Speccy repair
Posted: Thu Jan 11, 2018 3:24 pm
I have recently bought a non working ZX Spectrum 128 (toast rack) and I am struggling with the repair since. Maybe someone here has repair experience with these and has some ideas how to proceed...
It was pretty much dead when it arrived, but after changing the 5V regulator and TR4/TR5 it showed some life signs. To be safe I swapped all electrolytic capacitors as well.
From the symptoms it looked like some RAM chips were gone, to help in locating these I used the diagnostic ROM by Retroleum. It signaled a couple of malicious RAMs and I swapped them as well. It all worked well until I came to IC10 that should be responsible for Bit 3. I swapped it and nothing changed. On every test cycle I run it always shows this IC as faulty. Of course I swapped with (several) other chips, double checked the soldering even swapped the socket again just to be sure.
I checked continuity on the adress lines and looked at the data line, it all looks okay to me. Voltages are fine as well.
Two additional strange things:
1. Upper Ram tests completely fine.
2. a different diagnostic ROM shows bits 7,6,5,4 faulty... WTF?
The screen is garbled and barely readable, sound output looks fine. The ROM reacts as it should, but nevertheless I swapped the CPU, just to be safe. No change.
Right now I have swapped and socketed all RAM chips, the CPU and the ROM. All caps are replaced, as well as TR4, TR5 and the 7805.
What puzzles me, is that upper RAM works fine, only lower RAM seems to be faulty. When I look at the schematic, data busses are identical, so these are most likely not the fault. Has to be addressing then, and there seem to be two chips involved: the ZX8401 multiplexer and the ULA.
Both are hard to come by, so I hesitate to simply swap them without being sure.
So if anyone here has some ideas about this, it would be a great help
It was pretty much dead when it arrived, but after changing the 5V regulator and TR4/TR5 it showed some life signs. To be safe I swapped all electrolytic capacitors as well.
From the symptoms it looked like some RAM chips were gone, to help in locating these I used the diagnostic ROM by Retroleum. It signaled a couple of malicious RAMs and I swapped them as well. It all worked well until I came to IC10 that should be responsible for Bit 3. I swapped it and nothing changed. On every test cycle I run it always shows this IC as faulty. Of course I swapped with (several) other chips, double checked the soldering even swapped the socket again just to be sure.
I checked continuity on the adress lines and looked at the data line, it all looks okay to me. Voltages are fine as well.
Two additional strange things:
1. Upper Ram tests completely fine.
2. a different diagnostic ROM shows bits 7,6,5,4 faulty... WTF?
The screen is garbled and barely readable, sound output looks fine. The ROM reacts as it should, but nevertheless I swapped the CPU, just to be safe. No change.
Right now I have swapped and socketed all RAM chips, the CPU and the ROM. All caps are replaced, as well as TR4, TR5 and the 7805.
What puzzles me, is that upper RAM works fine, only lower RAM seems to be faulty. When I look at the schematic, data busses are identical, so these are most likely not the fault. Has to be addressing then, and there seem to be two chips involved: the ZX8401 multiplexer and the ULA.
Both are hard to come by, so I hesitate to simply swap them without being sure.
So if anyone here has some ideas about this, it would be a great help