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IBM 5150 full project

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VintageVic

Experienced Member
Joined
Dec 9, 2019
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Last summer I got this ibm 5150 motherboard.

I have considered this a spare for my two other xt machines, but I was recently
able to acquire xt keyboard (for 80 €). Most expensive keyboard I've ever bought.

But. I realized that this could now be a beginning of a full pc/xt computer! The mainboard is
untested and has suffered decades of Finnish weather in a cold shed. Wish me luck it has
survived that treatment.

It's not much for a beginning, I am missing xt case but I will try to find some random at case and
power to test the board. I might be looking for xt case if the motherboard is functional.

Along with the motherboard I received a number of extension cards. I haven't gone through those
treasures yet very carefully, there might be floppy card, hard drive card, i/o cards and I know there are
memory extension cards as well. There were HDD cables also, but unfortunately no HD itself.

So, let's see what I can come up with these parts.
 

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Remember that a PC motherboard is 5-slot. XT cases (and above) are generally 8-slot.
 
Right.

I hope this is not going to end up being the shortest full project ever.
I have my test setup arranged. AT power, ibm MDA adapter and green monitor.

Any life in it? No, dead as a doornail as they say. Power fan works when it is not attached
to the 5150 motherboard. If the motherboard is plugged in and I turn on power, fan starts and stops
right away and psu starts to give slight buzzing sound.

I seem to remember that these motherboards have fried tantalum caps at times. I measured
these caps (all?) marked C7 and indeed the outer legs of them all are shorted. I guess they
are in the same rail.

Any typical suspects here? Or do I just have to start pulling them out one by one in hope to find
a shorted one? I guess one other possibility could be shorted ram chip. I could pull them out easier
since most are socketed.

Tried to measure ohm values from each of the caps, but my ohm meter is not fine enough. They all
have the same 0,03 ohm resistance thus not giving me any lead which might be the shorted one.
Or maybe there are more than one shorted.
 

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VintageVic said:
Right.

I hope this is not going to end up being the shortest full project ever.
I have my test setup arranged. AT power, ibm MDA adapter and green monitor.

Any life in it? No, dead as a doornail as they say. Power fan works when it is not attached
to the 5150 motherboard. If the motherboard is plugged in and I turn on power, fan starts and stops
right away and psu starts to give slight buzzing sound.

I seem to remember that these motherboards have fried tantalum caps at times. I measured
these caps (all?) marked C7 and indeed the outer legs of them all are shorted. I guess they
are in the same rail.

Any typical suspects here? Or do I just have to start pulling them out one by one in hope to find
a shorted one? I guess one other possibility could be shorted ram chip. I could pull them out easier
since most are socketed.

Tried to measure ohm values from each of the caps, but my ohm meter is not fine enough. They all
have the same 0,03 ohm resistance thus not giving me any lead which might be the shorted one.
Or maybe there are more than one shorted.
Click to expand...

Tantalum caps are polarized and don't really do well when put in a circuit "backwards". Those three legged ones on your motherboard are a work around. The outside legs are in fact connected together in an attempt to make "backwards" insertion a non issue. You're going to need to meter from the center leg to either outer leg.
 
Thanks a lot for that tip!

This narrows it down a lot. All three legs of this cap between J7 and COMP text are shorted!
So this one comes out the mainboard.

Now I should order a replacement. The cap have these numbers: 106 and 16K written on it.
 

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106 = 1 µF, probably 16 volt. An ordinary electrolytic should be fine for non-timing critical stuff. Of course, you'll have to locate the shorted component...
 
And don't always assume it has to be a capacitor. Since many are filter caps, a shorted IC would show the same symptoms.
 
You have the 64KB-256KB type of IBM 5150 motherboard.

On that motherboard:
* One C7 on the the +12V line,
* One C7 on the the -12V line,
* Many C7's on the +5V line,
* Many C7's on the -5V line,

VintageVic said:
Any typical suspects here?
Click to expand...
Yes, per what is list at [here], in the vast majority of cases, it is either the C7 on the +12V line, or the C7 on the -12V line. A photo of those two caps is at [here].

