Share your knowledge on these two classic MPCs
By 9lifes Tue Feb 21, 2023 10:30 am
Since a few days the memory on my MPC-60 MK1 OS 3.10E has dropped down from 1.5Mb to 750Kb (26.2 to 13.1 sec).

I checked the 14256 DRAM IC's from the expansion by swapping them from expansion to the voice board. They all check out. I already checked continuity on all traces and looked for broken solder joints.

Can it be a faulty 74HC541P or 74hc125p ?
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By Telefunky Wed Feb 22, 2023 2:53 pm
It‘s not very likely, but of of course a cmos logic ic can fail.
There are different version of this expansion. Guessing from photos you may have the one with tantalium capacitors (the tiny ball like drops, not the pancake like ones).
Some times these fail leaving a small hole on the surface, but not always. They are infamous for causing shorts. Inspect with a magnifier.
(you probably know this, but just in case: tantalium caps are heat sensitive and don‘t survive wrong polarity connections).

On the other hand the failure may be in decoding memory adresses of the expansion versus the main board, which would probably be related to the the main board.
(I don‘t have a circuit diagram, you need to trace these lines)
As usual electrolytic and tantalium caps (near the decoding logic) are the most common parts to fail.
By 9lifes Thu Feb 23, 2023 1:09 pm
Thanks for the reply

I ruled out the 74HC541P or 74hc125p. I traced it back to a demultiplexer (74hc138p) and from there to the
processor. So failure in the decoding memory adresses could be a real possibility.

First thing to do is to make 100% sure the expansion works and test in an other machine.
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By Telefunky Thu Feb 23, 2023 2:22 pm
Yes, that‘s a good approach... if another machine is available. :-D
Fortunately there are only 2 small electrolytic caps on the cpu board. You may check/replace these anyway as such caps are known to age.
ps: and a single tantalium next to the Sanyo ic.