[robb]

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 and it blew one fuse
link. the fuse link was replaced with same and when powered all
three fuses blown.

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


..
Only thing working is transformer outputs and it seems like i get
continuity hits everywhere i check
the board was built about 1985 (no SMT) it is about 14" x 5" has
3 x 40 DIP ICs and 10 or so 16 DIP support ICs, handful of
transistors many resistors,diodes etc,

ideas and help would be greatly appreciated
robb

[Eeyore]

robb wrote:

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 and it blew one fuse
link. the fuse link was replaced with same and when powered all
three fuses blown.

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

.
Only thing working is transformer outputs and it seems like i get
continuity hits everywhere i check
the board was built about 1985 (no SMT) it is about 14" x 5" has
3 x 40 DIP ICs and 10 or so 16 DIP support ICs, handful of
transistors many resistors,diodes etc,

ideas and help would be greatly appreciated

Well .... if the 5V supply went seriously high, the chances are it's all fried.
:-(

Have you tried powering the pcb from an external stabilised 5V power supply ?

The very first thing to look for is a clock oscillator (and valid reset signal).
Get the data sheets for those 40 pin chips, identify which one is the micro and
scope the relevant pin(s).

What are the 40 pin chip numbers btw ?

Graham

[robb]

"Eeyore" wrote in
message ...

robb wrote:

i have a micro-controller board ....
i believe the 32v shorted with 5V
blew one fuse link. the fuse link was replaced with same
and when powered all three fuses blown.

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


Well .... if the 5V supply went seriously high, the chances are
it's all fried.
:-(

there is a 630 mA Busman type fuses on all the lines i hope that
saved things ? but the fact that i am getting a short detected
between 0V and 5V line has me worried maybe thats good i do not
know hopeful someone can give me an method for finding problems ?


Have you tried powering the pcb from an external stabilised 5V
power supply ?

The very first thing to look for is a clock oscillator (and
valid reset signal).
Get the data sheets for those 40 pin chips, identify which one
is the micro and
scope the relevant pin(s).

What are the 40 pin chip numbers btw ?


one is the Phillips MAB 8051 micro,
then SRM 2017 Static RAM,
and NEC 8547E7 (D23256) ?? - can't find data sheet

then dual full bridge driver - L298
comparator - LM393 LM339 etc
some nor buffers and and and invertor pkgs

thanks again graham i really appreciate some help,
i can handle small problems but potential catastrophics like this
is a bit over

thanks
robb


Graham

[Jamie]

robb wrote:

"Eeyore" wrote in
message ...

robb wrote:


i have a micro-controller board ....
i believe the 32v shorted with 5V
blew one fuse link. the fuse link was replaced with same
and when powered all three fuses blown.

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


Well .... if the 5V supply went seriously high, the chances are

it's all fried.

:-(


there is a 630 mA Busman type fuses on all the lines i hope that
saved things ? but the fact that i am getting a short detected
between 0V and 5V line has me worried maybe thats good i do not
know hopeful someone can give me an method for finding problems ?


Have you tried powering the pcb from an external stabilised 5V

power supply ?

The very first thing to look for is a clock oscillator (and

valid reset signal).

Get the data sheets for those 40 pin chips, identify which one

is the micro and

scope the relevant pin(s).

What are the 40 pin chip numbers btw ?



one is the Phillips MAB 8051 micro,
then SRM 2017 Static RAM,
and NEC 8547E7 (D23256) ?? - can't find data sheet

then dual full bridge driver - L298
comparator - LM393 LM339 etc
some nor buffers and and and invertor pkgs

thanks again graham i really appreciate some help,
i can handle small problems but potential catastrophics like this
is a bit over

thanks
robb


Graham


I use a low ohm meter (Kelvin), it's very good at
tracking down the nearest point of the short on the
board.
You may want to look at capacitors. Especially
tantalums, they short very nicely many times leaving
nothing but ash.





--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

[John Tserkezis]

Jamie wrote:

You may want to look at capacitors. Especially
tantalums, they short very nicely many times leaving
nothing but ash.

Or worse. I find many times they go short with no visible evidence at all.

I was pushing the boss for a thermal imaging camera for this very purpose,
just apply power till things get warm enough in the right spots to tell what
is shorted.

I thought I had put forward a very good argument for saving time during
repairs that are relatively rare where we were, but he still sadly didn't go
for it... Oh well, I tried.

