Sinclair ZX80 / ZX81 / Z88 Forums

Dear Forum,

I have just purchased a ZX80 on eBay, sold as not tested. It appears to power up and when I hook it up to Channel 36 on a normal CRT, I get strong blacl diagonal lines across the screens.

I have re-seated all the ICs, however, this did not seem to make much difference.

The image does appear to change, whenever I press on the PCB or "fiddle" with the power input.

My next steps are

1) Check horizontal / vertical hold on CRT, although, I am not sure this has one.
2) Tune the pot on the RF Ouput can.
3) Test all ICs, however, which means I need to buy a machine from China.

Does anyone have any other ideas?

Regards
SH

What test gear do you have, or have access to?

Is the +5V within spec (4.75V to 5.25V)?

Is the +9V input between +9V and +14V?

Mark

Good morning Mark,

Hmm, unfortunately, I only have Access to a multi-meter, which should help with the voltage you mentioned.

There is a great ZX80 repair video on YouTube, where the chap was using an oscilloscope, enabling to check the clock and video output.

I shall check all the voltage drops first and let you know.

Thank you.

Regards
Simon

SHZ88 wrote: ↑Thu Jun 25, 2020 9:18 pm The image does appear to change, whenever I press on the PCB or "fiddle" with the power input.

Check the voltage during "fiddeling" with your meter. Is it stable? Sometimes the power connector has dry or broken joint. Use a magnifier glass.
Heinz

Hello Mark,

The Input to the 7805 Regulator is +12v and the output +8v, so I suspect that this is not working correctly, as I can also measure this voltage on the Z80 CPU. Or do you think the high voltage is due to a low load?

Hello Heinz,

The voltage is a Steady, even after some *fiddling*.

So the next I shall do is buy a new 7805 and see if that does the trick first.

Regards
Simon

8V is well beyond the voltage that the chips on the board are rated at. Your 7805 voltage regulator is toast, and the resulting over-voltage output may have damaged some of the other chips.

The whole point of a voltage regulator is to keep the output voltage within the specified output range (4.75V to 5.25V) regardless of the load current (assuming the load current is never 1A or more for the 1A rated 7805, the ZX80 should never consume anywhere near this much).

So yes, replace the 7805. If you want to keep it looking original, use a 7805 voltage regulator. If you want it to run cooler, use a DC-DC switching regulator. If you need more info on these, search the forum for ‘Recom’ and/or ‘Traco’. Note that it does not have to be from one of these manufacturers, other good quality types are also okay. Although I do NOT recommend the cheap modules from auction and other online trading sites.

Mark

Good aternoon Mark,

Thank you, for the confirmation.

I shall either rip a 7805 from a spare 81 I have or go to the local electronics shop and buy one, as they cost next to nothing,

I do hope the other ICs are fine

Shall keep you posted.

Regards
Simon