[robb]

"robb" wrote in message
...
i have a micro-controller board with 3 big 40DIP ICs, 10 or so
16
DIP support ICs. with 32v/8v/5v transformer supply

i believe the 32v shorted with 5V line

So....where does one start to diagnose such a *big* problem ?


I can only work on this in short segments of time and am usually
tired ... so i am moving backwards.

Anyway the revisit.

The last diagnostic consensus was to remove voltage regulators
and inject a good 5v power into the 5v lines and look for
warming components to get a fix on problems to repair.

Since my 5.2V *aged* lantern battery power supply did not impress
many i cobbled a simple 5v power supply from a 18 VAC/2.5A wall
wart, a 100V/1.5A bridge and 7805 5V regulator plus some
resitors and a 6V bulb to test.

I connected the 5v (through 450 Ohm) to the PCB 5v and 0v lines.
i measured ~60 mA curent nothing warmed ...i idecremented the
resistance by 100 for each iteration of testing the measured
currernt increased slightly until i made it to (100 Ohms) and
then the current spiked to ~+ 1.5A and the 7805 fried ?

but still no heat on the board anywhere, no trace no component no
IC ?

do i need to do something different with power supply test ?
thanks for any help,
robb

[Arfa Daily]

"robb" wrote in message
...
"robb" wrote in message
...
i have a micro-controller board with 3 big 40DIP ICs, 10 or so
16
DIP support ICs. with 32v/8v/5v transformer supply

i believe the 32v shorted with 5V line

So....where does one start to diagnose such a *big* problem ?


I can only work on this in short segments of time and am usually
tired ... so i am moving backwards.

Anyway the revisit.

The last diagnostic consensus was to remove voltage regulators
and inject a good 5v power into the 5v lines and look for
warming components to get a fix on problems to repair.

Since my 5.2V *aged* lantern battery power supply did not impress
many i cobbled a simple 5v power supply from a 18 VAC/2.5A wall
wart, a 100V/1.5A bridge and 7805 5V regulator plus some
resitors and a 6V bulb to test.

I connected the 5v (through 450 Ohm) to the PCB 5v and 0v lines.
i measured ~60 mA curent nothing warmed ...i idecremented the
resistance by 100 for each iteration of testing the measured
currernt increased slightly until i made it to (100 Ohms) and
then the current spiked to ~+ 1.5A and the 7805 fried ?

but still no heat on the board anywhere, no trace no component no
IC ?

do i need to do something different with power supply test ?
thanks for any help,
robb

Do you mean that the 7805 got hot and shut down ? One of those should never
"fry", because they are SOA protected. I suspect that something else
happened there, because if you do the math, it is impossible for 1.5 amps to
flow through a 100 ohm resistor, with only 5v driving it ... Apart from
that, most variations of the 7805 are only rated to 1 amp, although there
are exceptions.

When you cobbled together your test supply, did you mount the regulator on a
heatsink, and most importantly, did you place a 4u7 cap, paralleled with a
0u1 cap directly across the output and ground pins of the regulator, as
close to the device as you could get them? This is *very* important, to
prevent the regulator bursting into vicious ultrasonic or higher
oscillation. It may be just that you reached a current level where this
happened, and the meter didn't know what to do with what it was measuring,
so just displayed some meaningless nonsense. These regs do get very hot when
they oscillate. Although not strictly necessary, it's considered good 'belt
and braces' to put a 0u1 cap between input and ground pins as well.

The more I think about the 'problem' that this board has, the more non
'real-world' it seems. I can't really see any reason why three unrelated
fuses, on three unrelated rails, should have blown, unless there was a
serious and easily visible problem. The only thing that did occur to me was
input voltage. You're not by any chance firing the thing up on UK 240v
power, and its transformer is actually rated for 100 or 110v ? That would do
it ...

Arfa

[robb]

"Arfa Daily" wrote in message
...

"robb" wrote in message
...
"robb" wrote in message
...
i have a micro-controller
i believe the 32v shorted with 5V line
So....where does one start to diagnose such a *big* problem
?


i cobbled a simple 5v power supply from a 18 VAC/2.5A wall
wart, a 100V/1.5A bridge and 7805 5V regulator plus some
resitors and a 6V bulb to test.

I connected the 5v (through 450 Ohm) to the PCB 5v and 0v
lines.
i measured ~60 mA curent nothing warmed ...i idecremented the
resistance by 100 for each iteration of testing the measured
currernt increased slightly until i made it to (100 Ohms) and
then the current spiked to ~+ 1.5A and the 7805 fried ?

but still no heat on the board anywhere, no trace no
component no
IC ?

do i need to do something different with power supply test ?
thanks for any help,
robb

Do you mean that the 7805 got hot and shut down ? One of those
should never
"fry", because they are SOA protected. I suspect that something
else
happened there, because if you do the math, it is impossible
for 1.5 amps to
flow through a 100 ohm resistor, with only 5v driving it ...
Apart from
that, most variations of the 7805 are only rated to 1 amp,
although there
are exceptions.