--
Linux Registered User # 302622
http://counter.li.org

[Jamie]

John Tserkezis wrote:

Jamie wrote:

You may want to look at capacitors. Especially
tantalums, they short very nicely many times leaving
nothing but ash.


Or worse. I find many times they go short with no visible evidence at
all.

I was pushing the boss for a thermal imaging camera for this very
purpose, just apply power till things get warm enough in the right spots
to tell what is shorted.

I thought I had put forward a very good argument for saving time during
repairs that are relatively rare where we were, but he still sadly
didn't go for it... Oh well, I tried.

I use a IR Laser guided Temp probe. that works nicely.

--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

[James Sweet]

I use a low ohm meter (Kelvin), it's very good at
tracking down the nearest point of the short on the
board.
You may want to look at capacitors. Especially
tantalums, they short very nicely many times leaving
nothing but ash.





Sometimes it's possible to isolate sections of the circuit fairly easily, a
jumper link that can be removed, or in more drastic situations, a trace can
be cut. It doesn't take too many cycles of that to narrow things down.

Keep in mind that 5V TTL boards can pull quite a bit of juice, on larger
boards it can appear to be a nearly dead short when using an ohm meter
between Vcc and Gnd.

[robb]

"Jamie" t wrote
in message ...
robb wrote:

"Eeyore" wrote in
message ...

I use a low ohm meter (Kelvin), it's very good at
tracking down the nearest point of the short on the
board.
You may want to look at capacitors. Especially
tantalums, they short very nicely many times leaving
nothing but ash.


Currently the only diagnostic tools at my immediate disposal are

Fluke 177 DMM, a Tek 2236 oscope and a usb/PC based all purpose
logic anal/spectrum anal/oscope

i am hopeing these will be sufficient to solve / track this
fault(s)
if low ohm meter is not terribly expensive then i'll try to get
one if it helps more than my current tools

[John Tserkezis]

robb wrote:

Currently the only diagnostic tools at my immediate disposal are

Fluke 177 DMM, a Tek 2236 oscope and a usb/PC based all purpose
logic anal/spectrum anal/oscope

i am hopeing these will be sufficient to solve / track this
fault(s)
if low ohm meter is not terribly expensive then i'll try to get
one if it helps more than my current tools

You're missing a hammer.

That might be a lot funnier if we weren't talking about 32v on a 5v line,
but at this stage, a hammer certainly seems a viable tool.

Last time I had nearly 12v on a 5v line, (most of the board was 5v logic) I
didn't waste time diagnosing. It was faster to change all the silicon and
worry about shorted tantalums later.
Checking the semis afterwards, I found nearly half of them were stuffed, and
the other half I wouldn't have trusted anyway.

That was at nearly 12 volts. Not 32.

In your shoes, (depending on the scale of the baord and availability of
replacement boards) I'd either outright replace it, or, if this is the only
one left in existence I'd turf the silicon, and hope the EPROM or flash or
wherever the code is stored still works.

--
Linux Registered User # 302622
http://counter.li.org

[Eeyore]

robb wrote:

"Eeyore" wrote
robb wrote:

i have a micro-controller board ....
i believe the 32v shorted with 5V
blew one fuse link. the fuse link was replaced with same
and when powered all three fuses blown.

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


Well .... if the 5V supply went seriously high, the chances are
it's all fried. ::-(

there is a 630 mA Busman type fuses on all the lines i hope that
saved things ?

I'm afraid fuses are merely for preventing fires, not preventing damage (except
for the case of 'extreme' fuse types like the ultra-fast types).


but the fact that i am getting a short detected
between 0V and 5V line has me worried maybe thats good i do not
know hopeful someone can give me an method for finding problems ?

No, I'm afraid that's actually bad news. I assume therefore it literally 'wont
do anything' at present and any power supply connected to it goes into current
limit ?


Have you tried powering the pcb from an external stabilised 5V
power supply ?

The very first thing to look for is a clock oscillator (and
valid reset signal).
Get the data sheets for those 40 pin chips, identify which one
is the micro and scope the relevant pin(s).

What are the 40 pin chip numbers btw ?


one is the Phillips MAB 8051 micro,

OK. That's a real easy one to check AND replace (at least with a modern CMOS
version).
Data sheet here.
http://www.datasheets.org.uk/search....ub.x=0&sub.y=0

If it was working I'd check for activity (pulses/wvaeforms) on the Xtal1/2 pins,
ALE and PSEN first.


then SRM 2017 Static RAM,

OK. Not sure how easy that would be to replace.
Data here for the pinouts..
http://www.datasheets.org.uk/search....ub.x=0&sub.y=0


and NEC 8547E7 (D23256) ?? - can't find data sheet

I expect that's a 256k bit EPROM. Does it have a glass window ?

