Gonbes GBS-8200 VGA converter with CPC

[bjt]

These converters are available on eBay for very little money, I have had a version 4 board for a while that I originally wanted to use with a CGA-equipped PC.


Tried it with the CPC and it works OK, using the LUM line as composite sync input to the GBS-8220.
I say "OK", because the image has some noise in it, which gets worse the higher the output res of the GBS-8220.
640x480 has little to no noise, but my VGA monitor applies some sort of filter that softens the image noticeably.
The higher resolutions are more crisp but have noticeable random white speckle noise.


There are people hacking these, using a Raspberry Pi or Arduino to program the scaler chip over I2C, but it's not something I want to get into.
For the money it works OK with the CPC and certainly well enough to play some games.


Will stick some pics up when I get a chance.

[Bryce]

Why do you use LUM as C-Sync when the CPC has a perfectly good C-Sync signal?

I also had problems with noise, but I improved the grounding on the PCB and this reduced it quite a bit.

Bryce.

[bjt]

Well, the common CPC-SCART wiring connects LUM to the composite sync pin of the SCART connect, and uses SYNC for RGB switching without a battery...


I then built a female SCART-VGA converter to connect to the DB15 input of the GBS-8820.
How did you improve the grounding of the GBS?

[Munchausen]

Yeah it seems nuts to use lum as c-sync! I have one of these and it gives an almost perfect picture using c-sync.

[Bryce]

Well, the common CPC-SCART wiring connects LUM to the composite sync pin of the SCART connect, and uses SYNC for RGB switching without a battery...


I then built a female SCART-VGA converter to connect to the DB15 input of the GBS-8820.
How did you improve the grounding of the GBS?

Strapping all the video grounds to the CPC Monitor port GND improved the picture.
The CPC SCART only used the LUM as sync because it had to, to allow the csync to be used for RGB switching, csync should be used.

Bryce.

[bjt]

Changed to using CSYNC now and the picture no longer has corruption at the top/bottom.

Also much less noise now, the big difference is that I'm now using a decent PC PSU to power both the CPC and GBS.

Previously I was using the CPC monitor to power the CPC and a cheap eBay PSU for the Gotek.

I guess having a true common ground and common power noise (if any) between CPC and GBS makes all the difference.