CGA-to-VGA converter troubleshooting

[g67wjjk]

Hi,

Discussing with Ewan via email but thought it might be useful to post in the forum for posterity.

Bought a CGA-to-VGA converter a few years ago but never got around to using it, but my plan was to use it with my Amstrad CPCs, BBC Model Bs and some DOS machines once I was properly set-up.

Finally getting myself organised.

The Amstrad CPC and BBCs use the same analogue RGB output via 6-pin DIN:

Pin 1 = red
Pin 2 = green
Pin 3 = blue
Pin 4 = composite sync
Pin 5 = ground
Pin 6 = luminance

There didn't seem to be any documentation about connecting CPC to CGA, so I came up with this wiring scheme:

Amstrad pin 1 (RED) -> CGA pin 3
Amstrad pin 2 (GREEN) -> CGA pin 4
Amstrad pin 3 (BLUE) -> CGA pin 5
Amstrad pin 4 (SYNC) -> CGA pin 8 (H_SYNC)
Amstrad pin 5 (GND) -> CGA pin 1
Amstrad pin 6 (LUM) -> CGA pin 6 (INTENSITY ... although, incorrect/irrelevant?)

I left CGA pins 2 (extra GND), 7 (reserved?) and 9 (V_SYNC) unconnected.

Input voltage was 8V and it was drawing about 1/3 A.  The GBS8200 OSD worked but it continued to say "no input".  I also tried with a Commodore 128D with straight-through DB9 cable.

I confirmed the cables using an Amstrad EGA display - both my CPC-to-CGA and the one I used for the C128D.  The Amstrad displayed slowly rolling gibbersh (no V_SYNC), but the colours were correct (yellow text on blue background).  The C128D display was perfect.

I also want to use it on my Amstrad CPC6128+ that has an 8-pin DIN.

Brett.

   

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For interest, this is what the Amstrad CPC464 output looked like on the EGA display using that wiring scheme.

   

[g67wjjk]

It seems that I had a DIY kit.  Removing the RGB+I-to-Analogue adaptor board I immediately noticed some dodgy soldering by, ahem, me.

Pin 9 of U2 (MAX4619 analogue multiplexer) was unsoldered, and the middle pin of the 7805 (ground) looked dodgy too (although the chips seemed to be powered okay).

   

[g67wjjk]

Making progress.

Now I get an image when I wiggle my dodgy DIN plug on the Amstrad ... but red-on-blue instead of yellow-on-blue.  And of course the image is lost if I stop wiggling.

   

[someone]

It seems that I had a DIY kit.  Removing the RGB+I-to-Analogue adaptor board I immediately noticed some dodgy soldering by, ahem, me.

Pin 9 of U2 (MAX4619 analogue multiplexer) was unsoldered, and the middle pin of the 7805 (ground) looked dodgy too (although the chips seemed to be powered okay).

For future projects with similar sized PCB pads someone suggests that you use a thinner gauge of solder such as 0.71mm and a suitably wide chisel or double chisel soldering tip.
Investing in a bottle of suitable liquid flux and flux remover is also very useful.

[g67wjjk]

Someone has good advice - I've improved since and I finally did get a pot of proper flux that I use all the time now.

Still hate soldering DIN plugs though.

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More progress today.

Made a new DIN-to-DB9 cable.  More stable image but shifted to the right.  Strapping the H_SYNC or V_SYNC invert pins shifts the image left but flickers more.

Colours are still wrong though - Ewan pointed out this was because green was missing.

The C128D shows a perfect image but also incorrect colours.  This is using a different cable so the green must be missing somewhere internally ... although, maybe I should also check the VGA cable I'm using.

   

[g67wjjk]

Used an oscilloscope to check the R/G/B output from CGA board to the GBS 8200 ... R & B showed expected activity, nothing on the G output.

Then I noticed some haphazardly soldered inductors that appeared to be on the R/G/B signal paths:

L1 - red - C5
L2 - green - C4
L3 - blue - C3

   

There was activity on L2 but dodgy continuity between L2 and the green wire.

Re-soldered ... with a dab of flux ... and the colours are now perfect!  Happy.

Remaining problem is the image is right-shifted.

Strapping either V_SYNC or H_SYNC invert pins moves the image to the left (but not fully), however the image becomes more unstable - even more so when loading software from tape.

So still some troubleshooting to do.  I'll also hook up a BBC Model B to compare.

   

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Success!

Ewan suggested pressing the auto-adjust on the GBS 8200 and also tying the unused V_SYNC on the CGA side to ground.

Working perfectly now!

Final configuration:

Code:

CGA pin 1 (GND) -------> Amstrad pin 5
CGA pin 2 (GND) -------> CGA pin 9 (V_SYNC)
CGA pin 3 (RED) -------> Amstrad pin 1
CGA pin 4 (GREEN) -----> Amstrad pin 2
CGA pin 5 (BLUE) ------> Amstrad pin 3
CGA pin 6 (INTENSITY) -> unconnected
CGA pin 7 -------------> unconnected
CGA pin 8 (H_SYNC) ----> Amstrad pin 4 (SYNC)
CGA pin 9 (V_SYNC) ----> CGA pin 2 (GND)

Amstrad pin 6 (LUMINANCE) was also unconnected.

The H_SYNC invert pins on the CGA adaptor board were strapped and then auto-adjust on the GBS 8200 was executed.

   

   

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BIG THANKS to Ewan for taking the time over this weekend to help me get it sorted!

[g67wjjk]

This is the end result.

[1] Fixed my soldering issues: 7805 ground pin, MAX4619 pin 9, inductor L2 for green output signal
[2] Increased panel hole for DC power plug to 11.5mm so my inline power switch will fit
[3] Added switch for C128 Luma fix
[4] Added switch for H_SYNC invert for Amstrad CPC (and BBC Model B?)
[5] Added push button for auto-adjust in case I need to force a re-sync when switching computers

I'm very happy and really appreciate Ewan's help over the weekend.

Brett.

   

[g67wjjk]

Confirmed working on Amstrad CPC+ range as well - tested on Amstrad CPC6128+

These use an 8-pin DIN instead of the 6-pin DIN on the regular CPC range as they output L/R audio as well.  But otherwise configuration is the same, except H_SYNC invert is inactive (edited) instead of active.

   

   

[someone]

Excellent stuff.

Are your CPCs "PLUS" models with PSG DMA?

[g67wjjk]

Excellent stuff.

Are your CPCs "PLUS" models with PSG DMA?

I believe this is standard in the Plus models - this is the first time I've powered it on and tried out the cartridge software.  Although I guess I could've wired up an 8-pin to 6-pin cable and connected it to a regular CPC monitor.