Author Topic: CTM640 has power but no picture, 5V out  (Read 189 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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Witchy
Binary Dinosaurs - where RIFA capacitors come to die.
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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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Witchy
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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.
 

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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Witchy
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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.