troubleshooting method for micro-controller board ?

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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.

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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!

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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

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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

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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

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.

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

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!

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

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

16

connects

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