode power supplies. Sky.

http://rocky.digikey.com/WebLib/On-S...0NCV3843BV.pdf

http://www.onsemi.com/pub/Collateral/UC3842B.PDF

http://home.eunet.cz/rysanek/pdf/mot-sw.pdf

"Arfa Daily" wrote in message
...

"Kirk S." wrote in message
...
Hi,

I'm trying to figure out a dead monitor power supply that uses a UC3842B
controller. I'd like to hook my scope to the chip and observe any
signals
during startup, etc. Since this is connected to rectified line voltage
on
the ground plane and a large power resistor on the vcc line, I'm unsure
where to hook my ground for the scope probe. I'm using an isolation
transformer, variac and series light bulb in addition to the scope.

I'm just a tinkerer used to scoping clock signals on TTL circuits and
hesitant to do something that will toast my scope.

Any suggestions? Oh yeah, no schematics for the monitor either. I'm
looking at a similar schematic using the same control chip. I see the
startup voltage of +16vdc and no reference voltage or output strobe
signals.

Help!

Kirk S.



The best place to connect the ground of your 'scope for primary side
monitoring, is the negative terminal of the main primary side smoothing
cap.
But please, please be very careful if you're not an experienced engineer.
I
know you say that you're using an isolation transformer, but the belief
that
this makes you totally safe against electrocution, can lead to
complacency.
If you manage to get the mains across you, even if it's being supplied by
an
isolation transformer, this can still at the very least give you a very
nasty shock, and under extreme conditions, result in death. Also be aware
that under certain circumstances, with SMPS's, the mains earth on your
scope
can be an issue.

If you have startup voltage, but no chopper drive output from the '3842,
suspect first any small electrolytic caps associated with the chip,
particularly the one connected to the supply pin. It's also not at all
uncommon for the chip itself to fail.

BE CAREFUL - Keep one hand in your pocket, whilst taking readings.

Arfa

Yeah, isolation transformer or not, it's still lethal amps. I don't like to
probe with the line voltage on. I'll power it off, hook things up, stand
back and slowly bring up the power. I checked the small electrolytic cap on
the supply pin and it checks fine. I have no reference voltage from the
chip either. The symptoms were longer and longer startup times which has me
thinking that although the esr reading on that cap is ok, it may still be
dead. Might as well replace it first and see what happens. I don't get a
tweet, tweet, tweet of a cycling supply. I hate just swapping parts however
it might be the easiest thing to do...

I know that working around things like this can lead to accidents. Line
voltage is nothing to play with.

I like reading the electronics repair faq as well. Lots of good info and
constant warnings about discharging caps, using isolation transformers, one
hand. A little reminding doesn't hurt, either...

Thanks for the info!

Kirk S.

Kirk S. wrote:
"Arfa Daily" wrote in message
...

"Kirk S." wrote in message
...
Hi,

I'm trying to figure out a dead monitor power supply that uses a UC3842B
controller. I'd like to hook my scope to the chip and observe any
signals
during startup, etc. Since this is connected to rectified line voltage
on
the ground plane and a large power resistor on the vcc line, I'm unsure
where to hook my ground for the scope probe. I'm using an isolation
transformer, variac and series light bulb in addition to the scope.

I'm just a tinkerer used to scoping clock signals on TTL circuits and
hesitant to do something that will toast my scope.

Any suggestions? Oh yeah, no schematics for the monitor either. I'm
looking at a similar schematic using the same control chip. I see the
startup voltage of +16vdc and no reference voltage or output strobe
signals.

Help!

Kirk S.



The best place to connect the ground of your 'scope for primary side
monitoring, is the negative terminal of the main primary side smoothing
cap.
But please, please be very careful if you're not an experienced engineer.
I
know you say that you're using an isolation transformer, but the belief
that
this makes you totally safe against electrocution, can lead to
complacency.
If you manage to get the mains across you, even if it's being supplied by
an
isolation transformer, this can still at the very least give you a very
nasty shock, and under extreme conditions, result in death. Also be aware
that under certain circumstances, with SMPS's, the mains earth on your
scope
can be an issue.

If you have startup voltage, but no chopper drive output from the '3842,
suspect first any small electrolytic caps associated with the chip,
particularly the one connected to the supply pin. It's also not at all
uncommon for the chip itself to fail.

