Question on CTM644 - Very dark picture

[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 ?

[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).

[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]

How to discharge them?

[skywalky]

How to discharge them?

Discharging a TV or CRT monitor - YouTube

[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! 

[Bryce]

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

Bryce.

[remax]

Yes, the one hand rule is always a good choice.

Bryce.

Out of curiosity, can you explain why ?

[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]

........... 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 

[Shining]

  


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.

[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]

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.

[Bryce]

You mean this one?


Demonstration of HS 20kA Arc in 100A Disconnect 15 Cycles - YouTube


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.

[pelrun]

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


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

[Shining]

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

Thanks @all  .

[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.

[roy bates]

How to check for high voltage and discharge - YouTube


i use this method...

[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]

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?