Adjusting pots on tube of monitor while switched on

[Andy]

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?

 

[[email protected]]

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.

 

[Clive]

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.

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.

 

[Dave D]

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.

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.

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?

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

 

[Adrian C]

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.

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 :-(

 

[Martin Underwood]

Adrian C wrote in
[email protected]:

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 :-(

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.

 

[Clive]

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 :-(

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.

 

[Bruce H]

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

 

[Chris Vowles]

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?

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 ......

 

[Dave D]

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.

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

 

[Martin Underwood]

Chris Vowles wrote in
[email protected]:

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 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.

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

Same here.

 

[Adrian C]

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?

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]

Martin Underwood wrote:
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.

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...

 

[charles]

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.

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

 

[Martin Underwood]

charles wrote in
[email protected]:

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

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.

 

[Sam Goldwasser]

Chris Vowles wrote in
[email protected]:


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

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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[Derek ^]

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

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.

http://www.r-type.org/exhib/aaa0021.htm

"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-((

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.

DG

 

[Roderick Stewart]

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.

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.

 

[Roderick Stewart]

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 ......

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.

 

[Chris Youlden]

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.

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