Sinclair ZX80 / ZX81 / Z88 Forums

Hi All,

I have another ZX81 (Issue 1) which I have fitted another composite mod kit from You Make Robots.

https://www.youmakerobots.com/sinclair/ ... it-without

After turning on the signal isn't locking and seems to think it's NTSC, I have buzzed the video connections and seem to have a bridge between R34 and the USA 1 connection, I am thinking this shouldn't exist and probably why I can't get a video signal.

But shouldn't I be able to see something or is the fact the video board is wired up for UK and the bridge exists causing the system confusion and no video output?

I'll clear the bridge but was interested in how R34 and USA 1 interact when the main signal for the composite board is wired up correctly.

Kind regards,

Peter.

R34 should only have connections to the Ear socket and to capacitor C10. The other lead should connect to 0V/GND.

The USA1 connection only goes to the PCB pad for the not fitted channel selection switch (in the corner near the voltage regulator and the 9V power input connector).

R30 is the resistor that when fitted signals to the system that it should produce a 60Hz NTSC video signal. For ZX81s in Europe (or in other areas that have 50Hz TV systems) R30 should not be fitted.

Mark