I have inherited a pair of 48k Sinclair ZX Spectrums. First a series 2 with (at least) a bad ULA, and a lifted trace to the Z80. And a series 6a with nearly everything fried… Believe it or not, I think I stand a better chance of getting the 6a running, so I started with that one.
Replace *all* the parts!
Here’s what has been replaced in the 6a so far:
- Bad voltage regulator – replaced with TRACO
- 2 bad rams were bringing down the 12v and -5v lines (IC7 and IC11)
- Replaced D19 as it wasn’t testing well
- Replaced TR4 and TR5 (TR4 was dead)
- Verified voltages are normal after these replacements
- Verified TR1, TR2, TR8, and TR9 are working normally (pulled from board and tested out of circuit)
- Did a full recap with axial caps
- Did the composite video mod
- Socketed and replaced LM1889, which improved the video quality
- Socketed and replaced all lower RAM
- Removed all upper RAM
- Pin 39 on the ULA had a very unstable 14Mhz. I replaced X1, and now I get proper, steady value. (pin 32 gives a good 3.5Mhz)
- Socketed and tested ROM with a known working replacement
And to add to the fun, at some point, someone messed with the 22uF lower RAM decoupling capacitors on this board, and C57 was missing entirely. C6 and C7 appeared to have been desoldered, but then not completely resoldered back into place.
Fixing these issues has brought the board back from the brink, and now all the voltages are correct. I’m getting +12.3v (pin 8), 4.9v (pin 9), and -5.1v (pin 1) measured on the 4116 memory. So far so good…
The Mystery Deepens
However… Currently, the ZX powers on, and I get a white border, (yaaay, the ROM and CPU seem ok) but the paper shows the screen garbage loop that is seen when the ZX has no memory installed at all. Here’s a YouTube video that exactly matches what I’m seeing:
I had a loaned ByteDelight diag board for a bit… When the board is inserted, I get the same no memory loop on the screen, the border changes color as expected, and eventually the border displays that all the RAM is bad. I have a couple of dozen 4116 chips, but they’re all untested. However, whatever RAM I try, the diag board always says that all the RAM is bad, which I find hard to believe, and it makes me think something else is going on…
Here’s a video of a run with the diag cart:
I was getting nowhere swapping memory chips, so I suspected a bad trace or something wrong on one or more of the address or data lines. To test further, I desoldered the Z80, the PCF1306, the ROM, and removed the upper RAM chips. (My objective was always to have these socketed anyway…) Everything else was already socketed, and I removed the remaining ICs from the board and did continuity/resistance tests on every data, address line etc., between every IC location on the board. Everything checked out, so at least it is unlikely I have any broken traces. Checking the board under the “microscope” (cheap AliExpress microscope camera) did not reveal any broken traces or shorts.
Stumpered and Patiently Waiting…
At this point, I’m a bit lost… I’m waiting on a DRAM tester to arrive, so I can check those 4116 RAMs. Could I ready have 20+ bad 4116 chips? Bummer!
I also reached out to the nice folks at sinclairzxworld.com, and it was suggested that IC27 (the PCF1306P aka ZX8401 chip) might be fried as that is what does the multiplexing for all the RAM on these 6a boards. Luckily, replacement PCF1306P are not hard to come by and not terribly expensive. I ordered a ZXMUX from https://myretrostore.co.uk/product/zxmux, and I’m also waiting for that one to arrive.
So this work in progress will need to wait for the RAM tester and ZXMUX to arrive. Stay tuned…