On Sun, 15 Jul 2007 17:21:20 -0700, billdeg wrote:

> I get a prompt, but it's very dark. In fact
>I have to bring the contrast and brightness up all of the way just to
>barely see the prompt.

I suspect this is perfectly "normal" operation for your computer.
The early issues did not produce the back porch part of the video.
None of my issue 1's do, but my issue 3 does, so only my issue 3
produces a display without having to turn up the brightness, as it
correctly has a black-level back-porch part of the signal.

The problem is that the few uS after a sync (ie. the back porch) is
used by the TV to set the black level (among other things). Sinclair
didn't have curcuitry to generate this in the earlier machines, so the
white was displayed immediately after the sync. However, TVs assume
the back-porch interval to always be black, so the whole screen is
made dark (ie. the TV assumes the white level on the ZX80/81 to be the
black level and automatically adjusts the brightness for you!)

In my homebuilt ZX80/81, I have designed a circuit to do this.
Basically, I stretch the SYNC pulse by about 5uS, and use this to gate
the video signal (ie. no white video during this stretched pulse), so
the first few uS after SYNC is always black, then the white signal is
enabled. This has produced an excellent display on my televisions
without having to adjust the brightness or contrast at all

Hava a look at my ZX80-81 conversion page. You'll see the circuit I
use there:

http://home.micros.users.btopenworld...0/zx80nmi.html

Hope this helps.

Grant.