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5160, weird power problem

S

snq

Experienced Member
Joined
Mar 29, 2009
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164
Location
Sweden, way up north
Thanks to the help I got here my 5170 is now up and running, so it was time to start with the 5160.

Its original power supply was dead and its contents have been replaced with a modified generic AT power supply - original wiring from the XT PSU hooked up to the inners of the newer PSU, a couple mounting brackets on the inside to mount the new contents, just by looking at it you wouldn't know the difference. Weight and noise wise - big difference!

Anyway, this machine is behaving quite weird. Wether or not it powers up seems to depend on what kind of stuff is attached to the mainboard. As soon as I attach the P8 connector to the mainboard, it will not power up. No disk spinning, the PSU fan doesn't start, nothing happens whatsoever. Without the P8, it works kind of fine :-? It will boot to floppy etc, everything seems fine. Until I put in a quadboard, then it's completely dead again. The same quadboard works fine in my AT.
I've tried removing everything from the system leaving just the mainboard with nothing connected to it (no keyboard, nothing) and it just will not power up when P8 is connected. With P8 disconnected, it's all fine again and I at least get a couple of beeps because nothing is connected.

Obviously the PSU is a bit of a hack, but I don't see why it wouldn't work. I don't think I'm the first person to replace it with something else. This PSU is rated at 145W, which should be plenty for an XT I think.
I've tested with another generic AT PSU with +5V on P8 removed, and that gives exactly the same result.

So I'm kind of looking for an explanation for why it's completely dead with P8 connected, and why it actually works in most cases with P8 disconnected. I wasn't exactly expecting it to do anything at all but I just had to test.
 
Well, let's see, what does P8 carrry?
  • Pins 6 and 5, ground
  • Pin 4 -12
  • Pin 3 +12
  • Pin 2 NC (this must be blank
  • Pin 1 Power Good

I assume with P8 disconnected, you've taken a voltmeter and checked the other leads. So, if the voltages are correct, then the motherboard probably has a short on it. You can quickly tell which of the remaining P8 connections is at fault by connecting only one at a time of the +12 and -12. (The PG pin shouldn't be able to take the system down, but if neither +12 or -12 connecting does, try it). Suppose you determine that the +12 causes a failure. You should trace the line and look for any shorted capacitors along the line. Note that the main board itself uses neither +12 nor -12, but routes them to the expansion connectors. However, there are decoupling capacitors on the motherboard. You can remove a suspect capacitor to test and replace it later.
 
Is the PG pin at all used on the XT? Because it doesn't seem to care that it's not getting anything there.
I have checked the leads they all give what they're supposed to give so I guess it's indeed the mainboard. I just checked by removing the 12V and -12V one at a time, and if either of them is connected (PG already removed since earlier testing), everything is dead. So right now I have a P8 connector with only 2 ground wires...
This sounds a little advanced for me, but I may just give it a shot.

Would the lack of +/-12V with P8 disconnected have any effect on the HD giving nothing but i/o errors? Because that would be the next thing that I need help with ;) No errors at POST and boots to floppy fine, but fdisk can't find a drive. Diskman sees 1 harddrive, controller tests okay, but any reading results in an error. Same thing with 3 different drives of which at least 2 are tested and working in my AT (those 2 are larger than 10M though and I have the controller that only supports 10). Tested different cables and different positions on them but everything else results in a 1701 error.
 
The +12 is used on the XT hard disk controller; so it's not going to work if the supply isn't there.

Okay, put everything back into the P8 connector and remove all expansion cards. Does the unit power up? (i.e. we're trying to determine if it's a card or the main board)
 
I tested that earlier (removed all cards and disconnected everything else including drives and keyboard, and also tested with only the mainboard and a harddrive attached to put some load on the power supply) and it did not power up, not even the power supply fan spins. That's when I discovered that disconnecting the P8 connector made it somewhat work.
I guess I should be looking at either a new mainboard or see if I can repair this one, I don't have a lot of experience with that kind of repairs though... If anyone has a mainboard to spare I'm interested!
 
QuantumII said:
Why not take a multimeter to the mainboard P8 connector to probe for shorts? (GND to +12v , GND to -12v)
Click to expand...

Yes, but the OP is unwilling to do board-level repairs.

I've said before that if you're going to get into this business of collecting antique technology, it's pretty much a good thing to learn how to repair it. Could you imagine an antique car collector who refused to get his hands dirty?
 
Actually there's some progress now, not sure if it actually helps me anything or if it just makes things worse.
I re-tested the empty system with the other PSU, and lo and behold, it started up with +12V connected. Not with -12V though. But I figured if I can get +12 that's better than nothing.
So I started putting the system back together, the plan was one card at a time and testing each time if it will power up. The first card to go in was the CGA card. That right away resulted in the system not powering up, however this time the PSU fan did spin. Some progress at least altho I obviously have no idea why it changed its behavior all of a sudden. If I disconnected P8 with only the CGA card in the system, it did power up.
I replaced the CGA card with an EGA card and changed the switches. Now it started up with P8 connected! So only -12V disconnected now, putting -12V back made it stop working again, so I think it's safe to say something is definitely wrong there but hopefully +12V is okay. I'm currently putting the system back together one card at a time. So far so good, the FDD controller is back and working (boots to floppy), and I just put the HDD controller and HD in and am about to check if maybe it will actually work now.