Important note. This is the most important part on the board. If it's fried so
are you. A dead EPROM will mean that the whole machine can't be repaired (unless
you can get the source file). Treat it with utmost respect.


then dual full bridge driver - L298
comparator - LM393 LM339 etc
some nor buffers and and and invertor pkgs

These are 'commodity' parts that are easily replaced. Not a problem here. First
you need to get the 'big' digital parts functioning.

General tip. Don't apply power to any high power I/O switching chips like the
L298 until the digital stuff is back to normal again.

Here are some ideas. You say you have a short across the 5V logic supply.
Firstly you need to clear that and find the cause. Are the chips socketed ? If
so it's easy to remove them and check for individually shorted chips. Check
across the power pins. Normally these are opposing corner pins like pin 20 and
40 on the 8051 for example ( or 7/14 8/16 and 10/20 on 74 series chips) but best
to get data sheets for anything unfamiliar. If you're lucky you may find one (or
two or threee) failed 'sacrificially' - saving the others but you need to find
the failed chips and get replacements for them.

If they aren't socketed now is the time to do this ! Removing ICs from PCBs
without damage is a skilled task so I would sacrifice anything that's easily
replaceable by cutting the pins at the IC body with fine cutters and removing
those pins one by one from the PCB. That way there's very little chance of doing
much damage. Be careful to fit sockets that firmly grip the leads especially if
this is some industrial controller used in areas where there's significant
vibration. That means DON'T use turned pin sockets, the pins will 'walk out' of
them.

How about you give is a complete list of the ICs on this pcb, plus make a scan
of it and post it where we can see it ? That'll help enormously.



Graham

[robb]

"Eeyore" wrote in
message ...


[trimmed lots of helpfull advice]

responses to helpfull questions in separate post


How about you give is a complete list of the ICs on this pcb,
plus make a scan
of it and post it where we can see it ? That'll help
enormously.


hi Graham thanks again for the help,

i posted pics of the board on
(alt.binaries.electronics.schematics)

there was no obvious visual evidence of any smoked parts that i
could see , only the middle busman fuse was blown after the short

and that seemed to be related to power going to the L298

thanks for all the help,
robb

[Tom2000]

On Tue, 6 Nov 2007 07:34:53 -0500, "robb" wrote:



there was no obvious visual evidence of any smoked parts that i
could see , only the middle busman fuse was blown after the short

and that seemed to be related to power going to the L298

thanks for all the help,
robb


Tough problem with all the ICs soldered in.

Chances are that the 32 volt supply is only used for the L298s, as you
suggest. It also appears that you can unsolder these without damaging
any other parts on the board. (Even if you have to cut them out,
they're still available and easy to replace.)

If your short doesn't disappear after pulling the 298s, try
unsoldering any of the caps near those chips.

Leave unsoldering any ICs as your very last resort.

I'm now bowing out of this discussion and leaving it to the experts.

Good luck, Robb.

Tom

[Andrew Erickson]

In article ,
"robb" wrote:

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 and it blew one fuse
link. the fuse link was replaced with same and when powered all
three fuses blown.

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


.
Only thing working is transformer outputs and it seems like i get
continuity hits everywhere i check
the board was built about 1985 (no SMT) it is about 14" x 5" has
3 x 40 DIP ICs and 10 or so 16 DIP support ICs, handful of
transistors many resistors,diodes etc,

ideas and help would be greatly appreciated
robb

Clearly there is one or more shorts somewhere on the board. Your
mission therefore is to determine what is shorted, and where it is.

If the ICs are socketed, I'd probably pull them (making careful note of
what was where, if it's not clearly labeled on the PCB). This avoids
any (additional) damage to parts that may be quite hard to replace.
From there, checking components in the on-board power supply section
might be reasonable, as would checking the resistance between
permutations of the various power supply rails and ground for shorts
(preferably with something like an ESR meter that won't damage
semiconductors, especially if the ICs can't be removed.) Shorts between
rails could be caused by things like failed de-spiking capacitors. If
everything looks OK with ICs removed, maybe power up the board without
them and see what happens; if the power supply is stable and current
draw reasonable, the problem would seem to be with one of the removed
ICs, or with a circuit driven by one of them.

