Another dodgy monitor CTM-644

Started by dtwaide, 14:30, 23 May 22

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dtwaide

Hi folks

I've just dragged my old 6128 out of storage and found the monitor has failed. Reading back through the forum I see another monitor from last month with a similar problem and the consensus appeared to be that it was due to a short between the deflection coils in the yoke. 

I have experience working with electronics but not repairing CRTs so I thought it would be best to ask the experts if I'm on the right path before I continue. This is my personal machine, owned from new, so I'm not keen on replacing it. I'd rather repair it if possible. 

Would anyone be able to confirm the fault, please? And also point me in the direction of a good service manual for CRTs. I'm aware of the safety issues involved but I do have a good deal of technical experience and take safety seriously so I'm determined to fix this myself if you kind people can point me in the right direction.

Thanks!

PS I did search the forum beforehand but it never returns any results, regardless of the terms used.

Bryce

If you are really lucky, the yoke coil has just fallen out of position. If you are unlucky, it has burnt one of the coils and will need to be replaced.

Bryce.

dtwaide

Thanks Bryce. There's every chance the fault has developed when the monitor got moved. It worked just fine the last time I had it out a few years ago.

How would I safely check whether or not the yoke is in the correct position?

Bryce

As long as you keep your fingers from the dangerous bit (big capacitors and fat wires going to a suction-cup-like connector on the back of the tube) you should be ok.

Bryce.

dtwaide

The yoke seems to be held solidly in place and it doesn't look like it has moved. I'd rather not remove it until I'm certain that's the problem so I tried testing the coils. I unplugged the connector for the deflection coils from the mainboard and resistance tested the coils with a multimeter. I got 15 ohms across the yellow/green pins (vertical, I think) and 7 ohms across the red/blue ones (horizontal). Testing between both coils and between each coil and the CRT ground strap gave an open circuit.

Would anyone be able to confirm what resistance functioning coils have? I'm guessing the figures should be the same and part of the horizontal coil is shorted out.

tjohnson

Looks just like my CM14 before it completely failed.  Repaired it by getting a 14" TV and swapping the tube and coils.  I recall it took out some component on the board too.  It was the coils, only obvious when taking them off and seeing the scorch marks.

dtwaide

Okay, I guess I'll bite the bullet and take the yoke off tomorrow to inspect the damage. I'd really like to try everything possible to repair this although I know there's little chance of success. It's good to know there's a plan b too. Could I take the tube and yoke from any 14in tv or is there a certain specification I would be looking for?

tjohnson

I got mine from a late Philips 14" CRT TV, not sure if any would do, I suspect it would actually be ok.  Someone said measure the resistance of the coils and try to match as closely as possible.  There was some sort of fuse component on the main board I had to replace too, tbh I can't remember what it was our even how I found it was blown, I think just by testing components on the board.

dtwaide

Okay, I got the yoke removed from the tube with a little finesse (only used the wooden mallet!) and encountered no black scorch marks from shorting.

Just these green ones instead.  :'( I guess from a few drops of water running through the vent on the top of the case and down the tube to the inside of the yoke. Quite likely knowing my garage.

I'll keep a look out for a TV I might be able to swap the yoke/tube out of but in the mean time I will try and clean up the corrosion and re-varnish what's left.

Does anyone have resistance or Q values for the coils I could use to check against, please?

tjohnson

I found my original post which might be some help.

https://www.cpcwiki.eu/forum/hardware-related/cm14-issues/

I bet my coils would have looked like that had I not run it until they burnt out.

dragon

#10
The specs of the yoke are determined by the nomenclature of the model. Thats something that I learn from a book google electronics book when mine burn time ago. And in her day I made a data base of amstrad yokes and I search equivalent yoke witouth lucky.  Always some resistance don't meet.

Nworc

Quote from: dragonAnd in her day I made a data base of amstrad yokes and I search equivalent yoke witouth lucky.  Always some resistance don't meet.

As the yoke is not operated with direct current, isn't the impedance more relevant than the resistance? I would measure the impedance at the typical frequency for the respective coil (50Hz / 15,6kHz) and use that measurement for comparison. By theory, if the size of the tube and the angle of deflection are equal, you should be able to swap the yoke, as the electrons don't care about the model, they care about the physics, which would be presumably equal then. As a rule of thumb, if the new yoke sits well on the tube, I would give it a try. But there is still the risk of electical incompatibility - such a modified monitor should not be operated unattended due to the risk of fire, you know. I will take no resposibility if you do that swap trick. Btw: I would not try a yoke from manufacturers who are known to implement their own standards, like Sony for instance.

dtwaide

Quote from: tjohnson on 10:04, 27 May 22I found my original post which might be some help.
https://www.cpcwiki.eu/forum/hardware-related/cm14-issues/
I bet my coils would have looked like that had I not run it until they burnt out.
Thank you for digging up that old thread tjohnson. The explanation bryce gave fits my situation exactly. The windings have corroded in 3 places right where they touch the rubber wedges. If only I'd read that thread beforehand it would have saved me a world of pain.  :(

dragon

#13
NWORK I tell it from memory, was time ago when I look into it:.


"Hola, seguro que más de uno tenéis algún monitor muerto sin imagen como yo y sin solución por el yugo muerto.

Estoy intentando recopilar información sobre los tubos de los ctm-644 y los yugos que llevan montados para ver si cae la breva y aparece alguno compatible de los que se venden hoy.

