ZX81 Comp. Mod, no Video

[1024MAK]

R30 10Ω - USA/NTSC only, not fitted otherwise. Used by the ROM code to select video mode.
R31 270Ω or 390Ω - Used on USA versions only, part of video circuit to modulator input.
R32 560Ω or 2.2kΩ - Used on USA or French versions only, part of video circuit to modulator input.

Mark

[Moggy]

I don't have a US version but found these, click to enlarge.

Issue 1 and 3.

The issue one boards have had work done but should be ok for your needs, click pictures to enlarge.

[p5s]

Work has been distracting me. However, I found and fixed the short. I am now getting a rolling/very unstable composite video signal that neither the comp->HDMI converter or Apple IIc CRT can lock on to. See below:

FYI. I have tested both the monitors with a TS1000 + the simple resistor-transistor comp mod and are able to display a signal - K-prompt appears. Albeit the video on the LCD display with the comp->HDMI converter is exponentially better than the Apple IIc CRT.

I'm going to continue to investigate and maybe make adjustments to the comp mod I built to see if it affects the video out signal/I can make it more stable. Ideas and experience with this issue welcome. Thanks.

[1024MAK]

The length (time) of the back porch and the trigger level have to be in the right range, otherwise it affects both line (horizontal) and field (vertical) synchronisation of the picture.

If you are using a NE555, I’ve found that cheap versions from trading sites are too slow. It’s impossible to set up and get a full stable image. Genuine NE555 or better still, genuine 7555 (CMOS version) do work. There is an entire topic here on this forum about this, link.

Mark

[p5s]

Hi Mark.

Thanks, this is helpful. I get my supplies from Jameco.com, reputable and local to me. This said, I’ll try swap out the 555 as I have a few and will see about getting a 7555, and continue on…

Alan

[p5s]

Hi. Does anyone happen to know the value of the two inductors on the ZX81 near the 9v pwr input and the regulator? I don’t see these on the schematic or the BOM in the kit assembly. Likely unrelated, but they are missing from my troubled ZX81 w/no video output — still working to eliminate all issues.

Thanks, Alan

[1024MAK]

They are only fitted on models sold in the USA. They are to reduce the level of EMI “leaking” out. They are not used on UK models. They do not affect the operation of the computer (well unless broken).

I don’t know the value of them.

Mark