CTM644 showing only green

[airamcruz]

Hello everybody
Triyin to fix an old CTM644 colour monitor that I got as a present from my brother in law. I took it to a technician 'cos didn't turn on. After changing the "lines transformer"?? and a vertical hold IC (LA7800) the monitor is back to life, but..... Showing only green, it's like a GT65. 
The tech also replaced the screen 'cos it was nearly gone (But kept it)
I've opened the monitor (aware of danger) and found that the problem could come from the colour wires going into a coil on the back of the screen. I think the tech mixed up with this...
Don't want to take it back to him or anybody else, It looks like they don't take care too much (scratches everywhere, missing bolts, strange add-ons, etc....)
I wonder if anyone could give me a hand with this
Thanks in advance

[Fessor]

I had once a similar Problem with my ctm640 - it had a broken wire in the Connection Cable.

[airamcruz]

Hi Fessor, I've pulled the cables where they connect to the PCB and look like they're well fixed.
I've also noticed that theres a black wire tied (not soldered) to a metallic wire that goes around the screen (GND??)
Could that bee the problem?

[Bryce]

Na, that sounds like a metal cable-tie. That's how it should be. Post a picture of it if you're not sure.

Bryce.

[Fessor]

Please post a screenshot of this one-liner (it displays red, green and blue squares) to see which colours/signals are missing.

Code Select Expand


10 mode 0:pen 3:print chr$(143);:pen 6:print chr$(143);:pen12:print chr$(143):pen 1


You could/should also download the Service Manual from the Wiki CTM640/CTM644 - CPCWiki

Could you also post an overview of that thingy on the blue wire please? Its hardly to recognize. Never seen such thing before.
Found some nice pictures of a disassembled ctm Amstrad CTM 644-2 Monitor and there is no such component on the blue wire...

It seemed, in comparison on the screenshot in the wiki that the cables are connected at the correct pins. (Even if the quality of the work looks pretty bad)

[Bryce]

The "thingy" is a variable inductor (a seriously vintage one too!), which the tech probably added to try to remove some sort of noise in the signal.

Bryce.

[Fessor]

Ahh... okay.... But I didnt want to call this tech a tech if i see this quality of work with this leftovers of his experimentations of which signal goes to which pin. (good to see at the green and the blue connection, leftovers of tryouts). Also the soldering itself doesn't look good.

[Bryce]

I have to agree. I don't put a lot of trust in the repair judging from the pictures. It looks like it was soldered with a cigarette lighter.

Bryce.

Edit: Also questionable: "He changed the screen"?? You can't just change a screen, it would have to be a compatible one. Where did that come from? Is it really compatible?

[airamcruz]

Thank you guys for helping me with this...

I have to admit that the work he did is horrible, but the thing is working.... Anyway this is why I don't trust very much in the techs round here. Hey, you haven't seen the scratches on the front cover?

It made me laugh the cigarette lighter solder 

So you think the left wires are tryouts. I tought they were the correct places.

Yep. He changed the screen for a colour TV screen. I bet He didn't know if compatible or not. At last the pins of the tube are the same 'cos same socket

I wrote the one liner basic code and only appeared a green square. I also noticed that if you type pen 1,3,5,... Odd numbers you can see what you're writing. Whereas if you type pen 2,4,6,.... You can't.

I keep the old screen. I also have a tube rejuvenator. The odd thing is that I don't know how to use it or change the screen, i can solder better than that tough

Should I change the screen???  I'm aware of danger, I'm trained to be an electrician, but not much idea on electronics.

I post some photos of another odd capacitor+bad soldering. Is this another add on?

[Bryce]

Why don't we have a Face-palm Omoticon??? WTF!!!!

The black wire is really attempting to ground the tube! But what's with the 3 capacitors in series?? What was he trying to do there!?!?! I hope he doesn't think you can add the values? 

Bryce.

