Resurrecting A ZX81, ZX Printer & Spectrum

[ADAM714]

OK, i built my ZX81 when i was 14. That was 39 years ago. I recently became nostalgic for these old computers and deciding to resurrect sone old none working units on purchased on ebay.

On my ZX81 i was getting nothing but i noticed a change in the screen pattern when i powered on the unit and also the heat sink was getting hot so i figured it was a video output issue. I purchased a ZXVid and installed it using my horrible soldering skills.

What i am getting now is the normal K cursor screen but it only appears for about a second then seems to power cycle for 5 seconds then comes back to the K for a second and it just cycles like that 1 second on 5 seconds off. But i determined its not power cycling , the video is just cutting out because if i type something it remains .

I sent some pics from the guys at SMR and it was suggested that it is my poor soldering or bad caps. I re-soldered but same issue. I believe it may be caps because if it was a connection/grounding issue the "cycling" would be random and it's not.

I have a multi-meter but i don't know what connections to measure for voltage.

For my non working Spectrum i tried the solderless composite mod (https://www.youtube.com/watch?v=0dbNowKZ00g) and irnically i get the same result except instead of the screen going blue for 5 seconds, it's only 3. Very weird

Any assistance for this novice would be greatly appreciated

[1024MAK]

The first thing to do is to try both the ZX81 and the ZX Spectrum with a different TV. Not all TVs work well with 1980s home computers.

In terms of the ZX81, there are only two electrolytic capacitors on the ZX81 PCB. One is only used for the power on reset circuit. The second decouples the +9V power rail. The issue 3 boards have a third capacitor, this again is decoupling the +9V power rail. All are non-critical and I can’t see any of these causing this issue.

There is also a large electrolytic capacitor in the PSU. With the ZX81 case open, connected to the PSU and turned on, test across the 9V DC connector pins (see photo) using your multimeter switched to the 20V range (or equivalent). Report back the voltage that you get.

Issue 1 board

Issue 3 board

Mark

[ADAM714]

Thanks Mark

I have an issue one and voltage measured 11.61v

[1024MAK]

Thanks Mark

I have an issue one and voltage measured 11.61v

So that’s in the normal range, so it’s unlikely that the capacitor in the PSU is faulty.

Mark

[Ed in SoDak]

My cheap flat screen tv acted the same way with my TS1000. It would sync for a second, then lose it and go blue screen as if it had no signal. If you don't have an old CRT TV, maybe you do have a VCR, you can try feeding the ZX through the VCR to your flat screen.
-Ed

[bola_dor]

May be the ULA is faulty?
The most strange Thing is that you have a similar issue with the spectrum.. That will point to the TV sets may be they loose sync.. check were did you take the video signal I know there is a point that gives you 60Hz instead of 50Hz

[ADAM714]

well, the consensus seems to be the display. i think my only option may be the VCR trick. at least for troubleshooting anyway.

I'll report back when i can. i sort of put this project on hold since my black friday 3d printer arrived

[1024MAK]

Although we would prefer if you could beg / borrow a CRT TV, it is also worth trying with any other LCD TVs that you have access to. I have a Sharp LCD TV that works with most, if not all 1980s and 1990s computers.

Some small low budget LCD TVs also are okay with 1980s computers.

Mark

[ADAM714]

I'm using this Hannspree:


Its the only thing i have with composite input. i'll keep my yes open for something else to try

my TS 1500 with original UHF modulator works fine.

[1024MAK]

Whereabouts in the world are you? In the U.K. in the used item (secondhand/cash converters) shops and charity shops, it is sometimes possible to obtain CRT TVs and the older generation of LCD TVs.

There may be similar shops in the country where you live.

Mark