Thanks for help Arfa,
I am describing from memory, the last thing i recall was
decrement too the 100 Ohm resistor and the amp meter's numbers
were jumping around and 1.5 is the largest number i caught with
my eye.

as for fried 7805 ... the 100 ohm resistor started to smoke and i
think i saw the 7805 let out a puff of smoke and now the 7805
does not produce output ? so i assumed it was fried

this all happened very quickly


When you cobbled together your test supply, did you mount the
regulator on a
heatsink,

the pcb i took it from had a small flat square of aluminum bolted
to it 1.5cm x 1.5cm x 1 mm

and most importantly, did you place a 4u7 cap, paralleled with
a
0u1 cap directly across the output and ground pins of the
regulator, as
close to the device as you could get them?

no


This is *very* important, to prevent the regulator bursting
into vicious ultrasonic or higher oscillation.

uh-oh,

It may be just that you reached a current level where this
happened, and the meter didn't know what to do with what it was
measuring,
so just displayed some meaningless nonsense. These regs do get
very hot when
they oscillate. Although not strictly necessary, it's
considered good 'belt
and braces' to put a 0u1 cap between input and ground pins as
well.

i'll try the test again with those mods

The more I think about the 'problem' that this board has, the
more non
'real-world' it seems. I can't really see any reason why three
unrelated
fuses, on three unrelated rails, should have blown, unless
there was a
serious and easily visible problem. The only thing that did
occur to me was
input voltage. You're not by any chance firing the thing up on
UK 240v
power, and its transformer is actually rated for 100 or 110v ?
That would do
it ...


it is 115v plugged into 115v supply
i suppose if i could address the issue of the seemingly unrelated
supply rails i could probably figure it out

if this info helps ....

the machine *was working* when i replaced a VFD display driver
chip. That was easy problem to fix as it was very specific. I
started with closest components, checked pin signals with oscope
and i had a datasheet for guidance. Everything was working i just
needed to bolt it together.

*BUT* i found a yucky ring in peaks of the clock signal and
thought i should do something to repair that (find the source of
ring) again more very easy specific tasks, just follow traces and
check for expected behavior what goes in and what comes out

well i forgot to put the oscope probe tip guard on (to prevent
shorting pins) and i bridged the 32v VFD supply line to the 5V
rail (a little snap) and all sorts fo jitters came out of the
connected devices, i disconnected all those devices and i found
one lone fuse between a bridge and the transformer was blown i
think the 8V or 16V line that supplied the voltage regulators and
servo controler chips {L387,L298}

i replaced fuse , powered on and all three fuses blew

now i am here with very nebulous problem with no specific ideas
about how to repair

looking for help

thanks again for your help arfa,
robb

[robb]

"Arfa Daily" wrote in message
...

"robb" wrote in message
...
"robb" wrote in message
...
i have a micro-controller
i believe the 32v shorted with 5V line
So....where does one start to diagnose such a *big* problem
?


i cobbled a simple 5v power supply from a 18 VAC/2.5A wall
wart, a 100V/1.5A bridge and 7805 5V regulator plus some
resitors and a 6V bulb to test.

I connected the 5v (through 450 Ohm) to the PCB 5v and 0v
lines.
i measured ~60 mA curent nothing warmed ...i idecremented the
resistance by 100 for each iteration of testing the measured
currernt increased slightly until i made it to (100 Ohms) and
then the current spiked to ~+ 1.5A and the 7805 fried ?

but still no heat on the board anywhere, no trace no
component no
IC ?

do i need to do something different with power supply test ?
thanks for any help,
robb

Do you mean that the 7805 got hot and shut down ? One of those
should never
"fry", because they are SOA protected. I suspect that something
else
happened there, because if you do the math, it is impossible
for 1.5 amps to
flow through a 100 ohm resistor, with only 5v driving it ...
Apart from
that, most variations of the 7805 are only rated to 1 amp,
although there
are exceptions.


