First things first, it's an issue 5 board, with some modifications (which I didn't make). Namely, all the usual TLC with resistors and stuff recommended by Sinclair, but also one 5V trace cut roughly between the microdrives and under the 7805 (but not where indicated by the Service Manual), plus a wire from the 5V out pin of the 7805 to another 5V trace, just under the left keyboard ribbon socket.
Second things second, I've checked all the removable ICs on another working QL and also tried the corresponding ICs from the working QL onto the 'faulty' one. All ICs from the faulty one work perfectly on the working one, and the ICs from the working one make no difference to the faulty. So we can be relatively sure it's not a problem of the ICs. Obviously, I'm talking about the 68008, the 8301 (well, 8310 in both cases), the 8302, the 1377, the 8049 and the ROMs (JM for the working one, AS for the faulty).
Moving on to the symptoms, the QL is stopping at the 'white screen' stage, after the typical random red and green pixels. It does react as it should to a reset, only to reach the white screen again.
That in itself indicates that everything is *almost* working, only the RAM write test fails -- at least, according to the service manual.
I've checked, with the rather basic multimetre that I have here, the voltages and well, all ICs appear to be getting the correct 5V that they should. I've also checked and double-checked the data lines from the RAM chips to the 8301, the 74LS245, the CPU and the ROM chips, all present and accounted for, also the lower address lines, again all present; no shorts between adjacent pins either.
Now the interesting part. After staring at the boards for hours with my nose practically 1 inch away from the chip tops and the multimetre probes in my contortionist hands (you know what I'm talking about, right?) I accidentally connected my Gold Card to the faulty QL (instead of the working one) and turned it on. Lo and behold... it worked. Kind of.
The random green-red test shows... twice... as it should with a Gold Card, then the "Gold Card v2.49" thing... and a second or two later, the "F1...monitor, F2...TV, Sinclair Research etc." prompt. Only, rather garbled, as you can see in the following image:
- Comparison of outputs.
My interpretation: bearing in mind that the RAM is implemented on 1-bit RAM chips, the fact that the general layout of the display is correct (i.e. you can still see the "Gold Card" or the "F1 monitor, F2 TV" pixels being in the right place), that means that the RAM output is, at least, all present. Also, the fact that the display is such, means that the ROM access from the Gold Card, i.e. 68000, is fine, otherwise, it would be reading 'garbage' from the ROM. Now, to me, that looks like... a timing error, somewhere. Given that the (whatever) CPU has access to the RAM array through the LS245 buffer, I was thinking that maybe that IC is malfunctioning, or 'bouncing' its output and producing artifacts on writes. Alternatively, the oscillator feeding the 8301 could be off? Of course, it could always be the RAM chips themselves, but somehow, I don't find it very probable.
In any case, I would welcome any insight you guys might have. I don't want to start unsoldering chips without being (reasonably) convinced.
Standby alert 
