Hi again Christian
We'll get there, I have faith!
Cristian wrote:1) the resistor looks better in the reality than in photo. I measured it and the resistance matches.
I assume you mean the zero-Ohm resistor/links JU2..4, measured at zero/short cct. Am I right? Or did you mean the resistor next to the CPUCLK oscillator?
Cristian wrote:2) I removed zx8302. Result: black screen.
Curious - I'll try this test myself shortly - had always thought you could get at least through the memory-test without the ZX8302, but could well be wrong about that!
Cristian wrote:I put a different zx8302 from a working QL: the same white screen.
There's something else at play then... I'm leaning more and more towards the 'stuck interrupt' possibility... I think I can test that as well - stand-by.
Cristian wrote:3) I also installed the original AH roms.
Were these EPROMs or ROMs? If EPROMS, you would have also needed to switch around the JU links and re-attach a 'flying-lead' to achieve any success with the picka-back EPROM arrangement that was typical when the AH was released.
Cristian wrote:5) I took my (little) risk and installed another zx8301 from another working QL. Nothing changed: the same white screen.
...I trust that the ZX8301 worked as expected when re-inserted in to the working QL...
Cristian wrote:7) I noticed that the IC just under the RF modulator got relatively hot, so I swapped it, and nothing changed.
If that's IC28 - MC1377 - the PAL encoder, then yes, it's normal for this IC to get quite warm - more so than the other ICS...
8) Maybe if I swap also the motherboard...
Its always an option (i always keep one 'spare'), but unless you have evidence of the replacement working before purchase, is likely to lead to more troubleshooting!
Cristian wrote:I noticed that in the working QL the zx8301 stays almost cold, while in the white screen QL it becomes warm (not so hot).
That matches my experience - whenever I've had a non-booting unit, the ZX8301 gets much hotter than usual and pretty rapidly! My hunch has always been that some 'bus-contention' is occurring in such a way that the ZX8301 is trying to drive a signal line (sink or source current) whilst another component is simultaneously trying to do the opposite...
Cristian wrote:Apparently the only chip getting clearly hot is the IC just under the RF modulator. The other chips behave as the ones in the working QL: some stay cold, other become warm (74LS245).
A warm bus-transceiver does sound suspicious - depends just how warm, of course - it could be normal. But now I'm leaning further towards bus-contention - both the ZX8301 and the bus-transceiver operate on the 'private' data-bus of the ZX8301 and need to be in-sync in terms of 'direction' - Pin 1 ('S') on the '245, Pin 4 ('RDWL') on the ZX8301 and all the other ICs connected to the CPU bus.)
Did you say that you had ever replaced the '245 bus=transceiver?