Repairing an IBM PC XT 5160

[Captain Chaos]

Hi everyone,

This is my first post, and I'm just getting into the vintage PC repair hobby. I have basic electrical knowledge and some basic equipment like a multimeter and soldering iron. I purchased an IBM PC XT, plus keyboard, monitor and printer. It was being sold as fully working (except the floppy drive), but it has very rapidly deteriorated in various ways and now won't boot at all any more.

It had been fully working days earlier (I was shown days old videos of it in the same location and I think the seller is trustworthy), but when I came to pick it up, the video had stopped working. There is a problem with either the monitor or the MDA card. The monitor just displays a bright green field with a brighter scanline dropping from above. We could tell that it still booted though, as you could press Enter twice for date and time and then enter commands like dir which resulted in exactly the harddrive activity one would expect. We could even get the printer to print.

Then, while turned off but connected to the wall, the two RIFA capacitors in the power supply blew with a loud bang and a lot of smoke. But even then, the PC would still boot. I decided to purchase the set anyway.

Since then it has deteriorated further, and it now no longer boots at all. The harddrive does spin up and its light flickers very briefly. It seems to get to the POST and then fails with two short beeps. I assume an error code would be showing on screen, but the display doesn't work so we can't tell.

I think it will be a nice challenge to get this thing working again for me and a friend who is helping me. Things we have already done: we have ordered an 8-bit ISA VGA card, so that we can use a VGA monitor and hopefully see what the POST error is, and leave the display problem for later. And we have ordered two 0.1μF 275V X2 filter capacitors to replace the RIFA capacitors.

Hopefully you guys can give us tips and pointers for the following questions/problems we have:

1. The C56 and C58 capacitors on the motherboard have been intentionally disabled by someone. Two of their leads have been cut and the capacitors tilted up. Why would this have been done? What problems could it cause? Should we restore/replace them? Since the PC was fully working at one point in this configuration they don't appear to be essential.
2. Is the display problem likely to be due to the MDA card, or the monitor? When we remove the card we do get the beeps that indicate it can't initialise the graphics card, so it seems that the BIOS can at least initialise the card. Where do we start to diagnose this?
3. The floppy drive turned out to have had its power lead disconnected by someone. When we connect the power, the power supply of the PC won't even turn on (the fan doesn't turn). Does this mean it likely has a short? Where should we start looking to diagnose the floppy drive?
4. When we disconnect the harddrive the power supply also will not even turn on. Is this because it needs a minimum load?

Is there any general advice you can give for things we should look for or that make sense to do?

Many thanks in advance for any help you can give us with getting this PC working again! Let me know what information, photos, etc. I can provide to clear things up.

 

[ajacocks]

Welcome to the forum, @Captain Chaos !

That is an excellent write up of the state of the XT. Here are my initial thoughts:

1. Those capacitors are tantalums, and likely removed because they are shorted. They’re not 100% critical to the board running, but should be replaced with new ones. See:

https://www.vcfed.org/forum/forum/genres/pcs-and-clones/34322-removing-c56-from-ibm-xt-system-board

2. Given that the monitor produces a valid screen image (and what you describe sounds normal for the monitor receiving no signal), I’d suspect the MDA card. With that said, those cards are extremely reliable, so I’d start with cleaning the edge connector and spraying some Deoxit in the card slot, on the motherboard, to help with possible corrosion. Have you tried the card in a different slot?

3. Yes, it sounds like either 5 of 12vdc is shorted on the floppy drive. It should be possible for you to poke around with your multimeter and find the source of the short. You might start with the capacitors on the unit.

4. Yes, the power supply will not turn on without a minimum load. You can use any generic (even modern) hard drive as the load, though. I suggest finding a known-bad, but running, hard drive to use as your test load. Those should be free.

Hope this helps some!
- Alex

 

[Timo W.]

4. Yes, the power supply will not turn on without a minimum load. You can use any generic (even modern) hard drive as the load, though. I suggest finding a known-bad, but running, hard drive to use as your test load. Those should be free.

Wasn't that only the case for the PC/AT, i.e. the 5170, which had a dummy load because of that? I don't think the 135w PSU of the XT needs a hard disk as a load to turn on at all. The bare mainboard should certainly be enough load.

 

[Eudimorphodon]

Wasn't that only the case for the PC/AT, i.e. the 5170, which had a dummy load because of that? I don't think the 135w PSU of the XT needs a hard disk as a load to turn on at all. The bare mainboard should certainly be enough load.

