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VGA BOX

Posted: Tue Sep 25, 2018 10:48 am
by mhanias
Does VGA BOX destroys the ZX8301 chip?

This happens to mine

Re: VGA BOX

Posted: Tue Sep 25, 2018 7:34 pm
by belg4rion67
It’s possible because the video signals from that ULA are direct and not protected to rgb socket! If you have some experience with soldering you can attach some zener diodes to protect output pins.

Re: VGA BOX

Posted: Wed Sep 26, 2018 7:43 am
by Nasta
Unlikely a zener problem, although that protection method will not hurt.
It should be checked if the VGA box has termination resistors on the input RGB lines, these are usually a low 75 or 150 ohms, which the ZX8301 cannot drive (almost a short circuit) and will eventually fail.
The RGB output on the QL is DIGITAL, i.e. 0-5V while most standards, including scart RGB require analog, approximately 0-1V. Connecting QL RGB and CSYNCHL signals directly to scart pins is NOT a good idea, there should be series resistors to drop the signal to the required level and reduce the load on the 8301.
I seem to remember this being discussed on the forum already...

Re: VGA BOX

Posted: Wed Sep 26, 2018 8:28 am
by belg4rion67
yes is true! see this picture: http://www.fruitcake.plus.com/Sinclair/Spectrum128/SCARTCable/Spectrum128SCARTCableQL.htm
QLCableCircuit.gif
QLCableCircuit.gif (5.25 KiB) Viewed 1045 times

Re: VGA BOX

Posted: Sat Sep 29, 2018 5:24 pm
by polka
Hi all,

This is my SCART cable, I built two of them (for both my original QLs) and they work well (on ancient TVs and also on all my recent flat-screen TVs).

qltosc10.jpg

Two things to be noted :
1/ you need a 5v on some pins of the SCART. I picked it inside the QLs and output it on a supplementary plug beside their video output.
2/ I added a "SWITCH" to exchange the green and blue signals, for stereo graphics using red/blue goggles.

May the forth be with you !

Polka