The strange case of the Commodore 64 colour bands.

[stephen_usher]

C70 is certainly out of spec, assuming I can trust my multimeter at that low level of capacitance. It should be 10pF +-5% and it's actually 17pF.

I've put in an order for some from RS, so they should be here on Tuesday.

[rubber_jonnie]

C70 is certainly out of spec, assuming I can trust my multimeter at that low level of capacitance. It should be 10pF +-5% and it's actually 17pF.

I've put in an order for some from RS, so they should be here on Tuesday.

Sounds good.

I had a big problem with the master clock in one of my BBC Bs, the caps were out of spec and the crystal wasn't right either, so ended up replacing the all of them and happy to say ended up with a good working BBC.

Fingers crossed it helps, but you just never know with these things.

[stephen_usher]

Fingers crossed it helps, but you just never know with these things.

Indeed. If it's not the cap then it'll have to wait until I get back from my Easter break at the end of the month.

It all depends upon where the resonance which is causing the tint is located. It could be that the value of the capacitor and the crystal have an unfortunate overtone, which gets closer to being in resonance as the machine warms. (It definitely gets worse as the machine warms up.)

It would still not explain the brightening after ~5 seconds issue. I'm guess then that that could be an electrolytic in the modulator. That would be a pain as to desolder those you have to remove the whole modulator unit and that's soldered onto a huge ground plain on the main PCB.

[Atari74user]

Just curious, isn't some of this what the LumaFix64 is supposed to help alleviate, or are you experiencing symptoms over and above?

https://www.breadbox64.com/blog/the-lumafix64-mod/

[stephen_usher]

Just curious, isn't some of this what the LumaFix64 is supposed to help alleviate, or are you experiencing symptoms over and above?

https://www.breadbox64.com/blog/the-lumafix64-mod/

Well, there are luma lines, but this is in the chroma signal and it doesn't line up with the luma jail bars.

[Atari74user]

I have not fitted my LumaFix64 so can't comment first hand how good or bad. However the LumaFix is supposed to be able to be used to reduce some of the Chroma voltage level with the third potentiometer just to note.

Good luck, I'm hoping the LumaFix can fix some of the faint jail bars I have, we'll see!

[stephen_usher]

This isn't quite as bad as it gets when it's been left on a while to warm up:

IMG_2662.jpg (70.65 KiB) Viewed 1889 times

The tint goes from strong cyan to strong magenta. Red words (such as 'BAD 'in the diagnostic ROM) have a red first letter, black second letter and red last letter. Blue words look multi-coloured.

It's a problem with the chroma frequency and not voltage level as PAL is frequency based colour encoding.

[Atari74user]

Ahah OK I see your point, OK that's interesting, now I am wanting to spend time with the 64!

[stephen_usher]

Well, for a lot of the team who designed the ST it was their previous project.

[stephen_usher]

Seeing as I have caps and a desoldering gun, I decided to recap the modulator... No change.

I'm going to have to get some shorter 470uF capacitors as the one I've used is too tall for me to install the top shield. I could put the original back though as it's obviously not bad.

P.S. It doesn't change the lightening after 5 seconds issue either. I've no clue what could be going on there.