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Connecting the Amstrad GT 64 (65) Green Monitor to the Model 1

LambdaMikel

LambdaMikel

Experienced Member
Joined
Jan 25, 2020
Messages
369
Folks, I am trying to connect my Amstrad GT 64 Green Monitor to the Model 1.

The GT 64 / 65 has R, G, B, and SYNC, LUM, and GND. Hence, it is "analog RGB", but only LUM and SYNC matter. The GT has no composite input.

The usual trick of connecting the Model 1's Composite output to both (or either) SYNC and LUM did not work for me - I tried. No synchronization; but visible characters (but unreadable).

Some other folks reported that running a sync splitter on the Composite signal before feeding "cleaned" LUM and then separate SYNC to the GT might solve that problem:

http://www.mmmonkey.co.uk/composite-sync-stripper-lm1881/

Anybody has any experience with that?
Or any other ideas / suggestions?
 
Alphasite said:
I haven't seen anyone mention doing that before.
Click to expand...

Hmm, rather than trying to extract Sync from the Composite signal, why not grab it directly from the board... the Model 1 really has excellent documentation. Never saw such good documentation.

The Technical Reference Manual Rev. 2 explains on Page 42 how the Composite Signal is "mixed" from combined sync signal and character generator dot output (Figure 9a). It seems I can just grab these signals from the board!

Will try that.
 
LambdaMikel said:
Hmm, rather than trying to extract Sync from the Composite signal, why not grab it directly from the board... the Model 1 really has excellent documentation. Never saw such good documentation.

The Technical Reference Manual Rev. 2 explains on Page 42 how the Composite Signal is "mixed" from combined sync signal and character generator dot output (Figure 9a). It seems I can just grab these signals from the board!

Will try that.
Click to expand...

For the record - this works well. Not a bad monitor for the Model 1!

DSC09687.JPG
DSC09688.JPG
DSC09689.JPG
 
Cool!, thanks for sharing this!.

I've a GT65 covered in dust and with such converter I can try to find some new uses for it. Did you finally assembled a working sync splitter?,
 
jltursan said:
Cool!, thanks for sharing this!.

I've a GT65 covered in dust and with such converter I can try to find some new uses for it. Did you finally assembled a working sync splitter?,
Click to expand...

My little GT64 connector box works *without* the LM1881 sync splitter.

I am grabbing the SYNC, LUMA, and power lines from the board, before they go into the video mixer transistors, and all the little circuit does is feed SYNC and LUMA into a 74LS04 for amplification and decoupling from the Model 1. I can then also invert LUMA as shown in the pictures if I wish to. This works well with my GT 64. The only catch though is that I need to turn on the GT64 AFTER the Model 1. Otherwise, it doesn't grab the sync. I also needed to adjust the inductor in the GT 64 a bit (this is L703 in the Service Manual for the GT64 - GT65 might be different number). It still works with the CPC as well.

http://www.cpcwiki.eu/index.php/Service_Manuals

The sync splitter LM1881 is another route you can try. I have the chip sitting here, but have not tried it because I got the other solution working before the chip arrived. Also, it seems of course cleaner to me to use the original signals in the first place rather than having to "split" them from the composite.

Good luck! And careful adjusting the inductor / coil in the GT64... hopefully it is not required. Don't zap yourself.

It is worth pursuing it - the GT 64/65 is a GREAT monitor for the Model 1. Very good picture quality.
 
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