Author Topic: CTM 640 Colour Monitor Repair - No Power  (Read 1203 times)

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Offline pledg

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CTM 640 Colour Monitor Repair - No Power
« on: 23:17, 06 September 21 »
Hi Everyone,


It's been a while since I've posted on the forum but I finally got hold of CTM 640 monitor to go with my restored Amstrad CPC464's.


I was fortunate to find a green screen GT 65 monitor a while back dumped at the bottle bank? I thoroughly dried it out as it had been outside for some time and cleaned it internally.
Fired it up and it worked!!!


Good as this is Iv'e always been after a colour screen and finally picked up spares or repair CTM 640 as a repair project. As you do???


I was told it worked initially, the owner walked away for a short time came back and it was dead. He told me he'd replaced a few parts but being as the unit was full of dust I couldn't see anything that looked like a replacement i.e. clean! everything was covered in dust.


So here's what I've done and found so far:


Checked the plug fuse all okay.


Thoroughly cleaned the unit blown out the dust.


Checked the fuse on the board okay.


There was a burn mark under R502 but it seemed the burn flash mark emitted from the bottom of C505. (I will check this resistor again)


I removed C505 100uf 400v capacitor and underneath was a black burn mark and damage to the negative pin and the wrapping on the neg side of the capacitor.


I replaced this capacitor.


IC401 the LA7800 looked like it maybe it had some discolouration to some of the legs so I just replaced it as I had some available.


I put the case back together and powered the unit on. Not the usual kind of static sound at power on and no 5V from the CPC power connector.




I'll refer to the schematic and check the power supplies.


Any suggestions for points to check appreciated.


Cheers


Pledg.










« Last Edit: 09:59, 07 September 21 by pledg »
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Offline pledg

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #1 on: 22:54, 07 September 21 »
So today I removed and checked R502 which was okay but I replaced with new anyway.


I will remove R501 and check that also.



D501 and D502 removed and checked okay, D503 and D504 to do next.


I've started checking voltages and they don't seem correct but i'll do a more methodical check when time allows.


Next I'll check the following:


IC401 (New I replaced this)
IC402
IC502
Q405
Q404


I'll run through these but I'm suspecting there's a problem with IC501 STK7308


Any thoughts please chip in...



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Offline Bryce

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #2 on: 10:06, 08 September 21 »
So today I removed and checked R502 which was okay but I replaced with new anyway.


I will remove R501 and check that also.



D501 and D502 removed and checked okay, D503 and D504 to do next.


I've started checking voltages and they don't seem correct but i'll do a more methodical check when time allows.


Next I'll check the following:


IC401 (New I replaced this)
IC402
IC502
Q405
Q404


I'll run through these but I'm suspecting there's a problem with IC501 STK7308


Any thoughts please chip in...

I'd suspect the STK7308. These tend to die first.

Bryce.
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Offline pledg

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #3 on: 12:09, 09 September 21 »
Hi Bryce,

Hope you're keeping well and thanks for your valued input.
Sometimes things are just staring you in the face an it takes a while to register. I'm no expert but took some time to understand what the rectifier circuit is supposed to be doing and how to check it's input and output voltages AC and DC.
I realised that this section of the board is right under the front of the screen and didn't get caked in dust as the rest of the board so I didn't spot that a diode and a smoothing capacitor had been previously replaced.
On reading this rectifier I'm getting 240v into the AC and 280v out DC?
This seems like a fail to me and what is next component down the line I should be checking for over voltage damage? Voltage regulator???

Pledg


« Last Edit: 12:28, 09 September 21 by pledg »
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Offline Bryce

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #4 on: 12:57, 09 September 21 »

On reading this rectifier I'm getting 240v into the AC and 280v out DC?
This seems like a fail to me and what is next component down the line I should be checking for over voltage damage? Voltage regulator???

Pledg

Why does that sound like a fail to you? Those are the values I'd expect.

