Adjusting pots on tube of monitor while switched on

I am in the UK.

I have been cleaning out the dust from the inside of my monitor and I just moved a few pots to make sure their tracks are clean and returned the pots to how they were.

In fact the only pots I can find are three right next to the coil on the neck of the picture tube and they are marked:

Y-HC: Y and in small letters, HC Delta V: capital delta symbol then V YV: Y and in a small letter, V

What do these do?

To be prfectly honest I think I may have not quite returned them to their original postion but might be 20 or 30 degrees out.

Can I adjust them with the monitor on? Of course I won't be touching any HT parts but I wonder if they are inherently too close to the HT to adjust while the monitor is on?

Must I use an insulated trimming tool rather than a meta scredriver?

Reply to
Andy
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Years ago, my Dad, a learned professional and well respected in his field, told me this story when I was a boy.

One day he was sitting with all the other well respected learned professionals in the lounge in the building where they worked.

They had just gotten a new and very expensive piece of equipment installed, and the installer was training them in the procedures for using their new equipment.

The equipment was a COLOR television. The first one that many of the people in the room had ever seen.

After the speech, asked the respected learned professionals in the room if they wanted the absolute best color picture they could get on this tv. "Well, of course.", somebody replied.

The installer opened a little door in the front of the device and pointed to a little row of colored knobs he'd just finished adjusting. See these little knobs here?

In rapt attention, they all stared...yes..

DON'T F*** WITH THEM.

Reply to
thrugoodmarshall

EEK, if it ain't broke, don't fix it.

One of my first childhood 'accidents' was messing with the myriad of alignment pots which were too easily accessable on the back of our old Philips colour TV.

Moving pots which have not been touched for years can also cause problems and can crack an old, seized shaft or rip up the carbon track.....

//Clive.

Reply to
Clive

With respect, that was an extremely foolish thing to do. If you don't understand the inside of mains operated equipment, you shouldn't be inside it, let alone turning adjustments. If the pots had been dirty but producing no symptoms, turning them could have caused a fault that wasn't there before by dragging the dirt under the wiper.

They are convergence pots. They alter the relationship of the geometries between the beams. The red, green and blue images which make up the colour picture on your monitor must overlap as close to perfectly as possible, otherwise you will see red, green or blue fringes on different parts of the image. They should only ever be altered by someone who understands how to set them up- they can be very difficult to get back if you don't know what you're doing.

If the picture looks OK, you've got away with it.

At least you didn't alter the static convergence rings on the CRT neck- that really would have caused problems.

Dave

Reply to
Dave D

Totally unnecessary operation but as you've fiddled now, it can't hurt to mess around with some insulated tools and some safety knowledge. Track down the Sci.Electronics.Rapair FAQ site and read.. (or skip the monitor if the picture is bad, and get a better one from freecycle.org)

Long time ago a friend bought (then) a quite nice secondhand VHS video recorder (JVC 3V43) with HiFi heads, still frame advance and all the toys. Plugged it into his television set and noticed the picture tearing across from the top. So he 'fixed' it. Took the VCR covers off, got some sand paper, scrubbed the head drum - found 4 black specks wouldn't shift. So took a screwdriver and neatly chipped them away...

Yes, this idiot computer programmer's TV was to blame (time constant not set for VCR) but the video head was now completely beyond service - and cost to replace as much as the whole recorder cost him :-(

-- Adrian C

Reply to
Adrian C

Adrian C wrote in snipped-for-privacy@individual.net:

LOL

I'm in two minds as to whether to dismiss this story as apocryphal or to accept that there really *are* people who would do this!

I once managed to resurrect my old VCR which started producing *very* snowy pictures after a tape got jammed inside it and must have shed some oxide onto the heads. However that was with the aid of a cotton-wool ear-bud soaked in isopropyl alcohol and very gently stroked over the heads - and only after a head-cleaning cassette had proved to have no effect. The amount of crud that came off was quite remarkable. After allowing time for the alcohol to evaporate (I didn't want the tape sticking to the drum!) I gingerly fired up the VCR and the picture improved over the course of a few minutes' playing. After a bit longer, the hi-fi sound came back as well.

But to attack a video head with sandpaper and a screwdriver... gulp!

I'm firmly of the school of thought that says "look as much as you like, but if it's not broken, don't try to 'fix' it". And when EHT is involved, I'd be very reluctant to open the case of a monitor, even after it's been switched off for a while - capacitors can store lethal voltages. If I was going to attempt to tweak the pots, I'd use a long nylon screwdriver and I'd just tickle each pot in case the setting was very critical. To be "almost right, but maybe 20 or 30 degrees out" sounds rather vague.

Reply to
Martin Underwood

Reminds me of when I walked into a friends room to find him armed with a carving knife in one hand and the bezel of his computer monitor in the other.