Thanks for help Arfa,
I am describing from memory, the last thing i recall was
decrement to the 100 Ohm resistor and the amp meter's numbers
were jumping around and 1.5 is the largest number i caught with
my eye.

as for fried 7805 ... the 100 ohm resistor started to smoke and i
think i saw the 7805 let out a puff of smoke and now the 7805
does not produce output ? so i assumed it was fried

this all happened very quickly


When you cobbled together your test supply, did you mount
the regulator on a heatsink,

well the regulator had a small flat square of aluminum bolted
to it 1.5cm x 1.5cm x 1 mm when i pulled it from some other pcb

and most importantly, did you place a 4u7 cap, paralleled
with 0u1 cap directly across the output and ground pins of
the regulator, as
close to the device as you could get them?

no


This is *very* important, to prevent the regulator bursting
into vicious ultrasonic or higher oscillation.

ohhhh

It may be just that you reached a current level where this
happened, and the meter didn't know what to do with what it
was measuring, so just displayed some meaningless
nonsense. These regs do get very hot when
they oscillate. Although not strictly necessary,
it's considered good 'belt and braces' to put a 0u1
cap between input and ground pins as well.

i'll try the test again with those cap mods


The more I think about the 'problem' that this board has,
the more non 'real-world' it seems. I can't really see
any reason why three unrelated
fuses, on three unrelated rails, should have blown,
unless there was a serious and easily visible
problem. The only thing
that did occur to me was input voltage.
You're not by any chance firing the thing up
on UK 240v power, and its transformer is actually
rated for 100 or 110v ?That would do
it ...


it is 115v plugged into 115v supply

i wish i knew enough to speculate the connection , for now i
can only report what i see and do. i suppose if i could address
the issue of the seemingly unrelated supply rails i could
probably figure it out

FWIW, if this info helps ....

the machine *was working* when i replaced a VFD display driver
chip. That was an easy problem to fix as it was very specific. I
started with components that were related to the VFD (ie. driver
chip) , i checked pin signals with oscope and i had a datasheet
for guidance. Driver chip had 6 bad lines that would not allow
user button presses to be seen by main board. I replace the DIP40
with a PLCC chip and Everything was working i just needed to
re-assemble and bolt the pieces together.

***BUT***
i stumbled across a "yucky" ring in the peaks of the clock signal
to the VFD driver chip and i thought i should do something to
repair that (ie find the source of ring) again more very easy
specific tasks, just follow traces and check for expected
behavior (in vs outs) what goes in and what comes out

well i forgot to put the oscope probe tip guard on (to prevent
shorting pins) and i bridged the 32v VFD supply line to the 5V
rail (a little arc snap) and all sorts of jitters came out of the
connected devices, i disconnected all those devices and i found
one lone fuse between a bridge and the transformer was blown. i
think the 8V or 16V line that supplied the voltage regulators and
servo controler chips {L387,L298, etc}

i replaced the one fuse, powered on and all three fuses blew

since then i have checked transformer output (ok), desoldered the
L298/L387 ics, continuity tests on various parts of PCB (73 Ohms
between 5v and 0V all around the board), tried to feed 5v into
the 5v line as suggested on the group to feel for warming
components,

now i am here with a very nebulous problem with no specific ideas
about how to repair

and looking for help

thanks again for your help arfa,
robb

BTW there are pictures of all this stuff on
(alt.binaries.schematic.electronics)

[robb]

"Arfa Daily" wrote in message
...

"robb" wrote in message
...
"robb" wrote in message
...
i have a micro-controller
i believe the 32v shorted with 5V line
So....where does one start to diagnose such a *big* problem
?


i cobbled a simple 5v power supply from a 18 VAC/2.5A wall
wart, a 100V/1.5A bridge and 7805 5V regulator plus some
resitors and a 6V bulb to test.

I connected the 5v (through 450 Ohm) to the PCB 5v and 0v
lines.
i measured ~60 mA curent nothing warmed ...i idecremented the
resistance by 100 for each iteration of testing the measured
currernt increased slightly until i made it to (100 Ohms) and
then the current spiked to ~+ 1.5A and the 7805 fried ?

but still no heat on the board anywhere, no trace no
component no
IC ?

do i need to do something different with power supply test ?
thanks for any help,
robb

Do you mean that the 7805 got hot and shut down ? One of those
should never
"fry", because they are SOA protected. I suspect that something
else
happened there, because if you do the math, it is impossible
for 1.5 amps to
flow through a 100 ohm resistor, with only 5v driving it ...
Apart from
that, most variations of the 7805 are only rated to 1 amp,
although there
are exceptions.


