flickering brightness knob on CTM644

[blackdalek]

Whenever I give the brightness knob at side a turn, or even just touch it lightly, the whole screen flickers bright white flashes.
This monitor has been doing this for past 15 years at least and it has never really bothered me as it's not a critical problem given that the monitor brightness never really needs adjusting once set to a desirable level.

I'm just wondering if this minor issue is an easy fix I could do myself and is it likely to cause a catastrophic failure if I ignore it for another 15 years?

[pelrun]

My monitor is the same. It's nothing serious, just that the brightness pot is dodgy - exactly the same as when a volume control on something gets crackly.


It's not a difficult fix. Some contact cleaner in the pot might improve things, but it'll only be temporary. Replacing the pot is a simple job if you're comfortable soldering things; if not, it's probably better just to leave it be.

[CanonMan]

Exact replacement pots aren't easy to get hold of, so when mine started to get like that I repaired the pot.


It's just a matter of carefully opening it up, cleaning the track, wiper and contact ring with alcohol and a cotton bud and then lubricating it with vaseline before reassembly.


It sounds like a bit of a faff but it really works! I do a bit of vintage radio restoration as a hobby and you have to do this when replacement pots aren't available.

[Gryzor]

Vaseline??? Won't it get even worse as it starts accumulating dirt?

[CanonMan]

Vaseline??? Won't it get even worse as it starts accumulating dirt?

In my experience,  no, it doesn't.

[||C|-|E||]

I have always used a drop of olive oil for the old non replaceable  pots, including those in our monitors, it works wonders    I never tried the vaseline, though, but it is good to know.

[Gryzor]

Actually, same with olive oil - in my experience it turns into gunk very easily!

[||C|-|E||]

Heh, but in most cases the pots do not work fine not because they are very dirty, but completely scratched inside  That is why a little bit of oil works so well by smoothing them. For audio pots it is something really great, and in my experience they work very well after many years. What you should not add is something that dries inside, like WD-40.

[Gryzor]

WD-40 - precisely, it dries out. Exactly like olive oil, that's what I'm saying But anyhow. I've heard so many different opinions over the years about this... Though concerning olive oil, I remember my grandfather teaching me almost four decades ago to never use it as a lubricant for that exact reason, so maybe that's why it stuck with me

[||C|-|E||]

Hehe, that is funny    I learnt the trick from my uncle, who always said to me that olive oil was the right choice. That is why I stuck with it    I never re-opened a repaired pot to see if it is dry inside or not though. However, the trick definitely works... There is one amp I fixed almost 20 years ago that never developed the problem again. Vaseline dedinitely does not dry, but it is more difficult to apply... at least if you don't want to open the pot.

[Gryzor]

Heh, that's funny; I just looked it up. Olive oil belongs to the category of non-drying oils so indeed it doesn't dry out. Vaseline is proven to not dry out either, according to the internets. However, I do recall using olive oil for door hinges and the results were definitely not good after a while. Who knows, maybe it's not the drying out that's the problem, but it behaves better in enclosed spaces?

[Lazy Dude]

Fantastic cooking tips. Olive oil, thats a new one for me.
Both my CTMs have the flickery brightness.
Nice to know a bit of cleaning can help rather than try to source a replacement pot.

[chinnyhill10]

Worth reading:


https://www.w8ji.com/dielectric_grease_vs_conductive_grease.htm

[CanonMan]

Worth reading:


https://www.w8ji.com/dielectric_grease_vs_conductive_grease.htm

Ta!


I'll read it later, when I've got the time.

[llopis]

I know this is an old topic, but it's exactly what I have happen in one CTM 644 monitor I own.


I was planning to clean up the adjustable pot like described here, but I've never opened up a CRT monitor and I've read all the warnings about it. Do I still need to discharge the tube and the capacitors before I service the pot, or is that separate enough from the high voltages that it's safe to access?

[Bryce]

The pot is on the other side and relatively far from the HT side. It's on its own little PCB with a flylead. You don't have to discharge the tube as long as you stay well away from the fat red cable going to the back of the tube. As for capacitors, some of them will still be holding a nasty >400VDC charge that can really hurt, but as long as you don't touch the bottom of the main PCB or the PCB on the neck of the tube you should be ok. Just make sure that the monitor isn't plugged in, because you'll be poking around very close to the power switch.

Bryce.

[Gryzor]

Tried to do it myself yesterday, actually...


https://www.nzherald.co.nz/world/news/article.cfm?c_id=2&objectid=12111821