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CTM 644 Monitor, no Yellow!

Started by mrbee, 15:48, 09 November 23

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Amstrad 6128. it works. but..yellow is missing on the monitor and the potentiometer for yellow is a duplicate of blue! - retrogaming post - Imgur

If you check this link you can see that the monitor is missing yellow. Putting the amstrad on another tv shows that the issue isnt with the amstrad, I also connected another amstrad to confirm.

I have adjusted the potentiometers on the yoke (the board has been fully recapped recently).

because I am colourblind, I did not notice the change in colour on the basic menu as the red was bright enough but it did look a little off.

I have since fixed my 6128 and put my turtles disc in and the turtles and the logo was black. The text also was just dots as it used yellow to mix with blue to get green.

it is as if yellow is totally non existent on the monitor, Adjusting the potentiometers on the yoke it will adjust red and blue and brightness and contrast just fine.
The real strange issue is turning the potentiometer for yellow, this only adjusts blue! and if i turn blue, another instance of blue will show, as if there are two blue layers and one red layer.

So If i turn them all down so the screen is black, if I turn up the yellow pot, blue will appear, if i turn up blue, blue will appear brighter, red operates just fine.

Do you know what has caused this?
Thank you


As the monitor is mixing light, it's working in additive(RGB) colour space, not subtractive(CMY). You're not missing yellow, you're missing green! :D


Quote from: mrbee on 15:48, 09 November 23Do you know what has caused this?
Since the CPC works fine on another monitor and your CTM has been recapped, I would check if GREEN is connected correctly. 

Disconnect it from power. Connect the CPC to the CTM. Check if you get a connection between GREEN inside of the CPC and inside of the monitor. If that's okay, check the lanes inside the CTM. 


This should be an easy one to diagnose

Consult your CPC6128 manual for the exact basic commands. Set the border and background colour (ink 0) to green colours

Then if you see black, this confirms none (or little green colour)

Points of failure usually start with the most obvious - which would be the RGB cable from the CTM644 to the CPC6128. Check that there is continuity on the green line between the DIN connector and the white plug. I would suspect a faulty wire in the curly cable firstly

I would not mess around with any adjustments from factory. There should be no need to. If adjusting anything, take photos before adjusting so you can set them back to normal calibration

Next diode test continuity on the input diode, and then after diode test on the transistor. That is a pretty HV transistor, so I would be surprised if it was dead. Replacements are available (see my CTM posts in this area) since my red diode and transistor died due to flyback failure

My other note here is that some computers in the 80/90s had good capacitors and others did not. I've done ESR checks on my CTMs and all caps were in range. IMHO changing caps on these provides no benefit. 

However other machines like the Amigas all need a full recap (not the A500) due to the different types of electrolytics used. There were bad brands/batches of caps made but I don't think this affects our Amstrads
CPC464/GT64, CPC464 Plus/CTM640, 2 x CPC6128/CTM644


It's extremely likely that it's just a broken wire in the spiral cable. The rest of the circuit is low voltage stuff that doesn't get stressed in any way, very unlikely that something failed on the board.


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