Stuck with repair of Spectrum+

[speccyplus]

Ok, have some other info.

I have another test rom, the one from retroleum. When I pull IC3 but leave all other memory logic chips for upper and lower ram, it has a corrupt screen, but I can read the message saying diagnostics.

It is able to check upper and lower ram and says all green.

When I put in the ic3, it says that the first 3 bits fail, second 3 bits are good and last bit fails from what I recall. The upper memory it says is good still.

I have put the ULA in another board, and it tested OK.

[1024MAK]

Keep in mind that with IC3 missing, the testing of the RAM that IC3 feeds address lines to will lead to misleading results, as some of the RAM address line inputs will no longer be driven, hence the Z80 and the test program may ‘think’ it is testing all of this RAM, but in fact, it is only testing half of it.

Because of the way the hardware works, the test program doesn’t know that when it tries to access the area of RAM that IC3 would normally be used for, instead the test program is actually accessing the half that it does have access to. So it tests one half twice. The half that needs IC3 is never tested.

Mark