Sinclair ZX80 / ZX81 / Z88 Forums

Hi all,

I've recently acquired an issue 2 Spectrum. It was listed as "untested" so I assumed dead.

It got here, I did Joules per Columb's pre-flight checks and then plugged it in. I checked voltages, they looked OK and huzah, it works.


I then did him a new voltage regulator replaced caps, DC-DC mod, composite mod and checked again, still good.
Except there's some pink/blue ghosting, I've read about that. I tweak the VRs and now it looks OK.

I then do the 48k upgrade. I check it with a diag ROM.

Oh my, the colours are terrible. Red is nearly black and green looks like neon green.

I revert the composite mod. But the RF is so bad that I can't recognise any colours at all. I reinstall the composite mod but try the transistor version instead of the capacitor. It looks exactly the same.

I then:

- I triple check capacitor values and orientations. All good.
- swap LM1889, no change.
- swap in a vla82, no change.
- I try my HDMI board and it looks perfect. (I assume that tells me the ULA is working properly and the problem is in the video circuitry?)
- I replace TR4/5 and TR1/2, and check voltages again. No change.
- I replace TR1/2 with BC549Cs, remembering to turn them 180 degrees. No change.

I gave up and decided it's just how it's going to be so I bolted it together, and tried to play Manic Miner, as you do. It was un-playable. Blech.

But now I'm back at it and am a bit out of my depth.

Does anyone have any ideas on further de-bugging? I do have an oscilloscope. I'm just learning how to use it but should be able to take any basic measurements.

Attached is a shot of the board in question looking rather poorly and another board dropped into the exact same setup, looking fantastic.


many thanks!
mikol

looks like you've done most things that I would try, mine looked like this
and no amount of VR tiddling would sort it. It needed a re-cap and then was great

I'd be tempted to continue to try to adjust the VR's to see if you can get the colours right. After that I'd be stumped too .....

I'm tempted to replace the caps again just in case one or more are bunk but I'm afraid that shotgunning things at this point just put the poor old thing through too much undue stress.

This is an analogue video fault.

Try setting up the colour adjustment presets (variable resistors) VR1 and VR2.

You may want to read through this recent topic where incorrect colours were also an issue.

I don’t recommend replacing the electrolytic capacitors again. But do recommend you check the polarity of each one, check that the leads are in the correct PCB pads, and that all your solder joints are good.

Note:
C46 - the polarity printed on board is incorrect. Fit the opposite way round.
C65 - near the modulator, it should be mounted with negative lead to the left.

Mark

mpryon wrote: ↑Mon Oct 09, 2023 4:28 pm I'm tempted to replace the caps again just in case one or more are bunk

You say that the colour issues happened after you recapped, can you confirm it looked OK before you did this? if so, it may point to an issue with one or more of the replacement caps - check they are the right values etc

Unfortunately, I cannot say that it happened after a re-cap.

The RF was terrible and I couldn't say if the colour issue was there or not.
It's reasonably subtle on the diag ROM so it's easy to chalk up to the terrible RF.
Or maybe it wasn't there at all?

And to be fair, I wasn't looking for that issue as I just wasn't expecting it.

In hindsight, the composite mod should have been the first thing done. After voltage checks of course.

1024MAK wrote: ↑Mon Oct 09, 2023 4:52 pm This is an analogue video fault.

Try setting up the colour adjustment presets (variable resistors) VR1 and VR2.

You may want to read through this recent topic where incorrect colours were also an issue.

Mark

Triple checked caps. All in the right way round and in the correct holes.

But that thread you mentioned, I think that has proven to be very interesting.
In it it's mentioned that pins 2, 3, and 4 in IC14 should all be the same, roughly.

I get 4.54v on pin #4
I forgot to check #3
and I get 10.10v on pin #2 !?!?!

**edit** I've checked pin #3 on IC14 and it's around 4.3v

That's with VR2 at fullstop clockwise. Going a bit anti-clockwise it rises up over 11v.

I've measured that trim pot and I get about 100ohm minimum and ~2.8k at max, so I think it's reasonable?

I'll keep looking at the schematic and see if it makes any sense to me.

Your pin 3 to 4 voltage offset, whist ok is not optimal, the service manual says pin 4 should be 50mV above pin 3. However pin 2 is very wrong. I'd be changing both VR1 and VR2 just in case.
I'd also check for shorts around R39 & R63, also make sure C53 has not shorted. Something is dragging the pin 2 line way too high. If you can adjust VR2 so that close to 12v is being applied to pin 2 then focus on R39/C53.

Cheers,

I did go straight to R39 yesterday and it checked OK. I didn't think to test C53 but will do tonight when I get home.

fingers crossed.

I think you are looking for a short pulling pin 2 high, or a failed VR2, but as it seems to vary the voltage from 10v upwards that seems to be at least working