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Question on CTM644 - Very dark picture

Started by Shining, 10:59, 11 March 15

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Shining

I never owned a CTM644 in the past and went to scart very early. But to have a complete cpc-look-alike colour solution I catched a CTM644 from ebay. But I'm wondering that, even with brightness full up, some of the darker colours (eg. red) are nearly invisible.


Is the monitor too old ?
Is this normal ?
Can I do something against this ?
TGS is back

Download my productions at:
cpc.scifinet.org

pelrun

It definitely sounds repairable. There's a couple of straightforward things to try - adjusting the internal sub-bright pot, or doing a (full!) electrolytic capacitor replacement - that are sort of the first thing anyone does when working on a monitor this old, before doing anything more involved.


BUT!


DO NOT take the back off a CRT unless you know what you are doing. They hold a significant charge even when unplugged, and can hurt you badly if you don't know how to discharge them safely. It's definitely doable, but if you can possibly find an ex-TV repairman to do it for you then that's a safer option than having a stab at it yourself.

ivarf

There is a knob hidden on the back of the monitor below the tube. Try this one before you try anything else. Its on the outside of the monitor so its safe to try

Shining

So will first try to find the additional bright-pot then.


Opening the case and replacing capacitors should be a job, I'm capable with. But I've never discharged a tube. (Little bit scared).
TGS is back

Download my productions at:
cpc.scifinet.org

Bryce

The knobs on the back off a CTM644 are to adjust V-Hold and H-Hold. They will not have any effect on the brightness of the picture and due to their age, only turn them if you really have to!
As suggested, the brightness could be due to old capacitors, but also dirt on the PCB. I would also implore the "don't open it if you don't know what you are doing" advice! There are seriously high voltages in there, even after it's been turned off for weeks.

Bryce.

P.s. The Tube doesn't hold a charge, so it doesn't need to be discharged. It's the HV capacitors that kill.

TFM

TFM of FutureSoft
Also visit the CPC and Plus users favorite OS: FutureOS - The Revolution on CPC6128 and 6128Plus


kevin.c

My name is Marek and this is my first post here so I say Hello to everybody!
CTM644 has sub britghness regulator VR402 and it is located on left edge of the board, close to service switch.
To avoid electric shock, good rule says to work with one hand, and the other on your back... 

TFM

Hi and welcome in the forum! Hope you have a good time here!  :)
TFM of FutureSoft
Also visit the CPC and Plus users favorite OS: FutureOS - The Revolution on CPC6128 and 6128Plus

Bryce

Yes, the one hand rule is always a good choice. I discharge the caps with a 220V light bulb usually.

Bryce.

remax

Quote from: Bryce on 20:45, 11 March 15
Yes, the one hand rule is always a good choice.

Bryce.

Out of curiosity, can you explain why ? :)
Brain Radioactivity

Bryce

When measuring voltages on a running device with high voltages, you usually connect the GND test lead to the chassis and probe with the other lead. Although you're careful not to touch something with the "probe hand", there's a very high chance that the "free" hand will accidentally touch something because it's not your focus of attention, so by putting it behind your back you remove it from the area of danger.
Electricians use similar rules such as the "hands in pockets" rule and their "one hand rule" for extremely high voltages. As the electrician is isolated (rubber shoes), touching one phase isn't fatal, but if the other hand is leaning on the wall / chassis / cabinet he's dead.

Bryce.

kevin.c

Quote from: Bryce on 21:48, 11 March 15
........... but if the other hand is leaning on the wall / chassis / cabinet he's dead.

Bryce.
A few more tutorial posts, and Shining will never touch his cpc  ;D

Shining

 :laugh: 


I've no fear about current or high-voltage. I worked at 20000 V high-tension power lines in the past.
But I've never discarged the caps from a monitor. So before getting in action with high voltages I want to always exactly know where is the risk and how to prevent problems.




