Sinclair ZX80 / ZX81 / Z88 Forums

Hey everyone, I'm new here so if I'm not following the proper etiquette please say!

Anyway I recently came into possession of a 16k zx spectrum issue 2, which I'm looking to restore. The main problem at the moment is the video, which is yellow rather than the grey it should be. I know this is a common issue and so I've followed some of the tips mentioned elsewhere, including adjusting VR1 and VR2, however no values of those seemed to help at all, I could only get either a dark yellow or a very bright yellow.

So I got out the multimeter and did the check on pins 2, 3 and 4 of the LMS1889 (I think that's what it's called) and the lowest I could get on VR1 and VR2 was 150mV and 1.1V respectively.

The other issue was that it's just very noisy when powered on. I expect this from an RF signal sure, but the board itself is making a lot of noise, presumably from the power supply? I've never seen this mentioned before, so I assume it's not normal, so if people know what's going on there I'd love to know.

If you've got any insight I'd love to hear it. Thanks!

Hello and welcome

The adjustment of VR1 and VR2 preset (resistors) is very sensitive. That is, an extremely small physical movement can have a surprisingly large affect. Plus there is some interaction between the two adjustments.

So adjust each, then readjust each again. Repeat until you do get white.

If the voltage from one preset does not cover the normal range, that preset may be faulty. Or there may be dirt and dust causing problems.

The audible noise comes from "the coil" (the inductor/transformer). It's vibrating at the switching frequency and yes, This is part of the on board DC/DC converter (power supply section). This is normal.

If you want to reduce the amount of buzzing/noise, very carefully apply a small amount of nail varnish or similar to the windings on "the coil".

Oh, and please note that when using the TV/RF/UHF, turn the sound right down. No audio is transmitted over this signal.

Mark

Ok, so I have had an improvement, but it's still not great, I've seen other people adjust the pots and it "snap" to the right colour but for me I could only get various amounts of yellowish grey. Is there anywhere I can go from here or is this as good as it gets?

Well, This adjustment is often found to be tricky/fiddly and even if you set it up perfectly, as the computer warms up, it may drift... And even if you set it up perfectly when warm, within weeks it may drift again.

Hence why Sinclair changed the design for the issue 3 board...

If you are still having problems, replace both VR1 and VR2 with new modern parts.

But a nearly white picture with just a slight tinge of yellow is not too bad. Remember, the ZX Spectrum was designed to work with colour portable CRT TV sets. And you could easily adjust the colour control on these.

Mark

Ok, done a composite mod and the all the colour issues are basically gone now. A little fringing on the text but that's normal for composite. I'd say it's a fairly crisp composite signal too! Thanks for your help Mark