I would second that. I used to test 5160s with nothing more than a video card plugged in and they'd work fine. IBM sold the machine in hard-disk-less configurations and I've never seen one with any kind of load resistor plugged into it.

Given that the monitor produces a valid screen image (and what you describe sounds normal for the monitor receiving no signal), I’d suspect the MDA card.

Is the monitor in question here a 5151, or some third-party unit? My recall is a little fuzzy on the details, but I don't believe the 5151 has any internal sweep generation, it relies on getting Hsync from the video card as a direct drive signal. So if it's painting a green screen I think that's a sign that the CRTC on the video card at least sort of works?

Just to sanity check, you've definitely got the MDA monitor plugged into an MDA *card*, not a CGA card? (Or a dual-mode card that may have for some reason been switched to CGA mode since those videos you saw of the machine working?) A 5151 plugged into a port outputting video at CGA frequencies is a no-no.

 

[modem7]

I purchased an IBM PC XT, plus keyboard, monitor and printer.

Lots of IBM XT (IBM 5160) information at [here].

It seems to get to the POST and then fails with two short beeps.

Two short beeps only is strange. That symptom does not appear at [here]. And an MDA card is not going to have a BIOS expansion ROM, something that can generate error beep patterns.

Does the motherboard have an IBM BIOS ROM set ?

The monitor just displays a bright green field with a brighter scanline dropping from above.

If the monitor is an IBM 5151, then see the 'No video, but a stable raster shows if brightness turned up' section of [here].

The C56 and C58 capacitors on the motherboard have been intentionally disabled by someone. Two of their leads have been cut and the capacitors tilted up. Why would this have been done? What problems could it cause? Should we restore/replace them? Since the PC was fully working at one point in this configuration they don't appear to be essential.

Those capacitors are tantalums, and likely removed because they are shorted. They’re not 100% critical to the board running, but should be replaced with new ones.

Replacement information at [here].

The floppy drive turned out to have had its power lead disconnected by someone. When we connect the power, the power supply of the PC won't even turn on (the fan doesn't turn). Does this mean it likely has a short? Where should we start looking to diagnose the floppy drive?

Yes, it sounds like either 5 of 12vdc is shorted on the floppy drive. It should be possible for you to poke around with your multimeter and find the source of the short. You might start with the capacitors on the unit.

If the floppy drive is a Tandon TM100-2, then based on history, the most likely cause is either C36 or C43, per the 'Short-circuit capacitors' section of [here].

When we disconnect the harddrive the power supply also will not even turn on. Is this because it needs a minimum load?

Yes. Assuming the stock IBM power supply, the IBM XT motherboard alone is normally enough of a load, but we have had IBM XT owners reporting that more than that was needed.
Possible causes of IBM XT motherboard alone being inadequate:
* The power supply 'guts' IBM used varied (different make-models of 'guts') and so some variation of minimum load is expected (and on what lines).
* Power supply deterioration.
* The effect of C56 and C58 on the motherboard being disconnected (they draw some current at power-on).

 

[Captain Chaos]

Thanks @ajacocks, @Timo W., @Eudimorphodon and @modem7, for your replies so far! They are extremely helpful and I'm sure we'll get a lot further now. Just to answer some of the questions:

I’d start with cleaning the edge connector and spraying some Deoxit in the card slot, on the motherboard, to help with possible corrosion. Have you tried the card in a different slot?

I have tried it in a different slot and it still didn't work, but to be sure I will find some Deoxit or similar and try that.

Is the monitor in question here a 5151, or some third-party unit?

It's a 5151. The symptom is exactly as described on that page, which would seem to indicate the monitor is at least getting the sync signals from the card.

Just to sanity check, you've definitely got the MDA monitor plugged into an MDA *card*, not a CGA card?

Yes, I'm sure it's an MDA card. It says "monochrome adapter" (or similar; I'm not near it currently) on it and it has the parallel port on it as well.

Does the motherboard have an IBM BIOS ROM set ?

Yes, judging by that page it has the 11/08/82 version of the IBM BIOS.

Thanks again and I'll keep you guys posted!

 

[Captain Chaos]

Just in case it's relevant: I am in the Netherlands, and this is a 240V machine.

 

[Captain Chaos]

Two short beeps only is strange. That symptom does not appear at [here].