Thanks for help Arfa,
I am describing from memory, the last thing i recall was
decrement to the 100 Ohm resistor and the amp meter's numbers
were jumping around and 1.5 is the largest number i caught with
my eye.

as for fried 7805 ... the 100 ohm resistor started to smoke and i
think i saw the 7805 let out a puff of smoke and now the 7805
does not produce output ? so i assumed it was fried

this all happened very quickly


When you cobbled together your test supply, did you mount
the regulator on a heatsink,

well the regulator had a small flat square of aluminum bolted
to it 1.5cm x 1.5cm x 1 mm when i pulled it from some other pcb

and most importantly, did you place a 4u7 cap, paralleled
with 0u1 cap directly across the output and ground pins of
the regulator, as
close to the device as you could get them?

no


This is *very* important, to prevent the regulator bursting
into vicious ultrasonic or higher oscillation.

ohhhh

It may be just that you reached a current level where this
happened, and the meter didn't know what to do with what it
was measuring, so just displayed some meaningless
nonsense. These regs do get very hot when
they oscillate. Although not strictly necessary,
it's considered good 'belt and braces' to put a 0u1
cap between input and ground pins as well.

i'll try the test again with those cap mods


The more I think about the 'problem' that this board has,
the more non 'real-world' it seems. I can't really see
any reason why three unrelated
fuses, on three unrelated rails, should have blown,
unless there was a serious and easily visible
problem. The only thing
that did occur to me was input voltage.
You're not by any chance firing the thing up
on UK 240v power, and its transformer is actually
rated for 100 or 110v ?That would do
it ...


it is 115v plugged into 115v supply

i wish i knew enough to speculate the connection , for now i
can only report what i see and do. i suppose if i could address
the issue of the seemingly unrelated supply rails i could
probably figure it out

FWIW, if this info helps ....

the machine *was working* when i replaced a VFD display driver
chip. That was an easy problem to fix as it was very specific. I
started with components that were related to the VFD (ie. driver
chip) , i checked pin signals with oscope and i had a datasheet
for guidance. Driver chip had 6 bad lines that would not allow
user button presses to be seen by main board. I replace the DIP40
with a PLCC chip and Everything was working i just needed to
re-assemble and bolt the pieces together.

***BUT***
i stumbled across a "yucky" ring in the peaks of the clock signal
to the VFD driver chip and i thought i should do something to
repair that (ie find the source of ring) again more very easy
specific tasks, just follow traces and check for expected
behavior (in vs outs) what goes in and what comes out

well i forgot to put the oscope probe tip guard on (to prevent
shorting pins) and i bridged the 32v VFD supply line to the 5V
rail (a little arc snap) and all sorts of jitters came out of the
connected devices, i disconnected all those devices and i found
one lone fuse between a bridge and the transformer was blown. i
think the 8V or 16V line that supplied the voltage regulators and
servo controler chips {L387,L298, etc}

i replaced the one fuse, powered on and all three fuses blew

since then i have checked transformer output (ok), desoldered the
L298/L387 ics, continuity tests on various parts of PCB (73 Ohms
between 5v and 0V all around the board), tried to feed 5v into
the 5v line as suggested on the group to feel for warming
components,

now i am here with a very nebulous problem with no specific ideas
about how to repair

and looking for help

thanks again for your help arfa,
robb

BTW there are pictures of all this stuff on
(alt.binaries.schematic.electronics)

[clifto]

robb wrote:
I connected the 5v (through 450 Ohm) to the PCB 5v and 0v lines.
i measured ~60 mA curent nothing warmed ...

That's pretty good. If you then shorted the PCB 5V and 0V lines, you would
have 450 ohms across 5V and should read 11.1 ma.

i idecremented the
resistance by 100 for each iteration of testing the measured
currernt increased slightly until i made it to (100 Ohms) and
then the current spiked to ~+ 1.5A and the 7805 fried ?

To get 1.5A through 100 ohms you need to apply at least 150V to it.

--
Angry American flags attack Hillary Clinton!

[robb]

"clifto" wrote in message
...
robb wrote:
I connected the 5v (through 450 Ohm) to the PCB 5v and 0v
lines.
i measured ~60 mA curent nothing warmed ...

That's pretty good. If you then shorted the PCB 5V and 0V
lines, you would
have 450 ohms across 5V and should read 11.1 ma.

i idecremented the
resistance by 100 for each iteration of testing the measured
currernt increased slightly until i made it to (100 Ohms) and
then the current spiked to ~+ 1.5A and the 7805 fried ?

To get 1.5A through 100 ohms you need to apply at least 150V to
it.


thanks for reply clifto,

i will have to believe you on the numbers

and although the numbers may seem incorrect from a electronic
formulaic analysis i can only report what i see on the equipment
i use and with the setup i use.

The DMM amp reading numbers were fluctuating wildly on my last
test and 1.57 ???was something i thought flashed by as highest #
maybe it was .57 but i do know that whatever the number the 100
ohm resistor had a stream of smoke going up and the 7805 appeared
to have a puff of smoke released before i could dis-connect now
i am hopping somebody can help me decipher what went wrong with
my testing

even operator errors would be useful as long as it helps nme
omove toward a repair/
robb

[[email protected]]

I'm reading this thread with some interest, but I'm more puzzled as it
goes on, there should be something we are (and the OP is) missing.

