Issue 2 with terrible colour

[1024MAK]

It’s more likely that R63 is open circuit or the PCB tracks to it have a track break (dry joint/damaged PCB pad or broken through hole plating).

R39, VR2 and R63 act as a voltage divider.

Ceramic capacitors like C53 don’t normally fail short circuit unless physically damaged. If the resistance of R39 is okay, C53 can’t be short circuit.

Oh, and if the +12V supply is getting to pin 16 of the ULA, the ULA may have been damaged.

Mark

[redgatemodels]

Fair points Mark well made

with all the replacement parts that have been fitted it's possible that tracks have been broken or pads lifted etc that might not be apparent on first inspection ....

[mpryon]

I'm even more confused now...

R63 checked out fine.

The voltage getting to pin 16 of the ULA varies with the setting of VR2, between (very roughly) 1.6v to 5v. Give or take.

I can find no shorts or solder blobs. I keep looking at VR2 but it _seems_ to be working.

I've ordered something that I hope will be a replacement for the VRs, hopefully they'll work but I'm not holding out much hope.

At least it doesn't seem to be trying to kill my ULA.

[mpryon]

I've had a bit of a result.

Out of desperation, swapped vr1 and vr2 to seem if the voltage problem moved from pin #2 to pin #4 on the lm1889.

To my surprise, both voltages are now in reasonable ranges.

Dry solder or solder whisker on vr2?

But, the colour is still the same.

But now that the pot that was in VR2 is in VR1's spot, it's noticeably twitchy.

I cannot get the balances nailed down as when the now VR1 pot seems to be just about in the correct spot, colour balance jumps around and the composite signal on the scope is a mess.

When my replacement trim pots turn up, maybe they'll sort it?

[redgatemodels]

See how it is with new pots

[mpryon]

Argh. This thing continues to be a bit odd.

I had another play with it today and the voltage on pin #2 of IC14 is back up to 11v.

But this is still after I swapped the VR pots.

Pin #4 is still good.

I guess both pots could be failing in the same way but to only mess up on the pin #2 side doesn't seem terribly likely to me.

I'll still replace them when the parts arrive and re-evaluate but my confidence level just dropped a bit.

[1024MAK]

Are you absolutely sure there are no PCB track breaks? Some can be small cracks.

Mark

[mpryon]

Taking that sound advice, I went at the board with a magnifying glass and a multimeter.

I couldn't find anything amiss.

I decided to measure all the passives around the pin #2 setup.

I started with R39. No problems.
Then I pulled C53 and it measured 150pf.

I put it back in and checked I didn't make it any worse and.....

Wot? Not only are the voltages decent again, I've got proper colour for the first time ever on this board.

[1024MAK]

You have definitely disturbed something, I’m just not sure what

Mark

[mpryon]

After a couple of days of the colour being back every single time I boot up the board, I _think_ the problems are/were

1) the VR2 trim pot is toast (and possibly VR1 as well).

2) there were dry solder joints on C53 and R39 and every time I checked continuity, I must have checked on the solder on the pads rather than the leg of the component.