And to respond to this: I seem to remember from my own experience that PCs always made two beeps on POST errors. I assumed that that page just didn't mention them for that reason and instead lists the different error codes you might see. I hope the VGA card will work and resolve this mystery.

 

[Captain Chaos]

Another question: have I ruined the harddisk by not parking the heads before transporting this PC (twice)? It did not occur to me at all that that might be necessary for this vintage.

 

[modem7]

Things we have already done: we have ordered an 8-bit ISA VGA card, ...

So, you are still waiting for that card to arrive.

Another question: have I ruined the hard disk by not parking the heads before transporting this PC (twice)? It did not occur to me at all that that might be necessary for this vintage.

I say: unlikely, but possible.

Since then it has deteriorated further, and it now no longer boots at all. The hard drive does spin up and its light flickers very briefly. It seems to get to the POST and then fails with two short beeps. I assume an error code would be showing on screen, but the display doesn't work so we can't tell.

I brought out my early IBM XT (has the 11/08/82 version of the motherboard BIOS), disconnected the floppy drive (as yours is), and powered up the XT.
On-screen was the expected RAM count-up, and then TWO BEEPS and:

256 KB OK
601
ERROR. (RESUME = "F1" KEY)

So that explains the two beeps.

Does there then appear to be progress when you press the F1 key after you hear the two beeps ?

BTW. At a DOS prompt, or in Cassette BASIC, you can beep the speaker by momentarily pressing the 'g key whilst the CTRL key is held down

 

[modem7]

Two short beeps only is strange. That symptom does not appear at [here].

And to respond to this: I seem to remember from my own experience that PCs always made two beeps on POST errors. I assumed that that page just didn't mention them for that reason and instead lists the different error codes you might see.

So that I don't get caught out by this again, I have added '2 short beeps' to the web page that I referred to.

 

[Captain Chaos]

So, you are still waiting for that card to arrive.

Yes. It has to come from Russia.

Does there then appear to be progress when you press the F1 key after you hear the two beeps ?

Currently the power supply is disassembled and we can't test that, but we certainly will.

BTW. At a DOS prompt, or in Cassette BASIC, you can beep the speaker by momentarily pressing the 'g key whilst the CTRL key is held down

Ah yes. I remember that, now that you mention it. ☺ #memories

 

[modem7]

BTW. At a DOS prompt, or in Cassette BASIC, you can beep the speaker by momentarily pressing the 'g key whilst the CTRL key is held down

Ah yes. I remember that, now that you mention it. ☺ #memories

Of course, being 'blind', a question may be, "Am I at a DOS prompt or at a Cassette BASIC prompt?"
If entering the following results in a beep, then one is in Cassette BASIC: print chr$(7)

 

[Captain Chaos]

Alright, we made some progress:

• We removed the two destroyed RIFA capacitors and replaced them with new ones. This went well and the power supply appears to be functioning well.
• We tried an EGA Wonder card, which should work with the 5151 monitor, but the monitor displayed exactly the same pattern as it did with the MDA card (a green field with a dropping scanline). So for now I'm assuming that the problem is with the video cable, or the monitor itself. Does anyone have any suggestions as to what it might be likely to be?
• We also tried an 8-bit VGA card with a VGA monitor, and that worked! Finally we could see what was going on. It turned out to indeed be a 601 error, and that we could proceed by pressing F1.
• It turned out the two suggested capacitors on the floppy drive were indeed shorted. We managed to temporarily get the floppy drive to work (as in, to initialize; we don't have floppies to test with) by cutting those two capacitors. We will obtain replacements (as well as for those on the motherboard) and replace them later. The PC now boots without a POST error.
• Unfortunately, we seem to have damaged the hard drive. At one point, before we had the display working, the hard drive was still working and we could boot to DOS by pressing F1 and then two times Enter (date and time). We could enter commands like dir and see the hard drive respond as expected. But I think we messed with it too much and have now damaged the surface, as now whenever we try to boot from the hard drive it just displays Disk Boot failure. I understand that this means it could at least read the boot sector and has started booting DOS, but that subsequently failed. Is this likely caused by a damaged hard drive? How might we diagnose it? Is it worth trying to get Spinrite to fix it?
• It boots to Basic (with the hard drive disconnected)! It was very nostalgic for me to see that screen again after thirty years or so...

