Im only getting a white screen.
Im reasonably sure its the last step of getting the video onto the screen that's the problem.
What do I have:
- UK ZX81 issue 3
- ULA: 2C210E (so backporch)
- CPU: Zilog Z80A
- ROM: D2364C 649
- RAM: 2114LS x 2 (2 half K chips)
What have I done to it?
- Replaced C3, C5, C13
- Replaced 7805 (temporarily - waiting on a TSR 1-2450 in the mail)
- Added a 10 ohm resistor to R30
- Tried a simple 2N2904 with 100 ohm resistor video mod.
White screen, but I think Im getting a K:
When I hook up the oscilloscope to the pin 16, I see a backporch and generally a flat line of white, but a flicker of what looks to be the K being drawn. My keyboard membrane is a bit screwed and Im waiting on the Royal Mail Cyber issue to get a new one.
But poking around the connectors with a wire, I can "type" a random keyword & get the video output to change and draw more non white on the scan line. So Im pretty sure that its working.
This is happy progress - as earlier today, I was thinking maybe the RAM or one of the chips was screwed...
Transferring the probe over to the RCA output (eg after the transistor), I get the following voltage levels: So its 3.52V peak to peak, and the back porch is about 2.4V (with 0V being the sync)
But on the USA TV its all just one big white screen...
Advice please:
- Do you agree its likely working and its just the last video out part - correct?
- Does the frequency look right at 15.72KHz?
- Should I be scaling the voltage peak to peak, down on the RCA plug output?
- Should I just order one of the video mods around - even though I have a back porch? Recommendations?
Mike