As for repairing the board, I'm not unwilling, in fact I'd love to be able to just repair it, but it's a steep learning curve for someone who never had to deal with anything like this before. Electronics are like wizardry to me... I don't even know how to go about for testing the capacitators for example. I'm sure all info is out there on the internet so it's just a matter of reading and learning. I guess I'm just afraid I'll break it even more, and in that case I would probably rather have a somewhat working system than a completely dead one. If we're going to use car analogies - I'd rather have a car without a stereo than a car that doesn't run at all because I ruined it while trying to fix the stereo.
And as for getting hands dirty, I do collect vintage snowmobiles and have a couple of old cars as well, and have no problem getting my hands dirty - most of the stuff I buy does need a lot of work ;)
 
If you're worried about learning to solder and recognize components, the learning curve isn't that steep. And you have the benefit of the web, with instructional videos on YouTube and tons of print articles.

Otherwise, why bother with the old systems? Much newer machines are easier to find and will boot MS-DOS--and they run faster.
 
I'm mostly into this stuff for nostalgic reasons rather than becoming an electronics expert, and I'm sure I'm not the only one that is into this for that reason. But I can see that it'd be a good skill to have.
I know how to hold a soldering iron but as soon as anything more than just wires is involved, I freak out because I don't know what I'm doing ;)

The drives are still all giving me nothing but I/O errors so I guess I'm going to have to bite the bullet and see if I can fix this. Could obviously still be a bad controller, but the the power problem is quite obvious so that needs to be solved first because it's impossible to say anything about the rest of the system until this is solved. I know it will give me a lot of satisfaction if I do manage to get it all working without just replacing the mainboard!
 
So there's only 2 for each line? That wasn't as bad as I expected. I was kind of expecting having to de-solder half the board.
I was going to inspect the board visually but for whatever reason the floppydrive (in the left position) won't come out. Kind of gotta solve that first now...
 
Okay, the board is out and measuring between the -12V pin and ground gives none or very little resistance, so something is definitely wrong there. +12V to ground gives me a lot of resistance and it's a bit unstable, I think that is what it's supposed to do? I also measured the CGA card and it seems to have a short between +12V and ground, so I think that explains why replacing it with the EGA card made things a little better.
The C57/C58 capacitors don't show any external damage as far as I can tell, see attached pic.

How should I proceed from here? My idea was to remove the 2 capacitors one at a time and measure -12V to ground. If the resistance is okay I found the bad one?
I've been searching for a way to test them off the board, but I don't have a capacitor tester and measuring using a multimeter only seems to work with larger capacitors according to what I've been reading.

IMG_0046.jpg
 
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The "instablility" is probably due to a capacitor charging. The resistance should slowly increase as the capacitor charges.

Remove the -12 caps one at a time. Try the large one first--it's more likely to fail.
 
I had already gotten started before I read your reply.. So I started out with the smaller one, and that didn't help. But after removing the bigger 3 legged one, -12V to GND resistance was now infinite.
I really need to get myself one of those solder suckers, but I managed even if it took forever.

Problem found, I guess?
Now I just need to find something to replace it with.

Is there any proper way of testing these things with a regular digital multimeter? Or at least a test to see if they're completely broken or not?
 
Capacitors are either good or bad--no in-between really. Since nothing is using your -12, go ahead and try powering the system up without it. You can always get another capacitor when you get around to needing it.
 
Are you sure the HDD controller isn't using -12V for something? Because that could explain the I/O errors.
All the other cards in there seemed to be doing their job just fine without the -12V though.

This wasn't so hard after all, so far anyway. Altho I wouldn't have been able to do it without the info you gave me, so thanks :)
I'm wondering how you knew C57/58 were the ones to look at, because I'd never have been able to figure that out by myself. Can't see any visible lines going from the -12V to there.
 
snq said:
Are you sure the HDD controller isn't using -12V for something? Because that could explain the I/O errors.
All the other cards in there seemed to be doing their job just fine without the -12V though.
Click to expand...

No, the 5160 HDC is a bizarre little thing. It needs -5, but it gets it by using a DC-DC inverter to get the -5, not from the -5 line.

This wasn't so hard after all, so far anyway. Altho I wouldn't have been able to do it without the info you gave me, so thanks :)
I'm wondering how you knew C57/58 were the ones to look at, because I'd never have been able to figure that out by myself. Can't see any visible lines going from the -12V to there.
Click to expand...

A schematic helps a lot.
 
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Okay, well I put everything back together for now with the -12V wire back in place, and it works like a charm now using either the modified XT PSU or the AT one with +5V disconnected from P8.

Unfortunately no improvement on the harddrive I/O problem, I'm afraid the controller is bad.
 
snq said:
Okay, well I put everything back together for now with the -12V wire back in place, and it works like a charm now using either the modified XT PSU or the AT one with +5V disconnected from P8.

Unfortunately no improvement on the harddrive I/O problem, I'm afraid the controller is bad.
Click to expand...
There you go :)
It could also be the HDD that is causing the I/O errors, not just the controller alone.
 
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