BE CAREFUL - Keep one hand in your pocket, whilst taking readings.

Arfa

Yeah, isolation transformer or not, it's still lethal amps. I don't like to
probe with the line voltage on. I'll power it off, hook things up, stand
back and slowly bring up the power. I checked the small electrolytic cap on
the supply pin and it checks fine. I have no reference voltage from the
chip either. The symptoms were longer and longer startup times which has me
thinking that although the esr reading on that cap is ok, it may still be
dead. Might as well replace it first and see what happens. I don't get a
tweet, tweet, tweet of a cycling supply. I hate just swapping parts however
it might be the easiest thing to do...

I know that working around things like this can lead to accidents. Line
voltage is nothing to play with.

I like reading the electronics repair faq as well. Lots of good info and
constant warnings about discharging caps, using isolation transformers, one
hand. A little reminding doesn't hurt, either...

Thanks for the info!

BTW, you better power your circuit, not the scope from that isolation
transformer. That will isolate your entire construction from the hot wire
thus making your work much safer.

---
************************************************** ****************
* KSI@home KOI8 Net The impossible we do immediately. *
* Las Vegas NV, USA Miracles require 24-hour notice. *
************************************************** ****************

Kirk S. wrote:
Hi,

I'm trying to figure out a dead monitor power supply that uses a UC3842B
controller. I'd like to hook my scope to the chip and observe any signals
during startup, etc. Since this is connected to rectified line voltage on
the ground plane and a large power resistor on the vcc line, I'm unsure
where to hook my ground for the scope probe. I'm using an isolation
transformer, variac and series light bulb in addition to the scope.

I'm just a tinkerer used to scoping clock signals on TTL circuits and
hesitant to do something that will toast my scope.

Any suggestions? Oh yeah, no schematics for the monitor either. I'm
looking at a similar schematic using the same control chip. I see the
startup voltage of +16vdc and no reference voltage or output strobe signals.

Help!

Kirk S.



I've worked on these a lot - I don't use the scope anymore.
When I did, I made sure the ground of the scope was floating -
use one of those 2 prong to 3 prong adapters to isolate the
ground. BUT - since the IC isn't working - there really aren't
any signals to see. The +16v is good - suppy voltage. No ref
voltage says the chip is shut down. Also, if memory is good, the
16v has to go up over 18v to turn the chip on. It gets this via
feedback from the flyback [usually]. No feedback voltage - chip
doesn't turn on. Check all the diodes around the 3842 chip. And
all the other electrolytic caps around the 3842 too.

The next thing I'd check would be all the rectifying diodes on the
secondary of the power supply circuit. If one of those is bad - no
low voltages - no flyback voltage - chip doesn't turn on. Then I'd
check the horizontal output transistor.

There were a couple of times I actually had to replace the 3842; but,
usually it was something else that caused the 3842 to shut down. They
used to be pretty cheap at Dalbani; but, it looks like they've gone
out of business....????

later, The Schneids

The Schneids wrote in
:

Kirk S. wrote:
Hi,

I'm trying to figure out a dead monitor power supply that uses a
UC3842B controller. I'd like to hook my scope to the chip and
observe any signals during startup, etc. Since this is connected to
rectified line voltage on the ground plane and a large power resistor
on the vcc line, I'm unsure where to hook my ground for the scope
probe. I'm using an isolation transformer, variac and series light
bulb in addition to the scope.

I'm just a tinkerer used to scoping clock signals on TTL circuits and
hesitant to do something that will toast my scope.

Any suggestions? Oh yeah, no schematics for the monitor either. I'm
looking at a similar schematic using the same control chip. I see
the startup voltage of +16vdc and no reference voltage or output
strobe signals.

Help!

Kirk S.



I've worked on these a lot - I don't use the scope anymore.
When I did, I made sure the ground of the scope was floating -

The WRONG way.
You're supposed to isolate the DUT,not the test equipment.

use one of those 2 prong to 3 prong adapters to isolate the
ground.

Leaving the scope chassis potentially HOT,a clear hazard.