Our plans:

• Find a place to test the 5151 monitor. Is anyone located near Amersfoort in the Netherlands who has known good hardware I could come and connect it to to verify whether it works?
• Find a boot floppy, and a copy of Spinrite that fits on a 360 K floppy and runs on an IBM PC XT. Can anyone help me with either or both? I already own Spinrite, but I have no way of writing to 5¼" floppies.
• Find and install replacement capacitors.

 

[modem7]

Alright, we made some progress:

Excellent.

We tried an EGA Wonder card, which should work with the 5151 monitor, but the monitor displayed exactly the same pattern as it did with the MDA card (a green field with a dropping scanline). So for now I'm assuming that the problem is with the video cable, or the monitor itself. Does anyone have any suggestions as to what it might be likely to be?

- Contrast (repeat: contrast) control set too low?
- You have basic electrical knowledge and a multimeter, and so you can check continuity of the video wire in the cable (we know that ground and sync wires must have continuity).
- Thorough visual inspection of inside the 5151 (I have found many problems simply from a good look).
- Per [here], you could take a punt and replace IC201.

Beyond that, I think you are going to need more test equipment.

We also tried an 8-bit VGA card with a VGA monitor, and that worked!

Is that particular make-model in the list at [here] ?
If not, I will add it.

Unfortunately, we seem to have damaged the hard drive.

In case required later, what is the make-model of both the controller and the drive?

How might we diagnose it? Is it worth trying to get Spinrite to fix it?

Yes. And there is other software that can help in diagnosis. For example, [RAYXTMFM] and SpeedStor.

I already own Spinrite, but I have no way of writing to 5¼" floppies.

Are you aware that you can connect a 1.44M diskette drive up to the stock floppy controller in your 5160, and use 720K diskettes in that drive (but not 1.44M diskettes) ?
Additional information at [here].

 

[Captain Chaos]

Is that particular make-model in the list at [here] ?

Yes, it is the one by "Sergey". His full name (per the PCB) is Sergey Kiselev. It has a Trident TVGA9000i chip and BIOS on it. I have version 1.1 of the PCB and can confirm that it works in an XT. It comes without a bracket though, which is slightly inconvenient.

In case required later, what is the make-model of both the controller and the drive?

The controller says SMS MODEL OMTI 5510 on the PCB. The hard drive is a Seagate ST-225.

Are you aware that you can connect a 1.44M diskette drive up to the stock floppy controller in your 5160, and use 720K diskettes in that drive (but not 1.44M diskettes) ?

I was not aware of that. That is extremely helpful information, thank you! Just to be clear: can I use 1.44M diskettes formatted to 720K? Or do they specifically have to be 720K diskettes?

 

[Malc]

Just to be clear: can I use 1.44M diskettes formatted to 720K? Or do they specifically have to be 720K diskettes?

For reliability it's obviously best to use 720k diskettes, But yes formatting a 1.44M to 720k and even 360k has worked for me before, Some info on Modem7's site: http://www.minuszerodegrees.net/5150_5160/720k/5150_5160_boot_using_720k.htm

 

[Captain Chaos]

Are you aware that you can connect a 1.44M diskette drive up to the stock floppy controller in your 5160, and use 720K diskettes in that drive (but not 1.44M diskettes) ?
Additional information at [here].

We've hit a snag. That page says:

For the cable, you can use a standard twisted floppy cable in reverse to how it is normally used.

However a standard floppy cable has pin connectors on both ends, but the floppy drive controller of the IBM PC XT has an edge connector! How do we handle this? Do there exist adapters to go from one to the other? Are they easy to get, or a cable with the right connectors? Would a simple 1:1 adapter do or is rewiring necessary? Or are we missing something?

 

[Lutiana]

We've hit a snag. That page says:

However a standard floppy cable has pin connectors on both ends, but the floppy drive controller of the IBM PC XT has an edge connector! How do we handle this? Do there exist adapters to go from one to the other? Are they easy to get, or a cable with the right connectors? Would a simple 1:1 adapter do or is rewiring necessary? Or are we missing something?

You can buy the correct cable off of Ebay, here is one example. They do make adapters, but in my experience those are usually to go from an edge connector cable to a pin connector on the drive.

 

[Captain Chaos]

You can buy the correct cable off of Ebay, here is one example. They do make adapters, but in my experience those are usually to go from an edge connector cable to a pin connector on the drive.

Thanks! It seems I just ordered the wrong cable. I'll get the right one and proceed. It's also a very interesting journey to find, and then find a way to write, 720K DD floppies...