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Need some pointers for next steps on 5160 testing

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MikeTwain

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Jul 31, 2019
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Apologies in advance if any of these questions are considered common knowledge. I've been trying to read and watch videos on how to rehab my recently acquired XT but there are a few things I don't understand too well. I'm planning to go to the September VC Federation repair workshop but I'm trying to make some progress in the meantime.

Here's where I'm at:

With the minimal diagnostic configuration (MB, speaker, MDA card, keyboard) I can boot to ROM BASIC.
Although I have an original power supply which seems at least partially functional I'm using an ATX power supply with a P8/P9 XT/AT style converter.

My next step was to try to see if I could get anything out of the Hard Drive so I plugged in the Hard Drive controller card (but didn't connect the ribbon cables or plug in power to the HDD) and tried to boot again. The machine got through the mem test and then seemed to pause for a long time at which point the ATX power supply blew and shut down.

My first assumption was that there was some kind of short in the HDD controller that somehow overloaded the power supply but I know virtually nothing about electronics so I have no idea if that's even possible. The power supply was admittedly a $5 find at a resale store so it could easily have just been a bad PSU.

Here's what I'd like advice on:

Does anyone have any advice or resources on how to test expansion cards for shorts, etc? I have watched one video where the resistance on the 12v and 5v lines are checked when the card is plugged in and removed. The resistance drops but doesn't go to zero.

Is there some other next step that I should be pursuing?

Thanks!
 
The resistance between the voltage (5V or 12V) and GND is a measure for the power consumption. Using Ohm's law (resistance = voltage/current) and the power definition (power = voltage*current), you can get a feel on whether your resistance is abnormal or not. A resistance of close-to-zero is a short circuit. The resistance is not necessarily constant, though (power consumption depends on what the chips actually do).

Capacitors can be not-completely-shorted (very low resistance), in which case they carry a high current. A high current means a lot of power, which means a lot of heat - which can blow out the capacitor (or the PSU, or the traces).

Use a current-limiting supply and slowly crank up the limit while watching the temperature. That gives you a good indication of where the power goes.
 
MikeTwain said:
Does anyone have any advice or resources on how to test expansion cards for shorts, etc? I have watched one video where the resistance on the 12v and 5v lines are checked when the card is plugged in and removed. The resistance drops but doesn't go to zero.
Click to expand...
I have used this method with great success. I don't even bother to check the actual resistance -- my ohmmeter squeals when a short is encountered.

A stand-alone motherboard is my goto when I want to test expansion cards for shorts. :)
 
MikeTwain said:
Does anyone have any advice or resources on how to test expansion cards for shorts, etc? I have watched one video where the resistance on the 12v and 5v lines are checked when the card is plugged in and removed. The resistance drops but doesn't go to zero.

Is there some other next step that I should be pursuing?

Thanks!
Click to expand...

Any card added to the system will increase the load (i.e.-decrease the resistance) on the power supply. With the power supply unplugged from the motherboard, and measuring (with an ohmmeter) across the 12v to ground and 5v to ground, note the resistance across each (I'd imagine it's something close to the video you watched). Insert each card individually in the ISA slots and see how much the resistance drops for each card. While it should drop some, a large drop (bringing one or both 12v-GND/5v-GND connections close to 0 ohms) would indicate a bad card.

As Stone mentions, you can try this with a standalone system but be prepared to act quickly if you power it up with a potentially shorted card inserted.
 
MikeTwain said:
My next step was to try to see if I could get anything out of the Hard Drive so I plugged in the Hard Drive controller card (but didn't connect the ribbon cables or plug in power to the HDD) and tried to boot again. The machine got through the mem test and then seemed to pause for a long time at which point the ATX power supply blew and shut down.
Click to expand...
That has got to be just a bad power supply. If there were anything wrong on the motherboard or controller you would not have been able to get that far without seeing smoke from either of those. ATX power supplies were usually built to handle much higher loads than an XT motherboard.

BTW, with no drive attached it should have sat for a minute or two and eventually given a "1701" error, or something similar indicating it could not find a hard drive.
 
T-R-A said:
As Stone mentions, you can try this with a standalone system but be prepared to act quickly if you power it up with a potentially shorted card inserted.
Click to expand...
I didn't say a standalone system.

I said a stand-alone motherboard. It is therefore... unpowered. There is no PSU attached.

If I insert a card into a slot and then test the ±12V and ±5V rails against ground and there's a short I know it's on the card I just inserted.

FWIW, if you power up a system with a short on the motherboard or one of its cards the switching power supply shuts down immediately just as described on modem7's site. So, even then, there's no urgency to act quickly.
 
Stone said:
I didn't say a standalone system.

I said a stand-alone motherboard. It is therefore... unpowered. There is no PSU attached.

If I insert a card into a slot and then test the ±12V and ±5V rails against ground and there's a short I know it's on the card I just inserted.
Click to expand...

Sorry, didn't catch that...and good point.
 
I did some more testing today and my floppy controller seemed good from the perspective of shorts so I connected the controller and the floppy drive. Floppy lights up but I don't hear much in the way of spinning or drive activity...eventually it boots to ROM basic. I'll be trying the HDD controller next I suppose to see if I can get anything out of that.
 
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