doubt about swapping ULA in an issue 2 board

Is it possible to swap a 6C001E-6 ULA type with a 5C112E ?

I have this issue 2 board that is not booting, it shows me a black border with horizontal yellow and black bars and a few random squares.
I'm suspecting a CPU problem, or ULA.
Diagnostic ROM doesn't work on this machine, so I would like to swap the ULA and Z80 to narrow the problem.
Z80 is soldered, so I first would like to try swapping the ULA as I have another working issue 2 machine.

The broken machine has a transistor mod on the Z80, the working machine has a diode/resistor mod on the Z80.




  • You can in general put a later ULA version in an earlier board, so a 6C001E-6 should work in an issue 2 board. But putting an earlier version ULA in a later board may not work properly.

    The transistor mod. (often called the spider) corrects a design limitation present in all ULA versions. It is commonly on top of the Z80, but it does not have to be. Some are mounted further towards the front of the machine on the PCB.

    The diode/resistor arrangement does the same thing, it’s the alternative arrangement. But the transistor version is much more common. On issue 3 boards and later, this extra transistor is allocated a proper position on the board as TR6.

    Note that the machine will still work if TR6 / the spider mod. is not fitted, but most expansions won’t work correctly. So you can leave it off for testing if you want. Then refit it once you get the board working.

    See also this guide.

  • edited September 27
    Ok, thanks for the info, very useful.
    I swapped ULA and the machine kept failing, put a new Z80 and socket and it now boots.
    yellow tint was fixed with the VR2 pot.
    The machine seems normal, but DiagROM gives me lower RAM errors, it says all lower RAM is faulty, or bad ULA or bad IC3 or IC4.
    IC3 and IC4 tested fine, voltages seems fine (4.8V, -4.6V and 11.6V at the RAMs), 4.8V at the 7805.

    RAM IC's are MM5290N
    I replaced just one RAM with a modded 4164 ( i read this one only needs 5V) but it still shows me an entire lower RAM failure.

    I'm guessing a bad ULA then? the other ULA i tested gave me the same problem but more garbage appears on the diagrom test screen, So maybe both ULA are bad? Not sure where to look to test for voltage drop culprit.

    DiagROM ULA test gives no problem, but I'm not sure how complete this test is.

    Post edited by gleegum on
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