Bryce.
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Offline pledg

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #5 on: 14:06, 09 September 21 »
I incorrectly assumed that it would be lower than that, clearly I need to research this more.  :doh:

Given it's the correct voltage output should I now check R501?


Would this need to be replaced with a cement resistor or could I use a metal oxide film resistor of the same value?

 
« Last Edit: 16:15, 09 September 21 by pledg »
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Offline Bryce

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #6 on: 16:44, 09 September 21 »
I incorrectly assumed that it would be lower than that, clearly I need to research this more.  :doh:

Given it's the correct voltage output should I now check R501?


Would this need to be replaced with a cement resistor or could I use a metal oxide film resistor of the same value?

Measure the voltage across C505. If it's anywhere near 250V or above, then R501 is fine. R501 is a high power resistor, so if you replace it, you'll need to use a resistor with the same or higher power capacity.
Regarding the higher voltage reading, this is due to the fact that it's AC (with at least a minor load) on the input but DC (possibly without a load) on the output. I also don't know what meter you are using, so RMS (Root mean square) may be a factor too.

Bryce.
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Offline pledg

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #7 on: 18:35, 09 September 21 »
Thanks.


I removed and tested R501 and it was in fact dead so I replaced it with new.


On power up I had the main 2amp power fuse blow.


I replaced the fuse and powered on again.


I'm now getting 240v DC across C505.
« Last Edit: 19:19, 09 September 21 by pledg »
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Offline Bryce

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #8 on: 21:40, 09 September 21 »
If the fuse blew the first time, there was a short which you have now nuked out of existence. So you are now looking for a component with an open circuit rather than a short circuit. Unfortunately I would guess that it's the HV transformer (LOPT).

Bryce.
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Offline pledg

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #9 on: 22:18, 09 September 21 »
So is there an easy way to check whether the flyback is perhaps in open circuit or could it be an associated transistor that's blown?


My next question would be if needed are replacement flyback transformers available?
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Offline pledg

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #10 on: 13:52, 10 September 21 »
I'm not going to give up on this just yet.


I'm in no hurry and the case of this unit is in such mint condition that if I end up buying another for parts then so be it. I just want the complete CPC464 package finished.


I'd like to look at why I'm not getting a 5v supply out of the monitor. As you said earlier STK7308 is a suspect and I do have a spare for that but I'll check a few other parts before replacing it.


I'm going to follow the schematic and the voltage I do have going to T501 and the components around the voltage regulator IC502 LM78MG.


Also I'm going to remove Q405 to test.
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Offline pledg

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #11 on: 18:05, 10 September 21 »
Transistor Q405 2SD1397 is a fail so I've ordered a replacement.


Question is when did it fail? I feel I should have tested it earlier on in the repair.


Am I correct that if this part was defective from the start it would have stopped the operation of FB401?
« Last Edit: 20:11, 10 September 21 by pledg »
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Offline Bryce

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #12 on: 20:25, 10 September 21 »
The problem is that these fails probably weren't the initial cause of the non-functionality, they failed because something else had already failed. So don't just blindly swap the failed parts out, because if something else is still broken they will fail again.

Bryce.
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Offline pledg

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #13 on: 13:13, 11 September 21 »
It may not appear so to a trained technician but I am trying to understand the theory behind how the CRT works and follow the voltages on the schematic etc.


I'm not going to replace and just power on again. I'm sure your right the flyback has gone so I'd be interested in any method to test this on the board. I'll give it another visual inspection for any damage first. I can test for shorts between windings etc. I think.


I'm interested to find out the lack of 5v from around T501 and IC502.


If nothing else it's all good practice reading schematics, removing components safely and without damaging them to test.
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Offline Bryce

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #14 on: 14:24, 11 September 21 »
Measure the resistance of each coil in the LOPT, but make sure everything has been completely discharged before you start!

Bryce.
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Offline pledg

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #15 on: 22:49, 12 September 21 »
You are right of course it seems that the flyback has blown.