--
Jim Yanik
jyanik
at
kua.net

"The Schneids" wrote in message
...
Kirk S. wrote:
Hi,

I'm trying to figure out a dead monitor power supply that uses a UC3842B
controller. I'd like to hook my scope to the chip and observe any
signals
during startup, etc. Since this is connected to rectified line voltage
on
the ground plane and a large power resistor on the vcc line, I'm unsure
where to hook my ground for the scope probe. I'm using an isolation
transformer, variac and series light bulb in addition to the scope.

I'm just a tinkerer used to scoping clock signals on TTL circuits and
hesitant to do something that will toast my scope.

Any suggestions? Oh yeah, no schematics for the monitor either. I'm
looking at a similar schematic using the same control chip. I see the
startup voltage of +16vdc and no reference voltage or output strobe
signals.

Help!

Kirk S.



I've worked on these a lot - I don't use the scope anymore.
When I did, I made sure the ground of the scope was floating -
use one of those 2 prong to 3 prong adapters to isolate the
ground. BUT - since the IC isn't working - there really aren't
any signals to see. The +16v is good - suppy voltage. No ref
voltage says the chip is shut down. Also, if memory is good, the
16v has to go up over 18v to turn the chip on. It gets this via
feedback from the flyback [usually]. No feedback voltage - chip
doesn't turn on. Check all the diodes around the 3842 chip. And
all the other electrolytic caps around the 3842 too.

The next thing I'd check would be all the rectifying diodes on the
secondary of the power supply circuit. If one of those is bad - no
low voltages - no flyback voltage - chip doesn't turn on. Then I'd
check the horizontal output transistor.

There were a couple of times I actually had to replace the 3842; but,
usually it was something else that caused the 3842 to shut down. They
used to be pretty cheap at Dalbani; but, it looks like they've gone
out of business....????

later, The Schneids

Interesting to see that you run your 'scope with the ground disconnected. I
do too, but hesitated to suggest that the OP did this, because unless you're
well experienced, as I am, and clearly you are too, it can be a dangerous
practice.

I actually have the ground wire disconnected from inside the plug, and
brought out through the crack between the two halves of the plug, so that it
can be clearly seen to be disconnected, and I also have a Dymo label on the
plug top saying " Caution. No Earth "

One day, we're both gonna fry .... |:-(

Arfa

WARNING... DANGER.... what is the point of floating the AC ground on
the scope, it is not only dangerous, but totally unneccessary as long
as the equipment that you are troubleshooting is being powered via an
isolation transformer..... that is the whole reason to use an isolation
transformer so that things like a hot chassis television power supply
can be easily hooked up to properly grounded test equipment without
making sparks fly and exposing yourself to unneccessary hot ground
hazards... keep the safety ground (3 prong plug) connected on your
test gear... in a large shop or manufacturing and testing facility that
has accountability to OSHA, disconnecting the safety ground for any
reason is a flagrant and dangerous violation.
electricitym
..
..
..

wrote in message
oups.com...
WARNING... DANGER.... what is the point of floating the AC ground on
the scope, it is not only dangerous, but totally unneccessary as long
as the equipment that you are troubleshooting is being powered via an
isolation transformer..... that is the whole reason to use an isolation
transformer so that things like a hot chassis television power supply
can be easily hooked up to properly grounded test equipment without
making sparks fly and exposing yourself to unneccessary hot ground
hazards... keep the safety ground (3 prong plug) connected on your
test gear... in a large shop or manufacturing and testing facility that
has accountability to OSHA, disconnecting the safety ground for any
reason is a flagrant and dangerous violation.
electricitym
.
.
.


I agree - isolation is best, but if the 'scope has a 3-wire power cord,
with a properly grounded chassis, and the outlet is properly grounded,
should you not be able to just use the positive probe of the 'scope to look
at the primary, and not even connect the test lead ground wire to anything?


Mark Z.

"Mark D. Zacharias" wrote in
:


wrote in message
oups.com...
WARNING... DANGER.... what is the point of floating the AC ground on
the scope, it is not only dangerous, but totally unneccessary as long
as the equipment that you are troubleshooting is being powered via an
isolation transformer..... that is the whole reason to use an
isolation transformer so that things like a hot chassis television
power supply can be easily hooked up to properly grounded test
equipment without making sparks fly and exposing yourself to
unneccessary hot ground hazards... keep the safety ground (3 prong
plug) connected on your test gear... in a large shop or manufacturing
and testing facility that has accountability to OSHA, disconnecting
the safety ground for any reason is a flagrant and dangerous
violation. electricitym
.
.
.


