Most likely a tantalum capacitor in short (if there is of course).. Apply to the input 5V from an adjustable powerful PSU and gradually increase the current.. Watch heating with a pyrometer or thermal imager. You can apply the maximum current smoothly - the element in the short circuit will "pop".. Previously, all the chips in the beds should be carefully pulled out..

Update on this. So I pulled out about 5 tantalums that were getting warmer than the rest. Now the short has changed. Previously with a multi-meter I would have a constant short between ground and 5V at the board connector. Now I only get a single beep than the multi-meter reads about 85 ohms. So I'm not sure what I'm seeing. I'm guessing one of the tantalums I pulled had the constant short and now I'm not sure what kind of short I'm seeing? Does this behavior point to any specific component?

Thanks.

AndrewBSSC4 wrote on 2021-01-16, 21:04:

Update on this. So I pulled out about 5 tantalums that were getting warmer than the rest. Now the short has changed. Previously with a multi-meter I would have a constant short between ground and 5V at the board connector. Now I only get a single beep than the multi-meter reads about 85 ohms. So I'm not sure what I'm seeing. I'm guessing one of the tantalums I pulled had the constant short and now I'm not sure what kind of short I'm seeing? Does this behavior point to any specific component?

Thanks.

The 85 ohms on your multimeter might be normal behaviour on the +5V line, your system is likely fine now. Try reversing the red and black lead of your multimeter. If the indicated resistance depends on polarity, the current is mainly flowing through semiconductors, which is what you want on a mainboard.

Hi All,

So getting back to this after putting it down to tackle a Compaq Portable, Mac Classic, and C64. So going back to poke around this board and before I start to remove all capacitors, I want to make sure that the following observed behavior is not something that someone may have seen before.

So right now I've noticed the following behavior. So is mentioned before there seems to be a short between the 5V and ground. So if the board is left for a while and tested between the two I'd get a small beep on my tester than it would register in the 85-90 Ohms range on the meter. What I've now noticed if I supply power to board for a short burst, a few seconds, and then test the meter will register a constant beep short in the 14-16 Ohms range. But if I leave it for a minute or so it will eventually "drain" back up to the 85-90 Ohms.

So has anyone seen this type of behavior? Does that point in any directions? Should I be concentrating on electrolytic, tantalums, or ceramic capacitors?

Thanks.

AndrewBSSC4 wrote on 2021-03-16, 17:10:

So right now I've noticed the following behavior. So is mentioned before there seems to be a short between the 5V and ground. So if the board is left for a while and tested between the two I'd get a small beep on my tester than it would register in the 85-90 Ohms range on the meter.

This is normal behaviour, as you would see it on a working computer, too.

AndrewBSSC4 wrote on 2021-03-16, 17:10:

What I've now noticed if I supply power to board for a short burst, a few seconds, and then test the meter will register a constant beep short in the 14-16 Ohms range. But if I leave it for a minute or so it will eventually "drain" back up to the 85-90 Ohms.

This might point to a capacitor that breaks down if full 5V is applied and recovers if the current stops flowing through it. Behaviour like this can happen in electrolytic caps, and possibly in tantalum caps, but would be very weird in ceramic caps.

Another plausible explanation is a broken chip that shorts the supply only if power is present. I expect the short persists as long as some voltage is present. As +5V is shorted, it has to be a different power line. The first candidate I suspect for behaviour like this would be the RS232 transmitter/receiver chips, MC1488/1489 or clones thereof, often also having 1488/1489 in their name. If you are lucky, these chips are socketed, and you could try pulling them.

To differentiate between the two possiblilities, you could try to apply 5V only, and test whether the short appears even without +12V/-12V.

Hi All,

Thanks for the feedback. Following the advice that what I'm seeing is probably correct, I went ahead and re-installed all the caps I pulled. The power supply appears to be running fine when connected and doesn't go into any sort of protection mode. The chips all appear to be getting their voltage.

Unfortunately it's still booting to a black screen. The monitor gets a signal just black. No beeps. The only two chips that get warm are the processor and FDC chip. I'm suspecting maybe a bad BIOS. I'm waiting on the programmer to see if I can write some new images and hope that was it.

Just an update on this. I had my hunch about a bad BIOS sort of confirmed today. I remembered that while digging through a bad of old ICs I remembered there was one with a Compaq sticker on it. Turns out it was a BIOS from one of their XT clones. So I figured "Compaq was an XT clone - Commodore is an XT clone. What's the worse that could happen". Well popped it in and it posted. Gibberish but it posted. So pretty sure the board is fixed just need to program a new BIOS.