Sorry, real life / work has kept me busy... still catching up...
dizzy33 wrote: ↑Thu Feb 27, 2020 7:37 pm
Thanks a lot Mark ! In the meantime "Bola_dor" helped me with the voltages measurements. They are all good.
But now something new happens : running the same memory test now always end up with "IC25 failure" ...
IC25 is a SN74LS157N which seems to be involved in the memory management. I am now trying to find out how to get a new working one ...
There are two 74LS157 (sometimes shown as ‘LS157) chips used to access the ‘lower’ RAM, these are IC3 and IC4.
And on 48K issue 2 to issue 4S boards there are another two 74LS157 chips used to access the ‘upper’ RAM, these are IC25 and IC26.
The 74LS157 chips are multiplexer chips. The DRAM chips require the address information to be supplied as 7 or 8 bit row and 7 or 8 bit column information, one after another. But to save chip pins, both the row address and the column address use the same pins.
Whereas the CPU address bus is 16 bits wide. The multiplexer chips act like a bunch of 2 position switches, to route the correct CPU address line to the correct DRAM chip row/column address line.
If a 74LS157 chip becomes faulty, all kinds of strange symptoms can occur, as often the result is that one memory location can be written to and read from at one than one unique address. And this happens for many, many addresses / memory locations. But the CPU and the software can’t tell that this is happening.
If IC25 and IC26 are in sockets, and / or the ‘upper’ RAM chips are in sockets, remove all of them. Then power up and see what happens. The machine should power up as a 16K model. If there is no change in symptoms, then the fault is elsewhere.
If none of these are in sockets, there is another way to attempt to disable the ‘upper’ RAM. I will leave that for another post.
As you have tried removing all the chips for the ‘upper’ RAM, the next logical thing to do is to replace IC3 and / or IC4 as you have already suggested. If you fit good quality sockets, you can try one of the 74LS157 chips that were in the IC25 or IC26 positions.