Sinclair ZX80 / ZX81 / Z88 Forums

Hi all,

I've just composite modded my (new) TS1500, using the info provided here :

http://www.zx81stuff.org.uk/zx81/ts1500_video.html

The mod works, however the picture is very bright and there does not seem to be much contrast between the black and the white in the picture.

Any idea what the problem could be? Does the TS1500 maybe need the back porch / black level converter like the first version of the ZX81 ULA does ?

Cheers.

Phill.

I can't say for certain but it does sound like a back-porch issue, I do know that some people have purchased the ZXVid from me to get composite video working on the TS1500

The back porch problem normally results in a dark picture with a lack of contrast between black and white.

You appear to have a lack of contrast on a light picture. I assume that you have tried adjusting the contrast and brightness controls...

If this does not work, a circuit that regenerates the levels / adds a back porch, can be used. Or alternately try fitting one or two 1N4148 diodes in the base circuit of the transistor. But if the back porch is missing and your VDU does not like the TS1500 video, the diodes may not help.

(I don't have a RS1500 to try any mods on).

Mark

Cheers for the replies......

I have tried fitting a 100uf cap in series with the output from the transistor as suggested on this : viewtopic.php?f=7&t=654&hilit=TS1500 thread.

That has helped somewhat, still not perfect but a definite improvement, I may try adjusting the value of the cap see if that makes any difference.

Cheers.

Phill.

Using this extremely simplified schematic, you will get a much too strong video signal, which will saturate the video input of your TV set both for the white and the black levels; back porch or not, this can't properly work, but perhaps with an already partly damaged ULA providing a much too weak signal on pin 16.

You should insert one or two diodes between the ULA (pin 16) and the base of the transistor so to lower the output level. I got excellent results with a BAT85 (Schottky diode, 0.4V voltage drop) and an 1N4148 (Silicon diode, 0.7V voltage drop) in series which provided a good level (still saturating for the white, thus getting the noise clamped to avoid "grey patterns" in the white background), but allowing a, clean, deep black.

I'd also recommend that you use a fast switching transistor, such as the 2N2369A, to avoid "trailing" white to black transitions.

RWAP wrote:I can't say for certain but it does sound like a back-porch issue, I do know that some people have purchased the ZXVid from me to get composite video working on the TS1500

Pretty sure I purchased one of those, and it works nicely. Did try the mod in the OP, but didn't work at all for me.