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IBM PS/2 Model 50 Floating point errors

Imperious

Member
Joined
Aug 17, 2015
Messages
14
A friend of mine has one of these models of 286. Recently I resurrected it from the dead for Him, blown capacitor on the 12v rail.

I thought it might be a good idea to upgrade it with a 287 FPU DIP ic. I got one from China and was getting stack underflow and Math errors.
Tried reseating it numerous times but didn't change. The reference diskette was run many times as well. Ultimately nothing worked
so removed it and assumed it was faulty and got a refund via Aliexpress.
Ordered another 2 this time just in case, but from another supplier. Tried both Yesterday and pretty much the same errors.

Does anyone here have any experience with upgrading one of these machines with a FPU? and if so, are there any jumpers that need
to be changed. I noticed there is a JP17 right next to the CPU and FPU socket. There is no information in the technical manuals about
fitting a 287 in one of these machines, so one would assume that it's just fit and run the reference diskette and all should be ok.

I believe there has to be some sort of hardware issue causing this, as I don't believe there would be 3 faulty chips sent. All are D80287-10 the same
speed as the 286.
 
I had a Model 60 which had a 287 added. I don't remember having any problems with it.

The only thing I can think of is that the 287 socket wasn't properly soldered so take a look at it.
 
The socket being dodgy has crossed my mind. I might get the computer off him on Saturday and pull the motherboard out again and
check it out properly. The socket having nothing in it for 28 years or so wouldn't help as far as oxidization and other deterioration
caused by humidity etc is concerned.
 
Got my friends Computer off of Him Saturday night to have a better look at this.

I found the problem, there is a BUSY signal either going to the 286 or coming from it, anyway one of the through holes
on the PCB had some corrosion in it and it was open circuit. The signal goes to IC ZM3. Was an easy fix, just solder a length
of wire from the 286 pin 54 to the pin on ZM3. Ran the auto configuration and worked perfectly.
There is also a BUSY signal on the 287, so likely a conflict between the 2 being caused by the missing BUSY from/to the 286.

I would edit the Title to "solved" but don't see how.
 
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