I get steady and similar resistance across all the coils until I get to down to 8,1,9


I'm also getting a short between 1, 11, 5 & 6.


Its's real shame that these parts are virtually impossible to replace with new.


I'll keep my eye out for a replacement flyback but as you've previously commented Donberg advertise them but they'll obviously be well used examples no doubt if they ever send one.


In the meantime I'll keep my eye open for spare parts but I'm thinking of a mad project of mounting an old flat panel inside the CRT case? 



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Offline pledg

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #16 on: 22:50, 12 September 21 »
Thanks for your help!!!
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Offline pledg

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #17 on: 21:54, 16 September 21 »
Had a bit of a revisit to this...


I replaced the power resistor R501 again after what I had used previously failed, It was the same rating but for some reason it had failed.


I replaced all the diodes in the rectifier. This had been repaired by the previous owner but I noticed one of the diodes was a different rating.


The main fuse has not blown at all after this...


So I'm now getting a steady 320v across C505 and on until pin 14/15 of IC501


Don't seem to be getting any voltage into or from IC502 and have checked back through Q504, Q505, Q506, Q503 and Q502


Probably clutching at straws but anywhere else to check voltage around T501 or IC502?


Thanks...











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Offline pledg

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #18 on: 10:44, 17 September 21 »
Hi Bryce,


Can you give me a bit of pointer please.


I've recapped the board, replaced a couple of dead diodes and one zener diode around IC501.


Previously I was getting 0v from pin 11 and 6 of the flyback transformer. I'm now getting 15v but I trace that back to T501.


Should I be getting 110v out of this side of the transformer? If so is that a fault with the transformer or it's associated components?


Any other component I should be checking?


Thanks...
« Last Edit: 22:46, 24 September 21 by pledg »
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Offline pledg

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #19 on: 14:27, 25 September 21 »
Update!


Progress maybe???


I found a very small piece of copper wire fragment bridging pins 4 & 5 of IC501. When I removed it and powered on I get power noise/buzzing, the first time Iv'e heard anything like this!


No static to monitor front though?


I then stupidly accidentally made a short checking Q502 and the main power resister blew rather annoying so I need to replace and test more carefully.


I then discharged the CRT and got a small crack sound so that tells me that some HV has finally reached the CRT tube.




 
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Offline pledg

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #20 on: 18:13, 07 November 21 »
So I replaced IC501 STK7308 again after stupidly shorting it the first time and I now have a stable system again ie. the main fuse isn't blowing.


I'm back to getting a high pitched whine and no voltage out of T501 8142004. The vibration and noise is definitely emitting from this part.


When I power down the system I get the winding down of this transformer audibly.


I'll try and see if I can find a replacement part but in the meantime if anyone has a replacement kicking around on an old board I'd be happy to purchase.
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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #21 on: 16:49, 10 November 21 »
Okay so I removed T501 and tested all the coils but I can't see anything wrong with it.


I can't understand why I'm not getting the 110v and 9v out of it. Must be something not right around the STK7308.


The buzzing sound must mean that T501 isn't getting the correct voltages?
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Offline Bryce

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Re: CTM 640 Colour Monitor Repair - No Power
« Reply #22 on: 10:29, 11 November 21 »
Okay so I removed T501 and tested all the coils but I can't see anything wrong with it.


I can't understand why I'm not getting the 110v and 9v out of it. Must be something not right around the STK7308.


The buzzing sound must mean that T501 isn't getting the correct voltages?

Maybe a silly question, but I'll ask it anyway: You are aware that these are all high frequency AC voltages? So if your meter is on DC you'll measure nothing? And if the frequency is higher than what your meter can handle on AC you will be getting inaccurate readings. The buzzing sound is normal for an old transformer. The coils loosen over time and vibrate at the frequency on the input voltage. Unfortunately, this vibration is also what causes shorts inside the coil, killing both the coil and the part feeding it (STK7308).

Bryce.
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