VintageVic said:
All three legs of this cap between J7 and COMP text are shorted!
Click to expand...
So the one on the +12V line. That is the only motherboard component on the +12V line, and because there no cards in the ISA slots, the short has to be in that component.

Replacement details at [here].

When replacing, consider using the technique shown in the lower half of [here].
 
modem7 said:
Lots of IBM 5150 information at http://minuszerodegrees.net
Click to expand...

Thanks everyone for your helpful comments.
I'll be studying next steps from here, this link is very useful.

I can now confirm, that the short was indeed on that suspected tantalum cap. Pulled it out and it was
still shorted. Motherboard was not shorted, when the cap was out. I put there 10uF electrolytic cap and
powered the unit on.

And there is life! Yes!

I had errors:
1008 201 (quick checked this is related to ram)
301 (keyboard error - obviously, since I did not even have it connected).

Followed by:
Parity error 1

I have not even checked any other switch settings besides video, which I set for MDA before testing.
So, next to check memory settings from sw1 or sw2 and if they are ok, then off to find out which bank
might have failed ram chip.
 

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Okay.

SW settings were correct, so there were error on memory on the board.
"1008" refers to bank 1, bit 3. From here I quickly found out that fourth ram chip
on the motherboard was defective.

Failed ram chip to the bin and working unit on the board. I still got errors 301 and 601
(no keyboard or floppies attached at the moment), but the unit booted to the basic!

Great! Nice to continue building from here on.
 

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I found this 16bit i/o card at my own stock (1.jpg).
Funny... looks to me floppy connector and IDE connector are miss labeled here.

I think the IDE connector is the left one. That won't be working with pc motherboard anyway,
since I can see that traces go to the 16bit area of the card connector.

So, I gave the floppy part a test. Will it work with a pair of 360kb drives? At first attempt nothing, but
trying out second setting from the i/o card floppy jumper the drive started to seek boot disc. In fact,
it read quite far the disc, but ended up giving error "bad or missing command interpreter".

I figured, dirty drives and/or floppy disc(s). This is the first time I attempt to use those two 360kb floppy drives,
since I bought them last spring (a found at facebook flea market). So, I cleaned both heads of the drive and
the boot floppy.

Second boot attempt ended up in "1040 201" error. Duh! I recognized 201 error from a couple of nights back.
Error pointed to bank1 ram, 6th slot. I just replaced that to a working replacement ram (wow, these seem to blow
quite rapidly!) and gave another boot a go.

Now we got somewhere ! The setup booted right into to DOS, yehey! (3.jpg).

But, I want to replace that second 360kb drive to 3.5" floppy drive. So would it work here?
Sure thing. Pulled out that second drive and plugged in 3.5" drive and it works as 720kb floppy drive (5.jpg).

Would it also work as HD floppy drive, as suggested here:

http://www.minuszerodegrees.net/2M-XBIOS/2M-XBIOS - 1.44M as B.htm

I think, maybe. I will be giving that a try later. There is a minor issue though. That DOS 3.2 floppy boot
disc seems to have a full 8kb of free space in it and 2M-XBIOS.EXE seems to be needing about 13kb
of space. Oh gee, what space requirement trouble, not to mention that I'd have to edit my config.sys
with edlin :).

Well, this is a practice run for another evening.
 

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VintageVic said:
But, I want to replace that second 360kb drive to 3.5" floppy drive. So would it work here?
Sure thing. Pulled out that second drive and plugged in 3.5" drive and it works as 720kb floppy drive (5.jpg).
Click to expand...
Formatting a 720K diskette in that drive will be a problem, but the options at [here] overcome that.

VintageVic said:
Would it also work as HD floppy drive, as suggested here:
http://www.minuszerodegrees.net/2M-X...M%20as%20B.htm
I think, maybe. I will be giving that a try later.
Click to expand...
DOS <--> BIOS <--> HARDWARE

* As for the DOS level, note that 1.44M 3.5" HD operation requires DOS version 3.3 or later.
* 2M-XBIOS software provides HD support at the BIOS level.
* The 16-bit floppy card provides HD support at the hardware level.
 
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