ZX81 Troubleshooting Help

[iamonlv426]

Apologies if this has been covered before but I couldn't find exactly what I wanted after looking around the forum. I have a ZX81 that I have restored:-

Replaced three capacitors.
Replaced the power regulator.
Replaced the membrane keyboard.
Replaced the ULA with a VLA81.
Replaced the Mic, Ear and Power inputs.
Replaced the RF Modulator with a ZX Renew ZX2020 Composite Mod.
Replaced the original PSU with a new one.
Placed a heatsink on the CPU.

Once completed the machine booted and worked fine. Although the video mod was a tad too high for the case and I had to jiggle a bit to insert the composite cable. Decided I'd come back later to fix that.

Anyway a couple of months passed as we were moving etc.. and I fired up the ZX81 to try something out and no video. I have opened the case and removed the board and done as much troubleshooting as I'm capable of.

The back-porch dip switches are correct and it was working previously. I did adjust them anyway to check.
The power regulator gets hot and I have tried to use a cheap multi-meter to check whether the key chips are getting the correct voltage. Not sure I am doing that correctly as I'm new to using a multi-meter.

Any help with how/what to troubleshoot the board further, and maybe more accurately with a multi-meter, to try and determine the cause would be much appreciated.

Thanks

[mrtinb]

Finding error on a computer is almost impossible with a multimeter.

A cheap oscilloscope like this one that Adrian is demonstrating is $57. https://youtu.be/Uqrel5fQpK4

I have a Hantek 2C72, I paid $130 for.

Maybe Mark can help you with just a multimeter, I don't know.

[mikeh_nz]

When you say no video, do you mean black screen, or white screen with no “K”?

Is this a different tv from before when it was working?

Adrian has a few cheap scopes on his channel - I have the Hantek 6022BE one - I think in later videos he found a better one.

What is the voltage of the outer most wire going into the RF modulator (ie pin 16 of the ULA) (black to ground on the board & red to wire)?

What is the voltage of the center pin of the rca jack?

[1024MAK]

You can measure the voltage on the various chips by using either the metal heatsink for the 7805 voltage regulator, or the modulator case for the 0V (ground/common) connection (this is the negative/black multimeter lead/probe).

Then carefully (because you don’t want to slip and short anything out) touch the red probe to the pins shown as +5V or VCC or VDD as shown in the pin-outs here (same as the link in my signature below) for the Z80, the ROM and the RAM chip(s).

The +5V supply should be between 4.75V and 5.25V. But normally with a good voltage regulator will be a lot closer to 5V.

Mark

[iamonlv426]

Thank you all so much for the advise. I am looking at buying a cheap oscillator and learning the basics of it and the multimeter.

I mean there is no video making it to the monitor. I use a Retrotink 2x to convert the composite signal to HDMI. This as I say was working fine. Now I get the coloured bars followed by no HDMI signal.

Its the same monitor as before but that said I have tried three monitors to no effect. I have a C64 and it continues to work fine so the monitor, cable and Retrolink are good.

Voltage of the ULA pin 16 is fluctuating from .663 to .731 - assuming I'm doing this correctly this is really odd?
I'm not sure how to find the centre pin of the RCA jack. I did measure the voltage of the power supply and it was 9.16
I measured the ROM and RAM chips to start and got weird readings as well. RAM Chip - .028 - .238 fluctuating. ROM Chip 025 - .245 fluctuating I stopped at that point as I'm either doing something wrong on maybe my power regulator is gone?

Thanks

[iamonlv426]

I thought it might be helpful to post some pics and happy to post more if useful.

Multimeter at rest



Multimeter testing a RAM module

[mrtinb]

The ZX81 is the retro computer I have, that is most non standard with it's video signal. So if other computers works fine, does not matter. Look at it this way: If your monitor setup works with the ZX81, it will work with anything else.

I have bought an old surveillance monitor, just to get a picture from my ZX81.

I don't know the ZX Renew ZX2020 Composite Mod, but I do know that most people have great succes with the ZX8-CCB

[1024MAK]

Your multimeter needs to be switched to the DC voltage position marked “V⎓”.

Mark

[1024MAK]

It’s also worthwhile seeing if you have, or someone you know has a TV with a composite video input (often called AV ). The connector is a Phono/RCA/Cinch type which is often coloured yellow, then trying your ZX81 using this connection. The Wikipedia page about this type of connector is here.

The Phono/RCA/Cinch connector has two electrical contacts. The outer part is the signal ground/0V/common. The contact in the centre, is for the signal. With a ZX81, this is the video signal. See photo.

TV Phono/RCA/Cinch video input socket

The phono connector on the ZX81 looks a bit different, but is the same. See photo.

ZX81 Phono/RCA/Cinch video output socket

Click on the photos for a better image.

Please note that your ZX81 board may look different to the one in my picture. Please don’t worry about that.

Mark