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Kanoen
September 23rd, 2013, 11:42 AM
Hi there,
I am new around here and I admit that I registered because I am looking for help.

Recently I've bought an IBM 5155 which was supposed to be in working condition. Well, it wasn't. After replacing two shorted caps on the original graphics card (C6 and C8 ) the machine is kind of powering up. The PSU fan is turning, and the CRT turns on but shows nothing but an orange glow. No POST beeps and no sign of life in the floppy drives.

I extracted the motherboard and checked every capacitor; no shorts there, they seem to be alright.

I then performed the "Minimum Diagostic Configuration" test as explained here. Only the PSU and the speaker connected to the motherboard (I had the CRT and PSU fan still internally connected to the power supply if that matters).
http://www.minuszerodegrees.net/5150_5160/MDC/minimum_diag_config.htm

Result: no beeps (the speaker is working BTW), the PSU fan is turning, and the voltages on the powered motherboard are as expected:
http://www.minuszerodegrees.net/5150_5160/MDC/no_beeps_2.htm

This implies that the PSU is OK, and according to minuszerdegrees the most likely cause for motherboard failure is a faulty RAM chip or a faulty BIOS.
How do I find out about that? How can I replace either of them?

Finally, here's one weird thing I've noticed: capacitor C56 on the motherboard seems fine on the bare, unconnected motherboard. However, as soon as I plug in P8 the capacitor becomes shorted. Could there be something wrong with the PSU after all?
edit: I've removed C56. Still the same.

Any input is appreciated. Thanks!

modem7
September 24th, 2013, 12:30 AM
Hi there,
I am new around here and I admit that I registered because I am looking for help.
Well, welcome to these forums.


Recently I've bought an IBM 5155
If you are not already aware of it, note that the 5155 contains an early 5160 (XT) motherboard.


This implies that the PSU is OK,
Yes, under an adequate load, the PSU is producing DC voltages within spec, and producing the POWER GOOD signal.


and according to minuszerdegrees the most likely cause for motherboard failure is a faulty RAM chip or a faulty BIOS.
How do I find out about that? How can I replace either of them?

Possibility: One or more faulty RAM chips in bank 0

This is much more likely than a faulty BIOS chip.

On a 5155 motherboard (very early 5160 motherboard), all four banks of RAM use the same type of RAM chip, and unlike in the 5150, all four banks are in sockets.
Diagram [here (http://www.minuszerodegrees.net/5150_5160/ram/5150_5160_ram_banks.jpg)].

First, try simply reseating the chips in bank 0 (in case of a poor connection).

If that doesn't get the motherboard going, using anti-static procedures, try swapping banks 0 and 1. That assumes that all of the RAM chips in bank 1 are good.

And if that doesn't get the motherboard going, for good measure, try swapping banks 0 and 2.

Possibility: Faulty BIOS chip

On a 5155 motherboard (very early 5160 motherboard), the BIOS revision is 11/08/82.
In that revision, the U18 chip contains the actual BIOS and POST (with BASIC spread over both U18 and U19).

What you could do is get someone to 'burn' you a replacement U18 chip, created from a 27256 EPROM.
That someone would get the image for the EPROM from [here (http://www.minuszerodegrees.net/bios/bios.htm)].

If that doesn't fix the problem, then you have a spare U18.

Other

Of course, there can be many other causes of motherboard failure

mikey99
September 24th, 2013, 07:22 AM
Supersoft Landmark diagnostics ROM would be a useful tool....if you have access to an EPROM burner.....or can find someone to burn a copy for you.

Kanoen
September 24th, 2013, 11:24 AM
Thanks for your replies!

I know some basic electronics but EPROM burning is something I cannot do myself. I will first try to swap the memory chips. I've also in the process of acquiring a spare AT power supply just to be sure. For now I had to stash the computer because its carcass claimed too much living room space ;) In the weekend I will have a chance to looking after it again. I will keep you updated if I make any progress.

Cheers!

Al Hartman
September 24th, 2013, 12:00 PM
You can find ISA POST Diagnostic Cards on eBay pretty affordably.

mikey99
September 24th, 2013, 02:50 PM
You can find ISA POST Diagnostic Cards on eBay pretty affordably.

I've been looking at those cards.....although it performs a quick check.... I would think the diagnostic ROM
would perform a much more extensive check. However, you do need a working display to see the results
of the diagnostic ROM wheras the POST card has an LED readout.

I was mentioning the diagnostic ROM .......because if you get someone to burn a new U18 ROM, they could
easily burn a second one with the diagnostics. The images and docs are on modem7's site. The docs
give a bit of basic troubleshooting info in the first few pages.

Al Hartman
September 24th, 2013, 03:25 PM
My understanding of the POST card is that it links into the ROM routines and gives you numeric codes for each stage of the POST. It itself does no checking. It is merely a display.

It would probably be great for the OP to have a diag ROM. I mentioned the POST card because they are inexpensive and a good tool to have.

modem7
September 24th, 2013, 11:33 PM
You can find ISA POST Diagnostic Cards on eBay pretty affordably.
Pretty much useless for an IBM 5155 (contains an early 5160 motherboard).
See the earlier thread [here (http://www.vintage-computer.com/vcforum/showthread.php?27859-POST-Diagnostic-Cards-8-Bit-ISA-Useful)].

Kanoen
November 20th, 2013, 02:55 AM
OK, it's been a while. In the mean time I have done the following things in a "Minimum Diagostic Configuration"

-Connected a tested (new old stock) AT power supply -> no beeps.
-Reseated all chips in sockets -> no beeps.
-Swapped the RAM chips of bank 0 and bank 1 -> no beeps.

I am afraid it's time to source some 27256 EPROMs and find someone who can burn them for me.

modem7
November 21st, 2013, 10:03 PM
-Swapped the RAM chips of bank 0 and bank 1 -> no beeps.
Because you do not know the history of the computer, there is the possibility that there are bad chips in both RAM banks 0 and 1.
Try swapping banks 0 and 2 (assumption: you have the original 5155 motherboard - uses 4164 type chips in all 4 banks)


I am afraid it's time to source some 27256 EPROMs and find someone who can burn them for me.
See [here (http://www.vintage-computer.com/vcforum/showthread.php?40043)]. Member Erwin-c64, in the Netherlands, might be able to help you.