Amstrad CTM-640 adventure

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Amstrad CTM-640 adventure

Postby SpidersWeb on Tue Oct 06, 2015 8:50 pm

So a few weeks back, I got a lovely CTM-640 from another member here for-parts-or-repair. I'd been informed the monitor was no longer outputting 5V or starting up and they remembered getting as far as identifying their was a fault in the power delivery side of things. Easy fix I thought - track down the bad component in the power supply, replace, and enjoy. What I didn't realise was how complex these things get when you get inside.

This is the short / simplified version - and doesn't including the week or so of me scratching my head looking blankly at a circuit diagram and poking it with a scope.

To start with, the 5V power supply (after you look up datasheets etc) is all powered off the feedback coil on the flyback - so it gets 0V until the flyback is oscillated.
Going in to the flyback is a 100V DC feed - but being DC and having no path to earth - there is no current flow nor does it induce a current on the flyback until it's pulsed (by switching the coil output to earth).

Q405 pulses the flyback, and following that backwards we see it leads back to the horizontal oscillator output of the 7800 IC. According to the datasheet, Vcc of this part is ~12V. This pin was reading 0V!
Fault found? OK follow that back, and you'll find that Vcc is powered by the feedback coil of the flyback?? So what came first, the chicken or the egg??!?

Turns out the 7800 IC appears to be two chips in one - and the horizontal oscillation section is powered off Pin 15 instead of it's normal VCC. On the circuit we can see Pin 15 is feed by a DC source NOT coming from the flyback for once - it's also filtered by C405 a 47uF capacitor. Using my $5 garbage dump oscilliscope I could see it had power on Pin 15, but nothing was coming out of the horizontal oscillator (needed to operate the flyback).

So I ordered a 7800 IC, which oddly you can find in New Zealand - order online - and get delivered overnight. I desoldered the old IC, and in the process noticed something I had not seen earlier:

Image

C405, the filtering cap for the IC, had lost it's pants. Thankfully I had a spare of that size - so swapped out both parts. Hit the power button and the CPC464 suddenly had a power light, a few dim dots appeared on the screen and their pattern changed when I pressed enter. Epic.

Turned up the sub-brightness on the flyback, which is rather un-nerving if I'm honest (almost 30 year old monitor, where most of the circuitry has not had power for many years, turned on with the lid off) and got this:
Image

Adjusted some of the hold settings and tada
Image

This is the second monitor I've repaired now that was caused by a fault in the horizontal oscillator circuit.

Just thought I'd post up because I find CRT's quite intriguing, and I suspect we'll see more and more of them failing with less and less people available to repair them. In Wellington I've only found one person - and I had to chase them up for several weeks to get any results.
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Re: Amstrad CTM-640 adventure

Postby SpidersWeb on Tue Oct 06, 2015 8:55 pm

Disclaimer: I'm excessively cautious when working on CRT's. Anyone else getting their hands dirty must be aware of the voltage levels in there and where they are before poking it with their fingers.


Also, check out the corrosion the electrolytic cap caused, looks very similar to battery damage. I'm sure that's been asked about here in the past.
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Re: Amstrad CTM-640 adventure

Postby Audronic on Tue Oct 06, 2015 9:14 pm

@SpidersWeb

Well done, another one saved.

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Re: Amstrad CTM-640 adventure

Postby tezza on Tue Oct 06, 2015 9:52 pm

Great work! I'll know who to contract when I need a monitor fixed :)

Looks like the 464 computer works just dandy too! Have you got/loaded any software into yet?

I've spent the last few months messing around with Amstrads. I've just finished getting a DD-1/FDD-1 drive up and going for my 464 and as a second drive for my 6128. It's the external drive I got given from the person in Wellington who donated some stuff when he had to shift flats. You got some PS/2s from him Jono. I can't recall his name (Paul?).

The drive needed a head clean and a new belt but now it's working just fine. I'll add a blog article on it in a week or two.
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Re: Amstrad CTM-640 adventure

Postby SpidersWeb on Tue Oct 06, 2015 10:40 pm

Cheers guys :) This is two monitors now, and I've got two more to learn from (Sprite and Taxan Supervision)
Funny thing is, I doubted myself, so I've got CTM-644 coming in the mail - perhaps I'll need to buy a 6128 base now.

tezza wrote:Great work! I'll know who to contract when I need a monitor fixed :)

Looks like the 464 computer works just dandy too! Have you got/loaded any software into yet?

I've spent the last few months messing around with Amstrads. I've just finished getting a DD-1/FDD-1 drive up and going for my 464 and as a second drive for my 6128. It's the external drive I got given from the person in Wellington who donated some stuff when he had to shift flats. You got some PS/2s from him Jono. I can't recall his name (Paul?).

The drive needed a head clean and a new belt but now it's working just fine. I'll add a blog article on it in a week or two.


I'd planned on buying a CPC at some point because it was my first-ever computer - so I'd been acquiring original tape games. There was two CPC 464's and a working GT65 green display - I combined them to make one good working unit and played a few rounds of Robocop but I'm yet to play games on the colour display. I had quite a nostalgia hit, hadn't used one in over 24-25 years and still remembered the CTRL+ENTER shortcut and wrote myself a small times-table program too.

There is quite a bit of hiss noise on the datasette, not sure if it's my tapes or the drive itself, but it's loading software just fine.

You're thinking of Ben :) 99% sure that's the one. That's where the Commodore 16 and plus 4 came from too, and the Dragon 32 worked too once I got power on it. Awesome guy - I don't think I got home that day until dark because I kept chatting with him. Great to hear the drives are running - it was certainly quite the upgrade!
Wanted - Dead or Alive - Reward $$$: Compaq Deskpro 8088 / 286 / 386 - IBM RT 6150/6151 parts - AT&T 3B2 parts
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Re: Amstrad CTM-640 adventure

Postby tezza on Wed Oct 07, 2015 7:56 am

SpidersWeb wrote:You're thinking of Ben :) 99% sure that's the one. That's where the Commodore 16 and plus 4 came from too, and the Dragon 32 worked too once I got power on it. Awesome guy - I don't think I got home that day until dark because I kept chatting with him. Great to hear the drives are running - it was certainly quite the upgrade!

Ben! Yea, that's the guy. A lot of his best gear went with him. A TRS-80 Model 3...I remember seeing a PET and some kind of minicomputer (a PDP-11 or was it a DEC?). I hope he found somewhere to put it all!
Tez (Terry Stewart) (Administrator)
Collection: http://www.classic-computers.org.nz/col ... /index.htm
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