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Re: Composite mod issue - left side of screen missing
Posted: Fri Mar 26, 2021 7:47 pm
by patters
1024MAK wrote: ↑Fri Mar 26, 2021 5:23 pm
All the various systems for adding the ‘back porch’ to the existing video signal work by cutting out the video signal from the ULA and replacing it with a darker than black level. So if either, one of these is fitted to a ZX81 that has a ULA that already does the back porch generation system,
or the adjustment is not correct for the timing and trigger, then the left part of the video signal will be lost and will appear black. This is what I think you are seeing.
This was my thought too. Presumably if those trimmer or resistor values are out of the circuit design spec this would be the outcome. Does my testing basically confirm this?
Re: Composite mod issue - left side of screen missing
Posted: Fri Mar 26, 2021 7:57 pm
by Moggy
I think you are right.
Without the 555 you seem to be getting the composite effect with the transistors doing the buffer between the ULA bit but no back porch hence the darker screen. It does seem as if it is out of spec for a zeddy and probably made for another computer but passed off as 81 compatible.
It's a bloody shame Pokemon isn't making ZX8-CCB's any more.
Re: Composite mod issue - left side of screen missing
Posted: Fri Mar 26, 2021 8:09 pm
by patters
Oh that's a shame. I just this minute messaged him on SellMyRetro to ask whether he expected to sell any more.
Re: Composite mod issue - left side of screen missing
Posted: Fri Mar 26, 2021 8:09 pm
by Lardo Boffin
Moggy wrote: ↑Fri Mar 26, 2021 7:57 pm
It's a bloody shame Pokemon isn't making ZX8-CCB's any more.
Has he stopped completely do we know or just having a break due to current circumstances?
Re: Composite mod issue - left side of screen missing
Posted: Fri Mar 26, 2021 8:13 pm
by Moggy
Circumstance I believe Lardo.
Re: Composite mod issue - left side of screen missing
Posted: Fri Mar 26, 2021 9:39 pm
by McKlaud
My composite mod board in photo:
If anyone is interested I can share Gerbers or send you a PCB for postage costs.
Re: Composite mod issue - left side of screen missing
Posted: Fri Mar 26, 2021 9:52 pm
by patters
I'd be interested in comparing the potentiometer specs. The ones in this mod both have a marking 103 which is 10K apparently. What are yours?
Your resistors seem to have quite different colour codes also.
Re: Composite mod issue - left side of screen missing
Posted: Fri Mar 26, 2021 10:09 pm
by McKlaud
Re: Composite mod issue - left side of screen missing
Posted: Fri Mar 26, 2021 10:32 pm
by Moggy
Being used with a later back porch providing ULA too which means both devices can work in harmony.
Re: Composite mod issue - left side of screen missing
Posted: Sat Mar 27, 2021 12:16 am
by 1024MAK
Moggy wrote: ↑Fri Mar 26, 2021 7:09 pm
I have seen these boards before and I'm sure they were being peddled for Atari not Sinclair.
The circuit is very similar to the design in section 3.1
here. That is linked from
here.
Some of the values are different.
Preset RV1 (10k) replaces R1
Preset RV2 (10k) replaces R2.
I don’t know where the idea came from originally.
Mark