I agree - isolation is best, but if the 'scope has a 3-wire power
cord, with a properly grounded chassis, and the outlet is properly
grounded, should you not be able to just use the positive probe of the
'scope to look at the primary, and not even connect the test lead
ground wire to anything?


Mark Z.




Not for the primary side of switchers.There is no "ground" for those,the
low side is a "common",and not connected to true earth ground,and it can be
elevated up to -170VDC .Some switchers use a doubler arrangement for 120VAC
operation and reconfigure for 240VAC operation.

Tektronix used to have a great booklet about isolation and why certain
methods like "ground-busting" should not be used.I used to include one with
every scope that I serviced that arrived with no ground pin on the plug.
Maybe they still have it,I do not know.

--
Jim Yanik
jyanik
at
kua.net

Since the equipment under test is being powered via an isolation
transformer you should ground the scope probe ground to the negative
side in the hot chassis in order to obtain proper waveform
measurements.... no problem, no sparks, not an issue.
Obviously voltage and current are still present and standard personal
safety proceedures are still required, but because the scope is safeyy
grounded there are not hot chassis issues since they are floating
thanks to the isolation transformer.
electricitym

wrote:
Since the equipment under test is being powered via an isolation
transformer you should ground the scope probe ground to the negative
side in the hot chassis in order to obtain proper waveform
measurements.... no problem, no sparks, not an issue.
Obviously voltage and current are still present and standard personal
safety proceedures are still required, but because the scope is safeyy
grounded there are not hot chassis issues since they are floating
thanks to the isolation transformer.
electricitym


All correct. An isolation transformer is always a good part of your
personal safety.

The UC384x type supplies I hadto repair have had these usual sympthoms:

1. Dead UC 3842. Change.
2. Freid resistor at chip output (in the connection to the gate of the
FET). Replace.
3. Dead FET. Replace.
4. Capacitor about Ucc of UC384x: Leaky or lost capacity. This is often
the "twit..twit...twit" sound of the startup.

google for the article of Keith Lofstrom (or similar , may be my memory
is not correctly remebering): "fixin scitching poer supplies".


hth,
Andreas

Hi all,

Thanks for the help...

I hooked the monitor up to the isolation transformer and variac, brought up
the voltage slowly while watching the VCC with the scope hooked to the (-)
of the large filter cap and the lead hooked to VCC on the 3842. Voltage
climbs to +17vdc and sits. If the chip was working, I would see some sort
of drop when the oscillator started running. Nothing on any of the output
lines either. All diodes around the IC test ok. All diodes on secondary
test ok.

Removed the 3842, put in strip sockets, installed new 3842 and the montor
fired up. Used the light bulb in place of fuse and as the monitor would
cycle up, the light bulb would get bright as the filter cap charged and then
settle down when the monitor came up.

The original was a KA3842A and I replaced it with a UC3842B. They both
cross to the same NTE and from the little I can garner from the specs, the B
is a higher tolerance version of the A.

Kirk S.
"tekman" wrote in message
oups.com...

wrote:
Since the equipment under test is being powered via an isolation
transformer you should ground the scope probe ground to the negative
side in the hot chassis in order to obtain proper waveform
measurements.... no problem, no sparks, not an issue.
Obviously voltage and current are still present and standard personal
safety proceedures are still required, but because the scope is safeyy
grounded there are not hot chassis issues since they are floating
thanks to the isolation transformer.
electricitym


All correct. An isolation transformer is always a good part of your
personal safety.

The UC384x type supplies I hadto repair have had these usual sympthoms:

1. Dead UC 3842. Change.
2. Freid resistor at chip output (in the connection to the gate of the
FET). Replace.
3. Dead FET. Replace.
4. Capacitor about Ucc of UC384x: Leaky or lost capacity. This is often
the "twit..twit...twit" sound of the startup.

google for the article of Keith Lofstrom (or similar , may be my memory
is not correctly remebering): "fixin scitching poer supplies".


hth,
Andreas