On 16 Nov, 19:11, "robb" wrote:
"clifto" wrote in message

....
....
it.

thanks for reply clifto,

i will have to believe you on the numbers

and although the numbers may seem incorrect from a electronic
formulaic analysis i can only report what i see on the equipment
i use and with the setup i use.

The DMM amp reading numbers were fluctuating wildly on my last
test and 1.57 ???was something i thought flashed by as highest #
maybe it was .57 but i do know that whatever the number the 100
ohm resistor had a stream of smoke going up and the 7805 appeared
to have a puff of smoke released before i could dis-connect now
i am hopping somebody can help me decipher what went wrong with
my testing


hmm 78xx don't smoke easily, maybe if you apply a too high input
voltage ( 30V I believe) or if it goes into self-oscillation if an
output capacitor is omitted. A 100 ohm resistor, assuming we are
talking about a 1/4 W one or bigger, shouldn't smoke easily with only
5V even on a short to ground, it should become quite hot but not smoke
instantly, if it smoked I believe the input voltage was higher.

Power supply problems shouldn't be that difficult to troubleshoot,
even if once I had almost the same problem with a simple three tubes
amplifier, but that was another story :-)

Regards
Francesco

[ian field]

wrote in message
...
I'm reading this thread with some interest, but I'm more puzzled as it
goes on, there should be something we are (and the OP is) missing.

AIUI the OP is trying to find which/how many 5V rail devices on a heavily
populated logic board are S/C after a short between 36V & 5V feeds.

A slightly risky strategy would be to use the 5V O/P from an old (pre-3.3V)
AT PSU, which typically can supply 200A or more via progressively lower
current limiting resistors, at some point the current will be high enough to
distinctly heat any S/C components enabling rapid identification, obvious
pitfalls are S/C Unobtainable Logic Arrays, firmware ROMs the supplier
refuse to sell as spares, the risk of blowing circuit traces (unrepairable
in multi-layer boards) and the risk to eyesight from exploding S/C
capacitors!

[clifto]

robb wrote:
"clifto" wrote...
robb wrote:
I connected the 5v (through 450 Ohm) to the PCB 5v and 0v lines.
i measured ~60 mA curent nothing warmed ...

That's pretty good. If you then shorted the PCB 5V and 0V lines, you would
have 450 ohms across 5V and should read 11.1 ma.

i idecremented the
resistance by 100 for each iteration of testing the measured
currernt increased slightly until i made it to (100 Ohms) and
then the current spiked to ~+ 1.5A and the 7805 fried ?

To get 1.5A through 100 ohms you need to apply at least 150V to it.

thanks for reply clifto,

i will have to believe you on the numbers

and although the numbers may seem incorrect from a electronic
formulaic analysis i can only report what i see on the equipment
i use and with the setup i use.

What I was trying to get you to realize is that there is probably a
problem with your equipment, or with your understanding of its use.
When the PCB supposedly draws over five times as much current as a
dead short, it should tell you that what you think you're seeing isn't
what is actually happening.

The DMM amp reading numbers were fluctuating wildly on my last
test and 1.57 ???was something i thought flashed by as highest #
maybe it was .57 but i do know that whatever the number the 100
ohm resistor had a stream of smoke going up and the 7805 appeared
to have a puff of smoke released before i could dis-connect now
i am hopping somebody can help me decipher what went wrong with
my testing

If you used a 1/10 watt 100 ohm resistor, putting 5 volts across it
should dissipate 2-1/2 times its capability, or 0.25 W. That might
make it release smoke.

--
Angry American flags attack Hillary Clinton!

[ian field]

"clifto" wrote in message
...

Angry American flags attack Hillary Clinton!

That's probably because most US flags are made in China these days!

[robb]

"clifto" wrote in message
...
robb wrote:
"clifto" wrote...
robb wrote:
I connected the 5v (through 450 Ohm) to the PCB 5v and 0v
lines.
i measured ~60 mA curent nothing warmed ...

That's pretty good. If you then shorted the PCB 5V and 0V
lines, you would
have 450 ohms across 5V and should read 11.1 ma.

i idecremented the
resistance by 100 for each iteration of testing the
measured
currernt increased slightly until i made it to (100 Ohms)
and
then the current spiked to ~+ 1.5A and the 7805 fried ?

To get 1.5A through 100 ohms you need to apply at least 150V
to it.

thanks for reply clifto,

i will have to believe you on the numbers

and although the numbers may seem incorrect from a
electronic
formulaic analysis i can only report what i see on the
equipment
i use and with the setup i use.

