I’ve acquired a ZX81 and I’m getting a poor picture (no contrast) on a modern TV. I’ve been following the well-known Video Conditioning mod instructions and trying to mount externally (leaving the modulator connected, to try and be as nondestructive as possible). I’ve run out +5v, 0v and Video through the side of the case.
Once or twice, on breadboard, I’ve got a beautiful clear bright picture, free of any ghosting etc, getting my hopes up. Every time I’ve transferred it to stripboard it fails, I just get a grey, barely visible picture again.
Now I can’t even get it to work on the breadboard (except, once or twice, I connected +5v to 555 pin 4 (reset) but not to pin 8, and strangely I got a clear picture but with the back porch encroaching on the left-hand side, similar to an effect I think others had seen in another thread where there were timing problems. This is no longer working at all though, so how it happened isn't clear.)
I’ve stuck as close to the spec (R, C, 1N4148, PN2222A) as per Zigg’s site as possible; I’ve tried replacing R1 and R2 with 10k pots and swivelling them around, switching out the 555 (I believe the ones I have currently from CPC are good TI NE555Ps), wiggling a bunch of components, and I’m wondering if I need to understand this more:
- Is there anything internal in the ZX81 that could be wrong with this machine and is known to possibly affect this? I’ve tried two TVs and never got a remotely usable picture from RF.
- Should I disconnect/remove the modulator? Should this work/not work while it is still there?
- Am I just bad at prototyping and should try again, I.e., is this most likely to be a contact/dead component problem, or rather a finicky timing one? I’ve tried a few times now with fresh parts.
- I don’t have an oscilloscope. Is it time to spend a few £100s now to diagnose a 95p mod?