[Fessor]

A professional would have used a soft underlaying to avoid scratches...
Those capacitators seemed to be a replacement for C430 for the line output transformator, in a row to get the correct capacity, but have nothing to do with the missing signals as it is seeable that red and blue are completely missing as the first two pens of the printed chars are defined with ink 6 (100% red) and ink 2 (100% blue). With ink 3,9 the first printed char should displayed half-bright (ink 9 is 50% green)

Based on the assumption that the cables are connected at the correct spots at the tube and even if bad soldered, red and blue should have given a picture.
What i would do at this point is: Let the tube discharge, grab a potentiometer and at check the signal path through the Din Cord to the Tube. Pin 1 of the DIN-Connector with the Ending of the red cable at the Tube, and Pin 3 of the Connector with the blue Cable. If that delivers no result then checking the Cord for itstelf as its unlikely to me that the Diodes D901 and D903 on the PCB are dead. Unlikely but not impossible. More likely is, as its the part with the only mechanical stress, that the Din Cord have broken wires.

[Bryce]

How do you know that they get the correct capacitance in a row if you can't see the values?

Bryce.

[Fessor]

I have not said that he got the capacitance right. I can only see what he have planned with this.

[airamcruz]

What i would do at this point is: Let the tube discharge, grab a potentiometer and at check the signal path through the Din Cord to the Tube. Pin 1 of the DIN-Connector with the Ending of the red cable at the Tube, and Pin 3 of the Connector with the blue Cable. If that delivers no result then checking the Cord for itstelf as its unlikely to me that the Diodes D901 and D903 on the PCB are dead. Unlikely but not impossible. More likely is, as its the part with the only mechanical stress, that the Din Cord have broken wires.

By potentiometer you mean multimeter? Do you want me to check continuity between the DIN pins and the terminals??
When I switch on the monitor, with the computer unplugged, it shows a green screen as well.

[Bryce]

I have not said that he got the capacitance right. I can only see what he have planned with this.

Ah, ok. I still doubt that someone with those soldering skills would have the knowledge to calculate the total of three electrolytic capacitors in series. I fear he may have just added up the values.

Bryce.

[Fessor]

By potentiometer you mean multimeter? Do you want me to check continuity between the DIN pins and the terminals??
When I switch on the monitor, with the computer unplugged, it shows a green screen as well.

As you mention that, then the brigthness of the tube must be to high, as you should have green Chars on black Background. You should have the usual whopping green of ink 18 on a black Background. Also the H-Size is not correct as there is no left or right Border reckognizable. V-Size seem nearly ok. You could try the circle-drawing-Program of the Manual to check the geometry.

Yes, i meant a Multimeter. Plus of the Multimeter to a Pin of the DIN-Plug, Minus of the Multimeter on the corresponding cable at the Tube and the switch to the Diode Symbol on the Multimeter. (I have uploaded a picture to illustrate the measuring points) This way you can test the Diode on the PCB as the connection in general. Changing the Direction (plus to the tube, minus to the pin)  the Multimeter should display an unchanged value as if not connected (because Diodes lets the current flow only in one direction). If you got a Value on the Multimeter then the Diode is defect. (But as it then let the current flow it should deliver the corresponding colour). If you got no Value at all you have to check each component of its own. Unplugging the Spiral Cord from the PCB and check Pin for Pin of the Cable. And from the PCB Connector of the Spiral Cord to the cables at the Tube.

(I dont think, that google translate is my friend; some of its results don't seem to be correct)

[Fessor]

Ah, ok. I still doubt that someone with those soldering skills would have the knowledge to calculate the total of three electrolytic capacitors in series. I fear he may have just added up the values.

Bryce.

Some form of Schroedingers Cat. Without knowledge of the values we cant say if the techy is only a bad worker with no quality standards or completely incompetent....

[Bryce]

They are 6.2V Zener diodes. The diode tester on most multimeters won't be able to test these properly because they can't supply enough voltage to trip them.

Bryce.