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Clym5
February 2nd, 2017, 11:47 AM
Hey all, first time poster on VCF!

I've been fixing up an IBM 5155, and I've noticed that there is some sort of a board modification. (http://i.imgur.com/bzenDWk.jpg) Unfortunately figuring out what this is doing is a bit beyond me..

Pin 13 (Y2) from the 74LS138 goes to pin 4 (2A) on the 74LS32
Pin 5 (2B) and 10 (3B) connect to each other on the 74LS32
And pin 6 (2Y) from the 74LS32 goes to pin 23 (-WR) on an Intel 8253 Programmable Interval Timer.

The computer seems to function normally since I fixed it, so I'm not sure what's going on and I'm not too keen on desoldering them for now.

If anyone has any idea what this might be, I'd appreciate any input, even speculation.
Cheers

modem7
February 2nd, 2017, 11:22 PM
Welcome to these forums.

That certainly looks like an IBM modification to me. You will see IBM doing the same to expansion cards as well.
Links to some examples follow:

* [5150 motherboard (http://minuszerodegrees.net/temp/3/temp_83157873fh2fh1.jpg)]
* [5160 motherboard (http://www.oldcomputers.arcula.co.uk/files/images/intl103.jpg)]
* [5160 motherboard (http://minuszerodegrees.net/5160/motherboard/5160_u18_u19_socket_mod.png)] (mod to properly accept 27256 EPROMs)
* [64/256KB Memory Expansion Option (http://minuszerodegrees.net/5150_5160/cards/5150_5160_cards.htm#64_256_adapter)]

Clym5
February 2nd, 2017, 11:33 PM
Thanks for the reply, I'll stop trying to figure it out for now and put the 5155 back together.
Cheers
Logan

mikey99
February 3rd, 2017, 05:38 AM
Yellow wires were often added to implement an engineering change or revision to the circuit.

Yellow wires may also have been added to repair a board if a connection or trace was bad.
A replacement 5150 motherboard cost over $1000 back then, so repairing the boards saved
some $

Mad-Mike
February 3rd, 2017, 08:12 AM
Normal IBM Practice - I had an IBM PS/2 Model 70 386 with the same sort of work done to the motherboard. Probably a circuit bug fix.

Great Hierophant
February 3rd, 2017, 10:20 AM
These appear to have been factory fixes. It seems it was cheaper to salvage flawed boards with a few patch wires than to scrap them and start anew. I see no reason to replicate them unless you know they fix a particular issue you are encountering.