video output switching between pal and secam

[KingtutUK]

I have 4 Vic 20's two of which I had to repair, and those two, although working, switch between pal and secam output. the two that did not need any work do not switch signals.I have changed bad chips and recapped one of the two machines. Chips like the 6561 and 6502 etc, all check out okay with the Backbit chip tester pro 2.

How it looks when first turned on (secam). Sometimes they will start in pal and then change to secam

This is when they switch. I can still use the computer when the screen goes blank.

This is after the switch (pal). then it goes back to secam. There is no regular time between switching modes, it's random.

Any help greatly appreciated.

[Mike]

First of all, it's not the VIC-20 that switches between PAL and SECAM, it's your display which mistakes the video signal of your VIC-20 for SECAM for some of the time. Real SECAM output requires an adapter PCB in the VIC-20. Do you own a contemporary, "non-intelligent" CRT monitor to cross-check?

I'd take the video circuitry in the RF box under good scrutiny. Maybe you created some inadvertent solder shorts during repair.

[KingtutUK]

First of all, it's not the VIC-20 that switches between PAL and SECAM, it's your display which mistakes the video signal of your VIC-20 for SECAM for some of the time. Real SECAM output requires an adapter PCB in the VIC-20. Do you own a contemporary, "non-intelligent" CRT monitor to cross-check?

I'd take the video circuitry in the RF box under good scrutiny. Maybe you created some inadvertent solder shorts during repair.

Hi, I am using an rca/phono to hdmi adapter to connect to my toshiba tv, the thinking it might be the display did not occur to me, thankyou, as I mentioned in my original post, the other two I have that work, do not have this problem. I have a crt on order and will check with that, but I will also recheck the boards and where I have soldered.
Thanks for the reply.

[KingtutUK]

you were right, thanks, the output from the Vic 20's is now good after checking on the crt i bought.

[jjne]

I've seen this fault with a cheap LCD TV, tweaking the VIC video gain sorted on that but the setting that TV needed caused a dim image on other TVs.

A Toshiba MSX was causing the same issue (and an issue 2 Spectrum was displaying yellow on this specific set as well) so it looks like different ways TVs are dealing with out-of-spec PAL signals.