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May I suggest a change to the memory test? When the test fails could the test stop at that location and print a map of which bits failed? This would allow the determination of which chip the fault is in. Also, is there space for a memory decode test for paged RAM. Maybe fill the "lower" 32...

Thanks Do carefully check that you get what you ordered, as sometimes the wrong components end up in the wrong storage bin, or the order picker makes a mistake and grabs the wrong item. One major supplier has even put the wrong chip in an anti-static bag, sealed it, put the sticker with the (wrong) ...

There is only one 2955, some datasheets are common to the 2955 and 3055, they are PNP and NPN. Might that be what you are looking at? Nope. Farnell (and RS) have two versions, one with +70V and the other with -60V, both manufactured by ST Electronics. I'm wary as in my other MTX I put a 2955 in whi...

Which version of the TIP2955 should I look for?

There seem to be two main types, one with a positive collector voltage and the other with a negative one.

OK, I think I may change that next, but that'll have to wait a couple of months now as I'm packing things away (and I don't have the part anyway).

What I have noticed while 'scoping is that the address bus seems to be only driven to ~3.4V. There are a very few times it goes above this. The data bus, however, is a lot cleaner and far nearer to 5V. This is consistent for all the data lines and all the address lines. This could be an issue if the...

With respect to 7A the inputs, they are: Address lines A5+6+7 outputting to LS04 6B and input to LS08 7E (which controls the GI input on the two LS138s, 7C and 7D) A4+/IORQ+/MI(via LS04 6B) outputting to the other input to LS08 7E Output of the first input via LS04 6B+A2+A4 outputting into LS00 5B a...

I've now tried this and thought that I'd found it, SN74LS27N, 7A as I had CP/M operating for the longest time I've been able to, if I kept the chip cold. So, I've swapped it for another of the brand new ones. It's certainly a bit better, it now takes 2-3 minutes rather than 1 minute to stop working ...

If the +5V rail is within spec (4.75V to 5.25V) and is clean and noise and ripple free, the power supply section is not your problem. Also, if the +5V supply was bad, it is very unlikely that the CPU would manage to run anything. If the fault appears to be heat sensitive, try using a can of freezer...

Silly question, did you replace the TIP2955 in the power regulation section ? (20 pages of thread is a lot to re-read, so I'll just ask instead :oops: ) :-) Nope. I'm guessing that might have to be the next thing to try given that I've replaced all the digital components which relate to the memory ...