Si tenéis un ctm 644/schneider muerto o no da igual, solo es quitarle la tapa y copiar el modelo de tubo del monitor+el modelo del yugo que se ven los 2 a simple vista sin tocar un cable.

Os agradecería mucho si podéis contribuir.

De momento la Info que tengo es esta solo:

MIO:

Orion Tube 3702B22-TC20
Yoke deflection coil: semco dse-1422BL  BEC


Cpcwiki:
Orion Tube: R70BV1BK1A-TC10
Yoke deflection coil:KYS? -60096    ggdk.

http://www.cpcwiki.eu/index.php/CTM640/CTM644

Schneider ctm 644 nighfall:
Orion Tube:3702B22-TC20
Yoke deflection coil:TCD-8701?  BEC

https://www.nightfallcrew.com/08/09/2013/amstrad-schneider-colour-monitor-ctm-644/?lang=it

Urusergi

Amstrad CTM 644 español
Orion Tube: 3702B22-TC20
Yoke deflection coil: TCD-8701 C B C

Orion 370lYB22

http://cpc.sylvestre.org/technique/technique_reglage_ecran.html

Gracias a todos por adelantado.

Lo que creo  que va a salir que mismos tubos montan diferentes yugos.

Valores de yugos conocidos:

http://www.jangkang.com/products/trans_dy_inductor.php?lang=&view=deflection_yoke

https://es.scribd.com/doc/61985466/Caracteristicas-yugos-TV

http://web.archive.org/web/20050127201130/http://www.korion.net/php/list.php3?lim=0&mode=yoke&option=

Si alguien se anima puede medir el suyo, pero solo si sabéis lo que hacéis.

https://www.jomac.net.au/tech-stuff-2/tech-stuff/

http://antel.cn/product_show.asp?CID1=16&CID2=165&ID=3843


"Dse=Samsung inline, crossarm type tube 29 diameter, 90 grades suitable for 14-20" tv. (pincushion free). "

http://webcache.googleusercontent.com/search?q=cache:jHtL-D53170J:www.szmie.com/product.asp+&cd=5&hl=es&ct=clnk&gl=es&client=ms-android-huawei-rev1&sxsrf

" I'n her day I found one samsung near it. Theorizing can be the same with diferent connector in the pcb.  (six four wires) the samsung dse-1422fl"

Thats info I throw in diferent posts in her day.

If I remember o.k the key are the last two letters because one means the type of connector I think.

dtwaide

Safety advice duly noted Nworc, thanks.

With regards to what is being measured resistance and impedance can both be useful diagnostic criteria, at least in motor windings, I have no experience repairing CRTs before now. From the small amount of reading around CRTs that I've managed to do so far the values I see quoted for deflection coils are the resistance and the Q-factor (which would take the reactance portion of the impedance into account). That said, any values to compare against would be handy at this point.

Judging by the comments above different models of Amstrad monitor, even, have different spec deflection coils. dragon mentioned a database. I wonder if this is available?

dragon

#15
That's it the upper post was the "database"
:D

Well not I also found in her day some tvs mount it from orion and matsuita. Of course out of range  tvs in Europe.

And there these people tell what I say about the two Last letters.

https://www.elektroda.pl/rtvforum/topic18399.html

dtwaide

Quote from: dragon on 12:49, 27 May 22That's it the upper post was the "database"
:D
Beat me to it  :D thank you!

dragon

#17
Only left I localize the book in Google books where I can view the info of dse let me search it.

I'n her day I stop finish investigate because alt change me the monitor.

And in theory. it in the antec web page you can made a inquiry a compatible yoke  with the dse 1422bl. But I never try it....

Bryce

Quote from: Nworc on 11:25, 27 May 22
Quote from: dragonAnd in her day I made a data base of amstrad yokes and I search equivalent yoke witouth lucky.  Always some resistance don't meet.

As the yoke is not operated with direct current, isn't the impedance more relevant than the resistance? I would measure the impedance at the typical frequency for the respective coil (50Hz / 15,6kHz) and use that measurement for comparison. By theory, if the size of the tube and the angle of deflection are equal, you should be able to swap the yoke, as the electrons don't care about the model, they care about the physics, which would be presumably equal then. As a rule of thumb, if the new yoke sits well on the tube, I would give it a try. But there is still the risk of electical incompatibility - such a modified monitor should not be operated unattended due to the risk of fire, you know. I will take no resposibility if you do that swap trick. Btw: I would not try a yoke from manufacturers who are known to implement their own standards, like Sony for instance.

Technically you're correct, when designing the circuit the impedance is the value you need to know, but when you are just looking to compare two coils then the resistance will tell you enough.

Bryce.

dtwaide

#19
Hi, just wanted to let everyone know this particular story had a happy outcome. Apologies for the delay, it took a week for the HV insulating varnish to arrive.

The horizontal deflector, on my monitor, comprises two coils wired in parallel of about 6 ohms each, 3 ohms total. When I separated the two it was clear one was fine and the other had a break in it, this was easy to find once the copper oxide was cleaned off. I soldered the two sections back together, applied a few coats of varnish, and after leaving for 24 hours to dry reassembled everything. A bit of fiddling with the convergence rings later and it's almost as good as new :)

Thank you very much to everyone for helping. I got this machine when I was 7 years old and learned to program on it so I really didn't want to replace it. Thanks again :)

PS I noticed the purity issue in the bottom right corner, this has been fixed :)

Bryce


dtwaide

Thanks Bryce, much appreciated.

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