(It's not a bad idea to figure out what the output interface circuitry
looks like that's driven by the ICs before powering it up without them;
maybe you'd be wise to tie some of the outputs to a fixed level rather
than letting them float.)

What's the purpose of this board? What sort of board-level I/Os does it
have? Based on the atypical power supply voltages, I'm guessing it must
drive or interface with some external devices; maybe those output
circuits are damaged, or the things driven by them or the wiring to them.

Just some thoughts; hopefully they're helpful.

--
Andrew Erickson

"He is no fool who gives what he cannot keep to gain that which he cannot
lose." -- Jim Elliot

[David L. Jones]

On Nov 6, 9:14 am, "robb" wrote:
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 and it blew one fuse
link. the fuse link was replaced with same and when powered all
three fuses blown.

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

.
Only thing working is transformer outputs and it seems like i get
continuity hits everywhere i check
the board was built about 1985 (no SMT) it is about 14" x 5" has
3 x 40 DIP ICs and 10 or so 16 DIP support ICs, handful of
transistors many resistors,diodes etc,

ideas and help would be greatly appreciated
robb


First thing you should do is check for shorts across the various power
rails. If a short is found, using a sufficient resolution low-ohms
meter might let you located the offending component. Sometimes the
bypass caps can go short on order gear like this, replace them all if
needed.

Dave.

[James Sweet]

First thing you should do is check for shorts across the various power
rails. If a short is found, using a sufficient resolution low-ohms
meter might let you located the offending component. Sometimes the
bypass caps can go short on order gear like this, replace them all if
needed.

Dave.


Second that on the caps, I recently repaired a Volkswagen ECU that had a
shorted electrolytic capacitor, I think that was the first time I'd seen a
lytic shorted, regulator was getting hot which clued me in. I've found
shorted tantalums in several things.

[BobG]

Look for date codes. If its only a couple years old, some smart EE
might have actually put Transient Voltage Suppressors in some
important places to guard against incompetent operators. Can anyone
rattle off three TVS brand names from memory he can look for?

[Arfa Daily]

"James Sweet" wrote in message
news:Y8RXi.2719$bm.1189@trndny08...


First thing you should do is check for shorts across the various power
rails. If a short is found, using a sufficient resolution low-ohms
meter might let you located the offending component. Sometimes the
bypass caps can go short on order gear like this, replace them all if
needed.

Dave.


Second that on the caps, I recently repaired a Volkswagen ECU that had a
shorted electrolytic capacitor, I think that was the first time I'd seen a
lytic shorted, regulator was getting hot which clued me in. I've found
shorted tantalums in several things.


Thirded (thirded ?!) on the caps. A few years back, Panasonic used to use
some purple 10u's in everything they made, and they were swines for going
short. Often used as decouplers around signal processing ICs, so you can
imagine what it was like with whole signal processing stages stopping
working, because a cap on an IC pin, decoupling some internal bias rail or
whatever, had failed ...

These days, I see a lot of shorted electrolytics of all manufacture, on
supply rails in amps and home cinema rigs, and so on.

Arfa

[sparky]

On Nov 5, 5:14 pm, "robb" wrote:
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 and it blew one fuse
link. the fuse link was replaced with same and when powered all
three fuses blown.

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

.
Only thing working is transformer outputs and it seems like i get
continuity hits everywhere i check
the board was built about 1985 (no SMT) it is about 14" x 5" has
3 x 40 DIP ICs and 10 or so 16 DIP support ICs, handful of
transistors many resistors,diodes etc,

ideas and help would be greatly appreciated
robb

A couple years ago I had a PCB with about 30 TTL chips on it that the
5 volt line had been shorted to the 12 volt line. I started out
removing the chips carefully but after I had taken out around 10 that
were shorted I just made a mass extraction and replaced them all.

[ian field]

"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 and it blew one fuse
link. the fuse link was replaced with same and when powered all
three fuses blown.

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


.
Only thing working is transformer outputs and it seems like i get
continuity hits everywhere i check
the board was built about 1985 (no SMT) it is about 14" x 5" has
3 x 40 DIP ICs and 10 or so 16 DIP support ICs, handful of
transistors many resistors,diodes etc,

ideas and help would be greatly appreciated
robb



Its phutted!

[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)

[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

[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!

[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

[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]

"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