troubleshooting method for micro-controller board ?

it

Jesus.. If only I could really direct that in proper context to you with out offending some one else here.

If you really knew how to debug things (something I learned from those that really knew what they were doing at age 13), the incandescent lamp has always been the choice of current regulators in debugging and in practical use of circuits. And in his case, 3 volt lamp is to low of voltage. come on , you can do better than that!.. You must have one that is at least rated to the voltage set you are connecting it into.. In this case, 5 VOLTS.. and that means a common

12 is fine for current testing. 6 volt at the smallest.

Please come back later when you have something that is worth absorbing. I may even thank you, although I doubt it.

As you pointed out indirectly or directly before, I can be just as rood an obnoxious as you can!

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

It depends on the type of IC's, but a couple hundred ohms is not uncommon for a fully functional circuit. "Shorted" implies near zero ohms, maybe 1 or

2 ohms of resistance in the traces and connectors leading into the thing.

Yeah - I was thinking that the fuses were on the DC rails, but reading your original post again you state that they are between the transformer and rectifiers. But you also said that there were two rectifiers (on 8 [or 12]V, and on 32 [or 30]V). What about the 5V rectifier? And did you check those bridges for shorted diodes? That continuity wouldn't necessarily show up downstream (especially with the fuses blown).

TM

Its phutted!

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Careful. He didn't say 73 ohms at 5V. If the 73 ohms is a DVM measurement, it was probably made with a low voltage applied. When the circuit is powered up with 5 V, other parts of the ICs are turned on and the current may be much more than 68 mA. The best use of the ohmeter measurement is to compare it to a good board. And yes, 73 ohms sounds low to me. Ken Fowler

16

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

?

lines.

component no

should never

else

for 1.5 amps to

Apart from

although there

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

regulator on a

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

a

regulator, as

no

uh-oh,

measuring,

very hot when

considered good 'belt

well.

i'll try the test again with those mods

more non

unrelated

there was a

occur to me was

UK 240v

That would do

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

?

lines.

component no

should never

else

for 1.5 amps to

Apart from

although there

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

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

no

ohhhh

i'll try the test again with those cap mods

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)

?

lines.

component no

should never

else

for 1.5 amps to

Apart from

although there

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

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

no

ohhhh

i'll try the test again with those cap mods

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)

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.

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

--
                 Angry American flags attack Hillary Clinton!

lines.

lines, you would

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

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

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!

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.

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!

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

lines.

lines, you would

measured

and

to it.

electronic

equipment

probably a

use.

as a

seeing isn't

[trim]

across it

might

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

puzzled as it

missing.

equipment

last

highest #

100

appeared

now

with

input

if an

are

with only

not smoke

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.

troubleshoot,

tubes

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

puzzled as it

missing.

a heavily

feeds.

old (pre-3.3V)

progressively lower

high enough to

identification, obvious

supplier

(unrepairable

S/C

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