What I was trying to get you to realize is that there is
probably a
problem with your equipment, or with your understanding of its
use.
When the PCB supposedly draws over five times as much current
as a
dead short, it should tell you that what you think you're
seeing isn't
what is actually happening.

[trim]

If you used a 1/10 watt 100 ohm resistor, putting 5 volts
across it
should dissipate 2-1/2 times its capability, or 0.25 W. That
might
make it release smoke.


Thanks for help clifto,

i do realize what you are saying which is why i said, "i
consider operator error to be a possible problem".
this amateur horse brain needs some help getting itself in front
of the cart.

i've been told that the 73 Ohm resistance between the 5V and 0v
is not so bad and is somewhat hopeful that i did not incinerate
all the ICs.

if i can diagnose without removing the ICs then that would be
great, otherwise i may need to fall back to brute force
diagnostics and remove ICs one at atime and check things and
iterate .

i was hoping to learn something a little more sophisticated.

i was uing 1/4 watt resistors.

i plan to try the capacitors to prevent oscillation,

thanks for help and for any more ideas you may have,
robb

[robb]

wrote in message
...
I'm reading this thread with some interest, but I'm more
puzzled as it
goes on, there should be something we are (and the OP is)
missing.

On 16 Nov, 19:11, "robb" wrote:
"clifto" wrote in message

and although the numbers may seem incorrect from a
electronic
formulaic analysis i can only report what i see on the
equipment
i use and with the setup i use.

The DMM amp reading numbers were fluctuating wildly on my
last
test and 1.57 ???was something i thought flashed by as
highest #
maybe it was .57 but i do know that whatever the number the
100
ohm resistor had a stream of smoke going up and the 7805
appeared
to have a puff of smoke released before i could dis-connect
now
i am hopping somebody can help me decipher what went wrong
with
my testing


hmm 78xx don't smoke easily, maybe if you apply a too high
input
voltage ( 30V I believe) or if it goes into self-oscillation
if an
output capacitor is omitted. A 100 ohm resistor, assuming we
are
talking about a 1/4 W one or bigger, shouldn't smoke easily
with only
5V even on a short to ground, it should become quite hot but
not smoke
instantly, if it smoked I believe the input voltage was higher.

thanks for help Francesco,

i do not under-estimate my ability to mis-understand.
so i consider myself missing something as distinct possibility.

but what ?

source is a 18-24v ac 1000mA wall wart that feeds the bridge and
then 7805.


Power supply problems shouldn't be that difficult to
troubleshoot,
even if once I had almost the same problem with a simple three
tubes
amplifier, but that was another story :-)

someoner suggested that i add caps to prevent oscillation i will
try and i hope to give more accurate report and numbers after i
make the change.

thanks for help francesco and any other ideas you may have,
robb

[robb]

"ian field" wrote in message
...

wrote in message

...
I'm reading this thread with some interest, but I'm more
puzzled as it
goes on, there should be something we are (and the OP is)
missing.

AIUI the OP is trying to find which/how many 5V rail devices on
a heavily
populated logic board are S/C after a short between 36V & 5V
feeds.

A slightly risky strategy would be to use the 5V O/P from an
old (pre-3.3V)
AT PSU, which typically can supply 200A or more via
progressively lower
current limiting resistors, at some point the current will be
high enough to
distinctly heat any S/C components enabling rapid
identification, obvious
pitfalls are S/C Unobtainable Logic Arrays, firmware ROMs the
supplier
refuse to sell as spares, the risk of blowing circuit traces
(unrepairable
in multi-layer boards) and the risk to eyesight from exploding
S/C
capacitors!


Ian has the problem packaged precisely in a nutshell.

but i don't remember the explosion risks better get my safety
glasses.

having said that ian i think i will desolder the rom, ram, and
8031 just in case they are ok and are not replaceable as you
point out.

the board was mfg mid 80's so i am pretty certain it is only two
layer board.

thanks for the nutshell version,
robb

[ian field]

"robb" wrote in message
...

"ian field" wrote in message
...

wrote in message

...
I'm reading this thread with some interest, but I'm more
puzzled as it
goes on, there should be something we are (and the OP is)
missing.

AIUI the OP is trying to find which/how many 5V rail devices on
a heavily
populated logic board are S/C after a short between 36V & 5V
feeds.

A slightly risky strategy would be to use the 5V O/P from an
old (pre-3.3V)
AT PSU, which typically can supply 200A or more via
progressively lower
current limiting resistors, at some point the current will be
high enough to
distinctly heat any S/C components enabling rapid
identification, obvious
pitfalls are S/C Unobtainable Logic Arrays, firmware ROMs the
supplier
refuse to sell as spares, the risk of blowing circuit traces
(unrepairable
in multi-layer boards) and the risk to eyesight from exploding
S/C
capacitors!


