Sinclair ZX80 / ZX81 / Z88 Forums

I have got some rough guide prices based on a 1KG package but they are quite high! DPD £24 Parcel force £20 Hermes £15 B2c Europe £12 I normally use DPD but they are very expensive. B2C are showing as the cheapest but I have never heard of them before so have no idea how good they are. Hermes are n...

I’ve got an issue 3 board (possibly produced for the French market given that it has a transistor inverting the video signal to the modulator) that has a resistor as mentioned in the posts above. Mark Just a silly question: on your pictures I see that the resonator is soldered in the opposite direc...

Maybe it’s just a broken ULA like 99% of all the repairs in this forum. It seems to be the weak spot. Yes it is possible but I want to explore first the "easy solutions" because the standard ULA is not easy to find and it seems that Andy has not more units ready at the moment. On the other hand min...

After testing with a digital probe I've found that the CLOCK signal on the pin 14 of the ULA is not present, its state is LOW and no PULSE. That would indicate a problem with the ceramic resonator, surrounding passive components, or ULA. But have a look at the board: there's some bodge resistor the...

Test the Z80 /MREQ, /RFSH, /RESET, A14, A15 and A6 pins with your logic probe. There is a pin out on the forum, check the link in my sig below. Mark Thank you Mark, I was aware about your post with the pin-out of the chips, in fact I made a PDF document with all the information in just one page and...

After testing with a digital probe I've found that the CLOCK signal on the pin 14 of the ULA is not present, its state is LOW and no PULSE. I don't know if these cheap test probes are reliables but I don't have oscilloscope. At the moment I ordered the 6.5MHz crystal because it's very cheap and easy...

flaviosr wrote: ↑Fri May 10, 2019 9:04 pm Heppy to help!
Tell me the results!

Unfortunatelly after replacing both transistors the problem remains. Now I'll start to check the ULA and CPU chips and see what happens..

Well, I already replaced TR1 and TR2 unsuccesfully. They were replaced by exactly the same transistor model but unfortunatelly the problem remains: white screen and no "K". This week-end I'll start to investigate with the logic probe if the ULA or Z80 are damaged. Bad luck

Obviously the RAM is ok. Having no more ideas I have decided simply to change the two transistors and the two capacitors and... the ZX81 is not perfectly working! I have chenged them all together so I will never know if it was a cold welding and which component was the bad one. The strange issue is...

The components density on the board is quite low so, most probably a smallest PCB can be done and installed into the ZX81 case, and the voltage taken from the ZX81 itself that will avoid the batteries use.

When I get my ZX8 repaired I'll investigate about this...