2e motherboard with 1 Ohm over the 12V

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Lowlander's picture
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2e motherboard with 1 Ohm over the 12V

Hi hi

 

Was testing a 2e yesterday. It booted but after a few seconds went down with the psu making worrying noises.

At first thought it was the PSU, so switched a working one on it, but only made same noises.

Then measured the V's of that one and compared them to the first one and all looks good.

So the 2e board itself...

Measured resistance over the power lines; - V's are good. +5 was just some 10 Ohm, but went high when i unplugged keyboard. Seems to be the light in there that makes the resistance low, bc it seems to be a an actual tiny lightbulb.

The 12 V however has a resistance of ~1 Ohm, which is effectively a short.

Took the board out, but i cant see anything wrong. The schematic isnt all that clear to me, what all uses 12 V. I know the expansion cars all but 0 and 1 i think have 12V and i saw the power on led is on 12-5V.

Anyone any tips before i spend another day(s) searching?

thank you!

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Nvm It was C12 that had given

Nvm

 

It was C12 that had given up after 40 years of service.

Replaced it. Working fine again.

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A common fault type

This type of fault is one of the applications of thermal imaging. By connecting only the shorted rail to a bench power supply set to a low current limit (100mA more or less) and looking for hot components, the cause of the short will heat up and be seen in infrared.

It is not completely fool-proof since there can be devices that depend on all supply voltages to prevent latchup. But +12V on an Apple II is fine.

 

Another way to find shorts (including shorted traces inside the board!) is a current tracer like the ToneOhm.

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That sounds nice ;]Im afraid

That sounds nice ;]

Im afraid i have plenty of devices for now, although im thinking about a new oscilloscope (since i have a load more old 8 bit computers to check and some will probably be malfunctioning by now). The scope i have is about as old as those computers and it hasnt been fired up in some 20 years...

For now i managed with a multi meter, the old fashioned way ;p

Got 2 more broken 2e's though, both giving vertical stripes. Ill figure it out, i hope ;]

All in all i cant complain: After some 30 years of storage got 6/8 working with little effort. All 6 monitors working, 13/14 disk ][ good. Actually most computers i took out of collection/storage so far were still functioning. I did forget a few batteries though (never knew they had batteries...) and that wrecked a few ;[

I did pop a couple of filter caps over the mains, bc they increased main power here in NL. But still, pleasantly suprised that even 40+ yo machines still work!

thx m8!

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