Scrolling screen with Composite and vLA81

[Rabs]

Good idea, thanks will see if I have another display with composite in. However it works OK with my spectrum, thanks again.

[Rabs]

Can't get over the fact that it looked OK for a moment.

So couple of things I am considering;

Old sockets and single leaf type
Old electrolytic caps

[Rabs]

Fixed. Turned BP OFF on the VLA81 and ON on the RF modulator replacement and DIP switch 1 on the VLA81 off. Then used the trim pots to get a stable picture.

[1024MAK]

Ahh, while I was being interrupted by putting the dogs to bed, you replied. Still, I’ll leave this here for information.

Can't get over the fact that it looked OK for a moment.

Not unusual with analogue video…

Old sockets and single leaf type

Maybe, but if the board is not being flexed, not convinced. Most poor connections prevent the computer from running.

Old electrolytic caps

There are either two or sometimes three electrolytic capacitors on a ZX81 board. Normally two on a issue one board. One is the capacitor in the reset circuit, it’s not very often that this gives problems. If it was faulty, the Z80 would not get reset correctly, hence it would appear dead.

The second (and sometimes third) electrolytic capacitors are on the +9V (nominal) line. And are primarily there for power hungry items (such as the ZX Printer) on the expansion bus / edge connector. Failure (open circuit or loss of capacitance, or high ESR) is very unlikely to cause the problems you are seeing. As all the logic and analogue circuitry in a ZX81 is run from the +5V supply from the 7805 voltage regulator.

Far more important is/are the electrolytic capacitor(s) in the PSU. Assuming you are using an original PSU. However, if this/these have degraded, again the computer would not run, or not for very long.

One of the problems with the ZX81, is that it’s video output is far from being close to the broadcast standard. Sinclair did just enough for it to work with monochrome (black and white) analogue CRT TV sets.

The second problem (if using an original ULA) is that the video voltage levels are too high for composite video, even when buffered by a transistor. This is not a problem when used with an ASTEC modulator as designed. But not all modern TVs (or colour CRT sets) are happy with the signal.

Sometimes adding a 47µF to 100µF capacitor in series with the composite video output helps (because it removes the DC level). If you were using circuitry that you had put together yourself, using two or more 1N4148 diodes in the base circuit of the NPN transistor can help.

The only other thing that I can suggest, is to remove the board that replaces the modulator. The vLA81 is designed to directly feed a composite video input on a TV / monitor.

Mark

[Rabs]

Thanks for the help.

Basically I did not read the instructions fully

So I bought the RF modulator replacement as originally it just looked like my monitor did not like the signal. Which turned out to be true.

I bought the vLA81 as a backup just in case and it was the last one in stock. The original ULA as it turns out is fine.

So I installed the RF modulator replacement first and got the same/similar poor signal but did not read the instructions. With back porch on, I should have simply adjusted the trim pots and it would just have worked.

Then I tried the vLA81 and again did not read the instructions fully. There is a theme here.

So I did not understand the implications of back porch on/off and the interaction between the vLA81 and the composite board. But what threw me is why with vLA81 back porch on and composite board off it did not work. Still not sure but probably something I had set wrong.

Anyway thanks very much for the help all working now.