Turned out he had a computer game with some text which scrolled at the very bottom on the screen and was partially obscured by the CRT surround which he intended to carve away. I quickly pointed out the V-height pot at the back of his monitor but it took hours to rebuild it as to get the bezel off he had unplugged every cable and HT lead he could see.

I really am surprised we don't see more people electrocuted whilst messing around in the back of TVs and monitors...

//Clive.

Reply to
Clive

Couldn't this also have been a result of the VCR's back tension being out of adjustment or was this occuring only with the regular cable signal?

Bruce

Reply to
Bruce H

I hate to admit this but it rang too much of a bell not to reply .... as a teenager I had a cheap 14" TV where the image was not in the correct place, I turned some very similar sounding pots with a non insulated screwdriver which came with a meccano set ! Everything went fine with the picture changing until I moved around to have a better look at the screen and the point of the screwdriver slipped out of the pot and onto a live pin on the board .... next thing I knew I was sat on the floor on the other side of the room with a sore arm and a blackened screwdriver

I did get an electrical engineering degree a few years later ......

Reply to
Chris Vowles

Cotton buds should not be used on video heads IMO. The fibres can snag and break off the heads, or become entangled. There's only two things I ever put near the drum to clean it- chamois swabs and copier paper, both with isopropyl alcohol. Both clean the heads very well indeed, and even dry copier paper does a safe job if one doesn't have the proper tools handy.

Dave

Reply to
Dave D

Chris Vowles wrote in e383b9$skh$1$ snipped-for-privacy@news.demon.co.uk:

I assume that this was mains voltage rather than EHT, otherwise you probably wouldn't be around to tell the tale.

I once made the elementary mistake of doing some work on an old mains-driven tape-recorder. I'd switched it off at the power-switch on the tape-recorder, but I'd forgotten to unplug it. All was fine until - you've guessed - my finger happened to touch the terminals of the switch. Likewise, I was left with a very sore, tingling arm - and a feeling that I had just lost one of my nine lives.

Same here.

Reply to
Martin Underwood

Nope. He got another video recorder and complained of the same problem. Didn't take us long to figure out what was up - around the mid eighties it was fairly common knowledge (here in the UK anyway) that one of the UHF preset channels (normally the last one of 4 or 8) on a TV set was reserved for VCR use - he'd used another.

--
Adrian C
Reply to
Adrian C

Martin Underwood wrote: All was fine until - you've guessed - my

Took apart (to save the bits) the SMPS from an old Sony betamax video recorder forgetting to discharge some heavy capacitors first. My stray fingers met some large DC voltage and by instinct / reflex I flung the module across the room cutting myself badly on a fold in the metal work.

That electronics degree taught me nothing...

--
Adrian C
Reply to
Adrian C

The last channel selector altered the time constant in the line sync locking so that it could cope with unstable vcr signals.

--
From KT24 - in drought-ridden Surrey

Using a RISC OS5 computer
Reply to
charles

charles wrote in snipped-for-privacy@charleshope.demon.co.uk:

What was the disadvantage of of using the VCR time constant setting for off-air reception? Presumably there must have been a disadvantage otherwise all the channel positions would have been permanently set to the VCR time constant.

I know my TV (bought in 2000) has different settings on its SCART sockets than on its phono and S-Video aux inputs on the front panel: the phono and S-video sockets have more problem locking onto the video output from my laptop which evidently isn't quite 625/50 specification, whereas the SCARTs are much more tolerant.

Reply to
Martin Underwood

Actually, this is a misconception that can prove quite lethal. Generally, the mains voltage - 115 VAC or 230 VAC - can be more dangerous than the

25 kV or whataver.

Touching the HV may throw you across the room due to the charge on the CRT capacitance, but probably won't kill you except on a really bad day. Aside from the capacitor, there is too little current to do any harm.

There are AMPs available from the mains voltage.

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Reply to
Sam Goldwasser

Naah. It's the volts that jolts but the mills that kills.

In a Vacuum tube colour set CA 1969 about the safest thing to get a shock off was the 25 Kv. Or so they taught us at BBC training school Evesham. IIRC effectively limited at about 2.5 ma. OTOH.

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"The normal anode voltage of 160 V is designed to be exceeded by pulses of up to 8,000 Volts and 1.4 Amps. 8-((

DG

Reply to
Derek ^

Excellent. First the practical, then the theory. If you survive the first and are still interested, you're qualified to have a go at the second. :-)

Rod.

Reply to
Roderick Stewart

Greater susceptibility to noise on received sugnals, which would cause raggedness of vertical lines due to variations in line timing. There's a reason for using what was called "flywheel sync". A shorter time constant effectively made it work with a smaller "flywheel" - OK for clean locally generated video signals, but not good for off-air material.

Rod.

Reply to
Roderick Stewart

In the late sixties I worked with a BBC Electronic Services engineer who regularly used to check whether EHT was present on a faulty monitor by removing the plug from the tube and sticking his thumb on it.

Chris Y

Reply to
Chris Youlden

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