Funny off-topic note: Tuesday my ebayed 6128+ arrived and scince I had no cable to connect it to TV or monitor I soldered an adaptor between ctm644 and plus yesterday. After pluggin it in I thaugt wtf it is not working coz the start-screen was black/white. I played a  little bit with pen-command and tadaa there was colour. This happens to people who see a working plus for the first time 25 years after its release  ;) . Side note: It seems that the plus-signal is stronger than the 6128-one coz it seems to be brighter.
TGS is back

Download my productions at:
cpc.scifinet.org

Bryce

Are you sure the monitor contacts on the old CPC aren't just very dirty? The Plus signals should be the same level as the classic CPC.

Discharging capacitors: It may sound low-tech, but I just have a standard 220V 100W lightbulb with two old multimeter test leads attached. You can touch these across the contacts of each of the HV capacitors on the main PCB to discharge them, but DO NOT USE THIS METHOD TO DISCHARGE THE OUTPUT OF THE FLYBACK TRANFORMER!!! There are specialist tools to do this, although, if you've already worked on 20KV lines, then you know the tool length and amount of insulation needed to work on voltages in this range (The CTM is at about 22KV I think).

The CTM schematic shows all the points where high voltage can be found (and how high it is).

Bryce.

Shining

#15
Thanks for your advices. Your bulb-method sounds cool to me. So I've finally a solution what to do with the box of outsorted bulbs  ;) .


Funny during apprenticeship where those horror-videos where you saw the electric arc on electrical accidents.




Two (off-topic) questions.


1) What is the best way to clean the old contacts ? I've contact spray but when I use this at the centronics-ports they look as oxidized as before.


2) Never used it before but are the cpc-keyboards (and the plus-case) clenable by using retrobrite or what is the best solution ? I don't want to loose the key caption like on my original 80s cpc where all the most common keys were later without caption.
TGS is back

Download my productions at:
cpc.scifinet.org

Bryce

You mean this one? :D




Is the case dirty or really yellowed? I'd give it a clean with normal hot water / washingup liquid first to see how it comes out. If it's still yellow, then you could consider retrobrite, but you will have to coat it with UV protection lacquer afterwards, otherwise it will go back yellow within weeks. NEVER use retrobrite on a classic CPC! But I assume you know this. Retrobrite won't stop the letters from wearing off the keys anyway.

Bryce.

Shining


Good one, when not the same I saw back then...

What looks a little bit ugly on the plus is, that it looks partly yellow and partly not. Same with the keys...


For the keys I think about replacing them with 6128-keys (also coz of the AZERTY) BUT the 6128 IS working and I've no good feeling about canibalizing a working CPC.
TGS is back

Download my productions at:
cpc.scifinet.org

pelrun

#18
Quote from: Bryce on 10:31, 12 March 15
If it's still yellow, then you could consider retrobrite, but you will have to coat it with UV protection lacquer afterwards, otherwise it will go back yellow within weeks. NEVER use retrobrite on a classic CPC!


Guess I'll just have to live with my greenish-yellow 6128 keys :P


Edit: dammit, I always get caught out with the gif for that emoji. Licking not intended!  ???

Shining

Still alive  ;D . The sub brightness regulator VR402 did the job very well.

Thanks @all  :) .
TGS is back

Download my productions at:
cpc.scifinet.org

Bryce

Good stuff. It's still a good idea to clean all contacts, switches and variable resistors. But good to hear that the CTM is still working.

Bryce.

CraigsBar

Hmmm. One of my cm14 screens is a bit dark, perhaps a look for the same solution if possible.
IRC:  #Retro4All on Freenode


Bryce

That's the specialist tool I mentioned earlier, but I've seen idiots do it with an uncovered screwdriver with a piece of wire wrapped around it!!

Bryce.

roy bates

Quote from: Bryce on 12:38, 13 March 15
That's the specialist tool I mentioned earlier, but I've seen idiots do it with an uncovered screwdriver with a piece of wire wrapped around it!!

Bryce.

whats wrong with that method?

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