Ian has the problem packaged precisely in a nutshell.

but i don't remember the explosion risks better get my safety
glasses.

having said that ian i think i will desolder the rom, ram, and
8031 just in case they are ok and are not replaceable as you
point out.

the board was mfg mid 80's so i am pretty certain it is only two
layer board.

thanks for the nutshell version,
robb

In my experience of situations like this (such as regulation failure in a
SMPSU) its quite common for a small number of devices - sometimes as few as
1, to go S/C and "crowbar" the rail effectively protecting the remainder of
devices. Occasionally a few devices will blow O/C instantaneously after
going S/C, which can lead to further problems after the S/C devices are
replaced - if the Vcc bond wire to the S/C die blows, the I/P & O/P pins cn
the S/C die can load the O/Ps of other chips connected to it, this can be
tedious to trace on bussed logic, its possible to snip the tops of DIL chip
pins to isolate the chip while leaving enough pin on the chip body to solder
the pin back on to (you can always replace the chip later if you want to)
with the pin isolated you can use a pulse injector and scope to find if the
snipped chip or something else on the bus line is still holding it down.

[Arfa Daily]

snip much

i plan to try the capacitors to prevent oscillation,


That is the most important thing that you must do before going any further.
If not properly bypassed, 78 series regulators will *readily* oscillate
violently, particularly if you have long leads attached to them. Connect 0u1
between "in" and ground, and 4u7 in parallel with 0u1 between "out" and
ground. It is *essential* that these are soldered *directly* to the pins of
the device, not some distance away down the connecting wires.

Arfa

[clifto]

Arfa Daily wrote:
That is the most important thing that you must do before going any further.
If not properly bypassed, 78 series regulators will *readily* oscillate
violently, particularly if you have long leads attached to them. Connect 0u1
between "in" and ground, and 4u7 in parallel with 0u1 between "out" and
ground. It is *essential* that these are soldered *directly* to the pins of
the device, not some distance away down the connecting wires.

Might as well add the back-bias protection diode with anode at "out" and
cathode at "in" while he's at it.

--
Angry American flags attack Hillary Clinton!

[Arfa Daily]

"clifto" wrote in message
...
Arfa Daily wrote:
That is the most important thing that you must do before going any
further.
If not properly bypassed, 78 series regulators will *readily* oscillate
violently, particularly if you have long leads attached to them. Connect
0u1
between "in" and ground, and 4u7 in parallel with 0u1 between "out" and
ground. It is *essential* that these are soldered *directly* to the pins
of
the device, not some distance away down the connecting wires.

Might as well add the back-bias protection diode with anode at "out" and
cathode at "in" while he's at it.


Yeah, if you like, though probably an unnecessary complication for this test
setup. Going back to the story of how this unfortunate situation arose, I
think that there is a valuable lesson for Robb to add to what he's learning
here, and that is "If it ain't broke, don't fix it ..." The fact that it was
working after replacing the VFD driver chip, should have been an end to the
job. It obviously didn't matter that the clock was a bit scruffy, as
observed on the 'scope, as the item was managing to use it satisfactorily.
Depending on how fast the clock is, and how good a probe was being used on
the 'scope, the signal may not even have been 'wrong' in the first place. It
might just have *looked* as though it was. It's pretty likely that any clock
inputs on the chips are internally schmitt'd anyway, to make sure that a
clean signal is being passed on into the chip's circuitry.

Arfa

[Eeyore]

Arfa Daily wrote:

Apart from that, most variations of the 7805 are only rated to 1 amp

Not any more.

1.5A is the norm. Look at any up-to-date data sheet.

Graham

[Arfa Daily]

"Eeyore" wrote in message
...


Arfa Daily wrote:

Apart from that, most variations of the 7805 are only rated to 1 amp

Not any more.

1.5A is the norm. Look at any up-to-date data sheet.

Graham


I wouldn't say it was "the norm". A quick look at Farnell's catalogue shows
at least as many 1 amp variations of the 7805, as 1.5 amp ones. Maplin,
probably the biggest supplier in the UK to the amateur market, as well as
having a professional supply division, doesn't even list any 1.5 amp
versions of the standard 78xx and 79xx series devices, although they do keep
a small range of 2 amp devices. I would also contend that any device older
than 5 years is pretty much certain to be only 1 amp rated, and Robb's
original fitted to a board from the 80s will definitely be.

