Author Topic: CTM640 has power but no picture, 5V out  (Read 884 times)

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Offline Witchy

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CTM640 has power but no picture, 5V out
« on: 10:39, 27 May 21 »
Hi foks,


First actual post though I've been a longtime lurker thanks to necessary fixing of my CPC collection over the years. This is the first time I've had a failed monitor though.


This is a CTM640 and it came to me utterly dead with an exploded main fuse. History of the unit is unknown so I don't know if its death was component or fluid related but the bottom of the board certainly has evidence of damp.


Swapped main fuse and naturally it blew again so looking around with the DMM and I discovered D502 and D503 (IN4007 1A diodes) were open circuit so they were swapped and the main fuse no longer blows. I hear the degauss coil energising at power up so SOME power is making it through. However there's no picture when hooked up to the 464 (tested OK with an MP2 modulator) and I can't see heater glow in the tube neck. 'Screen' adjustment on the flyback also makes no difference, nor brightness control.


Is there a common failure mode with these models beyond the usual flyback/HOT blowing? I notice donberg.ie claim to have these in stock but I need to buy 2 of them because unit cost is €17 but their minimum order is €35,  ::)


Any hints before I delve further into the schematic?


Cheers,
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Offline Witchy

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Re: CTM640 has power but no picture, 5V out
« Reply #1 on: 14:55, 01 June 21 »
*bump*


Has nobody else come across this one before? I've gone round the flyback and checked for dry joints etc, maybe I should concentrate on why the 5V output is dead since that implies a power fault elsewhere.


Cheers
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Offline Bryce

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Re: CTM640 has power but no picture, 5V out
« Reply #2 on: 15:12, 01 June 21 »
If you've checked that the tube neck is glowing and the HT voltages are present, then the next step is to trace the signals. Is the tube getting H-Sync and V-Sync, if yes, are the colour guns getting data etc.

Bryce.
 
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Offline Witchy

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Re: CTM640 has power but no picture, 5V out
« Reply #3 on: 23:00, 01 June 21 »
Hi Bryce,


Thanks for replying. I can't see a glow in the tube neck which is why I'm wondering if there's a power fault somewhere else that's also taking down the 5V output. I'll get the meter on it tomorrow, assuming the day job isn't as crazy as today was.


Cheers!
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Offline Bryce

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Re: CTM640 has power but no picture, 5V out
« Reply #4 on: 09:56, 02 June 21 »
All the voltages are mentioned on the schematic, so check which one is missing.

Bryce.
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Offline Witchy

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Re: CTM640 has power but no picture, 5V out
« Reply #5 on: 20:13, 26 June 21 »
Sorry for a lack of updates, I ended up back in hospital a couple of days after posting thanks to my kidneys. Fun times.

Checked all the voltages listed in the service manual today and there's obviously still a problem with the PSU since all the test points are 0v. Since it was D502 and D503 in the rectifier circuit that failed dead short could they have taken something else out before the main fuse blew? If IC501 has been nuked then that could explain why there's no voltages anywhere. What can I use as a negative reference for the DMM to meter out IC501?

Cheers

*edited for clarity*
« Last Edit: 21:38, 26 June 21 by Witchy »
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Offline Witchy

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Re: CTM640 has power but no picture, 5V out
« Reply #6 on: 18:10, 27 June 21 »
Ok, R501 is dead so that helps to explain lack of power anywhere else. D507 is also dead so I guess that's pointing fingers at the STK letting go?

*edit* also D507 is listed as an RD3.6FB which doesn't appear to exist any more. Can I substitute any 1W 3.6V Zener I wonder...

Cheers,
« Last Edit: 19:11, 27 June 21 by Witchy »
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Offline Witchy

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Re: CTM640 has power but no picture, 5V out
« Reply #7 on: 19:44, 10 July 21 »
Hello folks, a further update in amongst various hospital visits, who said getting old was fun?  :laugh:

Pins 10/11 on the STK were dead short so the total casualty so far is R501 (open), IC501 (short on 2 pins), D507 (short), C506/C507 (below tolerance), D502/D503 (short).

Having replaced all of those I got a nice wisp of smoke next to the flyback and with a high speed camera I could see what's marked on the schematic as R521 having a bit of a spark. This fusible 1 ohm resistor is the path to pin 6 of the flyback and sync circuits via L503. Testing it out of circuit shows it's dead (now) so while I can replace that too do I also need to think about replacing the flyback? Donberg claim to have them or at least have knowledge of them.

Can't add attachments or insert images for some reason so here they are.

https://binarydinosaurs.co.uk/burnyResistor.png
https://binarydinosaurs.co.uk/burnyResistor2.png


Cheers!
« Last Edit: 11:31, 11 July 21 by Witchy »
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Offline Bryce

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Re: CTM640 has power but no picture, 5V out
« Reply #8 on: 19:47, 10 July 21 »
Hello folks, a further update in amongst various hospital visits, who said getting old was fun?  :laugh:

Pins 10/11 on the STK were dead short so the total casualty so far is R501 (open), IC501 (short on 2 pins), D507 (short), C506/C507 (below tolerance), D502/D503 (short).

Having replaced all of those I got a nice wisp of smoke next to the flyback and with a high speed camera I could see what's marked on the schematic as R521 having a bit of a spark. This fusible 1 ohm resistor is the path to pin 6 of the flyback and sync circuits via L503. Testing it out of circuit shows it's dead (now) so while I can replace that too do I also need to think about replacing the flyback? Donberg claim to have them or at least have knowledge of them.

Can't add attachments or insert images for some reason so here they are.

https://binarydinosaurs.co.uk/burnyResistor.jpg
https://binarydinosaurs.co.uk/burnyResistor2.jpg


Cheers!

Without even looking at the photos (cos the links don't work), this repair is going to cost you more time, money and effort than it's worth. I'd recommend looking for a replacement or an alternative solution (SCART TV).

Bryce.
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Offline Witchy

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Re: CTM640 has power but no picture, 5V out
« Reply #9 on: 20:05, 10 July 21 »
Oops, that's what I get for using PNG instead of JPG. Changed links now.

That's the the way I was heading I guess. I only started looking at it originally so I could clean it up and sell it for a mate, it's not even mine. I've only spent ~£15 in parts so far and don't like being beaten by dead tech but might have to admit defeat this time. If it was one of my own I might make the effort but prices are varying wildly at present for even working ones so...

At least the 464 works.

Cheers!
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Online Animalgril987

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Re: CTM640 has power but no picture, 5V out
« Reply #10 on: 00:26, 11 July 21 »
Hi @Witchy .
The links still appear to not work. :(
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Offline Witchy

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Re: CTM640 has power but no picture, 5V out
« Reply #11 on: 11:32, 11 July 21 »
Indeed. When I edited them first time it only changes the text and not the link. I've got to go into HTML mode to actually edit the links. Done now.
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