White screen, no K? - now Okay.

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mikeh_nz
Posts: 67
Joined: Sat Feb 04, 2023 8:03 am
Location: Seattle

White screen, no K? - now Okay.

Post by mikeh_nz »

Im located in the US and have a ZX81 UK Series 3 that I am trying to get going.
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)
Original state
Original state

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.
Current state (with temp 7805 installed)
Current state (with temp 7805 installed)

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.
Pin16: Sync, back porch, and solid white line.
Pin16: Sync, back porch, and solid white line.
Pin16: Looks like a K is being drawn
Pin16: Looks like a K is 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:
RCA connector: K being drawn.
RCA connector: K being drawn.
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:
  1. Do you agree its likely working and its just the last video out part - correct?
  2. Does the frequency look right at 15.72KHz?
  3. Should I be scaling the voltage peak to peak, down on the RCA plug output?
  4. Should I just order one of the video mods around - even though I have a back porch? Recommendations?
Cheers
Mike
Last edited by 1024MAK on Mon Feb 06, 2023 12:47 am, edited 2 times in total.
Reason: Title updated
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1024MAK
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Re: White screen, no K?

Post by 1024MAK »

Hello Mike, welcome to our forum :D
mikeh_nz wrote: Sun Feb 05, 2023 12:26 am Advice please:
  1. Do you agree its likely working and its just the last video out part - correct?
Yes
mikeh_nz wrote: Sun Feb 05, 2023 12:26 am
  • Does the frequency look right at 15.72KHz?
Yes, the frequency will not be completely accurate, due to the ZX81 using a ceramic resonator rather than a crystal.
mikeh_nz wrote: Sun Feb 05, 2023 12:26 am
  • Should I be scaling the voltage peak to peak, down on the RCA plug output?
Either add a 47µF to 100µF capacitor in series with the video output from the transistor, positive leg to the transistor, negative to the TV/monitor, or add three (or more) 1N4148 diodes in series with the base of the transistor to reduce the output voltage. Cathode (k) to the base of the transistor.

To meet the normal video levels, the p-p voltage should be around 1V. However, a lot of TVs are not fussy as long as the DC input level is low enough. That’s the point of adding the capacitor.
mikeh_nz wrote: Sun Feb 05, 2023 12:26 am
  • Should I just order one of the video mods around - even though I have a back porch? Recommendations?
No. First, see the link below in my signature to see my version of a composite video circuit.
Secondly, you do not need the extra circuitry to generate a back porch, so the only benefit (depending on which device you buy) would be to clean up the noise. So, I suggest you wait while you continue experimenting.

Mark
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mikeh_nz
Posts: 67
Joined: Sat Feb 04, 2023 8:03 am
Location: Seattle

Re: White screen, no K?

Post by mikeh_nz »

Thanks Mark - I appreciate the advice

(It’s good that you use the same handle as over on the stardot forums :) )

I think it’s time to break out the breadboard and have a play!..
mikeh_nz
Posts: 67
Joined: Sat Feb 04, 2023 8:03 am
Location: Seattle

Re: White screen, no K?

Post by mikeh_nz »

Yup - that did the trick.

Post title can now be changed to: "White screen, now OK"

Now that I can get a display, I now feel more confident to work on the 16KB upgrade.
I also just saw ZX-WESPI today. I have some ESP32 boards around from another project, so I might give that a go.

It lives again.
It lives again.
K looks pretty sharp.
K looks pretty sharp.
Thanks for your help on this - another ZX81 lives again!
Mike
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