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Troubleshooting an XT

if you're in an absolute bind, let me know. I keep a small stockpile of mobos around. If all else fails, I could test em and send it to you. You'd have to pay the postage though. Don't ask me how much that would cost.
 
The TTL chips on the motherboard require +5 volts. I've seen different tolerances listed for TTL chips (eg. +/- 0.2V, +/- 0.25V, +/- 0.5V) but personally (I used to repair personal and mini computers) I wouldn't be happy with anything below 4.9V. Remember too that there'll be some voltage drop across the motherboard.

Anyhow, an authoritive source for the IBM XT system is IBM's technical reference for the XT. For the IBM 130W PSU, the tolerance on the +5V is +5%/-4% over the allowable current range. Thus the +5V should be +4.8V minimum.

Earlier I thought, the HDD is making a rattling sound but there's still a chance it will work. But with the HDD dragging down the PSU voltages as you've indicated, you can safely write off your HDD.

Well, unless you've got an under-powered PSU. I presume your PSU is at least 130W (what was supplied with the IBM XT).

I can see an ST-412 advertised at "http://www.labx.com/v2/newad.cfm?CatID=65&Page=254"

With only the motherboard (with no cards) connected to the PSU, the voltages (and 'power good') are fine.
And in that configuration you heard no beeps at all.

How unlucky - a stuffed motherboard and a stuffed HDD ?
 
According to the Seagate web site, the ST-412's power consuption ranges from 26W (typical) to 50W (max).
Therefore (even allowing for inefficiency ratio of the PSU), if you do have a 65W power suppy, it will be able to power just the ST-412 alone (assuming the ST-412 is serviceable).

Looking back, with only the HDD attached, interesting that your PSU is generating a 'power good' signal with the -5V sitting at -4V, the +12v at +10V, and the -12V at -10V. If it's a clone PSU you've got, it wouldn't surprise me. It's possible that within the PSU, only the +5v is looked at when generating the 'power good' signal.

In my IBM XT 130W PSU and ST-412 configuration (the HDD uses +5V and +12V), the +12v rightly stays at +12V. You could just have a cheap PSU where the +5V is regulated but not the other voltages. That would explain why the -5V and -12V has dropped as well (neither is supplied to the HDD).

And so on reflection, I wouldn't right-off the ST-412 just yet. Yes, it's making a 'rattling' sound (loose screw?), but all the other sounds it's producing are normal. However, with it dragging the +12v down to +10V, it doesn't look good. I'm sure even an engineer designing a cheap PSU wouldn't design the PSU so that the +12V dropped to +10V under normal load. But does that indicate that the PSU has failed in strange way.

Re your motherboard. Another test. See if it works when removed from the chassis.
 
Check the drive on the other computer. Connect only power input and you will see: it's power problem or disk problem.
 
Chris2005 said:
What you might want to do is place a (more or less) local ad in a newspaper or these little booklets they often give away (at least over here across the pond) for nuffin. There's bound to be people not too far away with old puters serving as tenements for silver fish and whatnot.
AND IF IF YOU SHOULD COME ACROSS A RESEARCH MACHINES NIMBUS PUTER PLEASE PLEASE SNIFFUL KEEP ME IN MIND. They're pretty common over there as far as things go I'm told. I'd love to see you find one. You could scrounge one of those GASP for parts, but I wouldn't advise it. Incompatible as all hell too lol, like a Tandy 2000, and n fact uses the same cpu (80186).
Click to expand...

RM Nimbus? Most old schools had those, sadly I haven't seen any for years but I'll keep an eye out :) My University still used modern RM machines, and probably still do.
 
Hmm it doesn't sound good for the hard drive. But at least you don't NEED one of those to use the XT :)

The power supply has a sticker on it saying 150W, how to check if it is a clone one or not? Maybe I should try doing the voltage tests again with another drive attached? Will try and test the MB removed from the case later on today.
 
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Just tried the HDD on my 333MHz AMD, and it still sounds the same. The grinding sounds have stopped now though, on both computers.
 
I don't ever recall seeing a genuine IBM PSU for the XT that differed from the 'standard' wiring colours. Most genuine IBM equipment will have a 7 digit part number on them. The part number is either the actual part number or the Field Replaceable Unit (FRU) part number. Some parts I've seen had both. My 130W PSU has a grey sticker labelled 'IBM 1501438' on the top.

Why did I suggest removing the board from the chassis? In the late eighties, I saw a few clone computers where someone's attempt to assemble a computer, or upgrade the motherboard, failed. Sometimes it was because something had been shorted out on the motherboard: In some cases a screw with an oversized head was used. In some cases the manufacture required that a non-conductive washer be used with the screw, and no washer was used. Sometimes the shorting was caused by a metal standoff under the motherboard.

Something else worth mentioning. I also saw the occasional genuine IBM that had problems after someone replaced the genuine XT motherboard with a clone AT motherboard. From memory, the boards started up okay but were unstable. Replacing the IBM PSU with a clone 'AT' PSU (one that fitted in the XT case) fixed the problem. I recall hearing that in moving to the AT, IBM changed the design of the 'AT' PSU so that certain voltages came up before others (because of something on the genuine AT motherboard.) Maybe some reader can elaborate.
 
Thanks to Jorg who sold me his old XT mainboard, this XT is back alive :D

The mainboard is a 256-640 version, but only has 256kB (four banks of 64k) so I have pushed the total up to 640kB with a couple of expansion cards as I don't have any spare 256K RAM chips to swap in. Had a few problems with bad memory on the cards, but luckily there were a couple of good banks on the dead mainboard I could salvage.

10MB Seagate ST412 is out of my 5150 (which is getting a hard card when it arrives) booting to DOS 2.10. I don't have any spare working FH drives, so the HH drive is good for now, and besides, the later XT's did come with HH drives so it is not all incorrect ;) I'll have to look in the spares box at work tomorrow to try and find a blanking plate to fill the gap!

One weird thing I noticed is that the mainboard is dated 8647 but has a BIOS date of 11-8-82, whereas the 640k mainboard in my other XT is dated 8611 and has a BIOS date of 1-10-86?!?

xt_working01.JPG

xt_working02.JPG

xt_hellovcf.JPG
 
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I'm not sure if to laugh or cry :) :| :(

Nice job, and I'm happy it found a good home!

I'll post some pictures when I am done with my building-an-Asus-txp4+Pentium 233-in-my-empty-XT5160-case.

Its going to have 192 MB ram, FAST SCSI harddisk and cdrom, and a wireless card- looking to run it under FreeBSD 6.
Oh, and Hercules graphics on a 5151 monitor :D

Picky thing is to find a wireless card that goes in a 5V PCI slot, is supported by FreeBSD, and does WPA.
But I think I found one, just have to order it.
 
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Where is the schematic for the IBM 1501438 power supply located?

A parts list (electrolytic, foil and tantalum capacitors!) would be even better.........


Okay, I have a schematic on Pages 8&9 of Sams Computerfacts for the 5150, the parts list as per above would be better.

Somone must have recapped it?
 
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