Arfa

Arfa

[robb]

"robb" wrote in message
...
i have a micro-controller board with 3 big 40DIP ICs, 10 or so
16
DIP support ICs. with 32v/8v/5v transformer supply that
connects
to two bridge rectifiers {32v, 8v}a fuse links sit between
transformer and rectifiers, no schematic.

i believe the 32v shorted with 5V line all
three fuses blown.

So....where does one start to diagnose such a *big* problem ?


Thanks to all who have helped in the past too many to name ..

seems my sacrificial chips so far were the MCU (8031) and ROM
chip (TMM23256) (((((( very sad

Synop:
--------
microcontroler board , 32v and 5v rails meet. a single snap
later and she is dead.
5v rail to 0v rail shows 63 Ohms and should probably be about 400
Ohms

To diagnose using advice fom *s.e.b*
removed the power regs, made a 5 Volt 1A power supply,
connected to 5V and monitored current (~450 milliAmps) and
monitor heat on all components

Nothing really panned out here, no heat or crazy current

So i removed the MCU (8031) and the super important ROM chip
(TMM23256P) and the Ohms jumped to about ~456 Ohms and the
current was down to ~ 275 milliAmps. so those must be the
sacrificial chips and of course the measured Ohms of the ROM
Vcc to Vgrd was 72 Ohms very sad : ..... ((

so i put the 5 volt Reg back and powered up then added fuses for
the 36 volt line and no fuses blown so now i need to get a new
8031 replacement and a ROM chip and of course the ROM IMAGE ((

Semi-Good News...
i have access to a a similar main board (an upgraded one
actually) but it has a 8051 MCU with on board ROM and not
external ROM

so my choices now are
A. get an 8051 and copy the ROM over and install maybe that will
work ?

B. get a 27c256 ROM chip and a 8051/8031 with or w/o ROM then
force external memory use pin to appropriate setting and copy
the borrowed 8051 ROM memory onto the 27c256 ... hope that works

Any advice here and ideas are highly welcomed as i do not want to
cause even more troubles

thanks to all that have helped so far,
robb

[Arfa Daily]

"robb" wrote in message
...

"robb" wrote in message
...
i have a micro-controller board with 3 big 40DIP ICs, 10 or so
16
DIP support ICs. with 32v/8v/5v transformer supply that
connects
to two bridge rectifiers {32v, 8v}a fuse links sit between
transformer and rectifiers, no schematic.

i believe the 32v shorted with 5V line all
three fuses blown.

So....where does one start to diagnose such a *big* problem ?


Thanks to all who have helped in the past too many to name ..

seems my sacrificial chips so far were the MCU (8031) and ROM
chip (TMM23256) (((((( very sad

Synop:
--------
microcontroler board , 32v and 5v rails meet. a single snap
later and she is dead.
5v rail to 0v rail shows 63 Ohms and should probably be about 400
Ohms

To diagnose using advice fom *s.e.b*
removed the power regs, made a 5 Volt 1A power supply,
connected to 5V and monitored current (~450 milliAmps) and
monitor heat on all components

Nothing really panned out here, no heat or crazy current

So i removed the MCU (8031) and the super important ROM chip
(TMM23256P) and the Ohms jumped to about ~456 Ohms and the
current was down to ~ 275 milliAmps. so those must be the
sacrificial chips and of course the measured Ohms of the ROM
Vcc to Vgrd was 72 Ohms very sad : ..... ((

so i put the 5 volt Reg back and powered up then added fuses for
the 36 volt line and no fuses blown so now i need to get a new
8031 replacement and a ROM chip and of course the ROM IMAGE ((

Semi-Good News...
i have access to a a similar main board (an upgraded one
actually) but it has a 8051 MCU with on board ROM and not
external ROM

so my choices now are
A. get an 8051 and copy the ROM over and install maybe that will
work ?

B. get a 27c256 ROM chip and a 8051/8031 with or w/o ROM then
force external memory use pin to appropriate setting and copy
the borrowed 8051 ROM memory onto the 27c256 ... hope that works

Any advice here and ideas are highly welcomed as i do not want to
cause even more troubles

thanks to all that have helped so far,
robb


I think that it would be a good idea to restart this thread at the top of
the list again, as it is now a long way down. The only way that I knew that
you had posted again, was that I still had a 'watch' set on, that I had
forgotten to remove. Just as a matter of interest, have you actually tried
sticking the ROM (I take it that it's a standard 27 series EP-ROM ?) into a
PROM blower just to see what it makes of it? They usually do an evaluation
of the chip first, and will put up an error message regarding what, if
anything, they find wrong. 72 ohms does, I agree, seem a bit low though.
Have you compared this figure to a known good chip ? Remind me again what
the board is out of. Is there absolutely no possibility of finding another,
even if still in service, anywhere in the world? The 'net is a very powerful
tool in this regard.

Arfa