Degauss circuit is always on..problem?

[shess]

My problem is that my monitor was vertically mounted and then I rotated it and now the colors are messed up.  The degauss coil didn't seem to be doing anything so I tried the drill+magnet trick and that didn't work either.  I even tried a different degauss coil and still no improvement.  I thought maybe the degauss circuit wasn't working so I hooked up my multimeter and turned it on.  It went to 120V AC and never came back down.  From other reading, it looks like it's supposed to only fire for a second and then turn off? 

What else can I do to degauss my monitor?  Take the extra degauss coil and hook it straight into the wall AC?  Does it also make any difference that I removed the metal sides from the chassis frame to rotate it?  The frame was designed to only be vertically mounted and I'm trying to mount it in a bartop so space is at a premium and the old frame didn't fit when rotated.  I removed the sides from the frame and attached the tube to some corner brackets to keep it in place.

Scott

[grantspain]

buy a manual degauss wand on ebay,they are cheap and handy
don't whatever you do plug a degauss coil to a wall socket-you will see it melt within a few seconds and then blow your fusebox

a degauss circuit is designed to switch voltage to the coil for a matter of seconds when the monitor powers from cold(i.e has been switched off for a few hours)

are you sure your degauss is automatic and does not use a manual switch?

[Kevin Mullins]

I thought maybe the degauss circuit wasn't working so I hooked up my multimeter and turned it on.  It went to 120V AC and never came back down. 

I'm surprised it hasn't burnt something up or blown fuses yet.

There is a PTC component that activates and deactivates the degauss coil, it may be shorted. (should be a short cycle as you have read)
Without knowing what make/model monitor it is I can't give ya component numbers or locations, etc.

Grantspain has a good point too..... if it has a manual degauss switch then there may be other stuff involved. (not much, just a few things)

[shess]

Should have given the model number, it's a WG 13k4851.  There is no remote control board so I don't *think* it has a manual degauss button.  I don't think the coil got abnormally hot when it stayed plugged in, so not sure how it's staying at a constant 120V.

Thanks for all the advice so far.

Scott

[lilshawn]

the "auto" degauss circuit consists of 2 resistors (they are housed in a small black box next the where the degausss coil plugs onto the board) and the coil of wire.

the first resistor is across the line voltage (effectively shorting it out with the resistor)
on power up, the resistor begins to get hot.

this in turn heats up a second resistor... in line with the hot wire of the line voltage... as you may or may not know, as temperature increases, so does resistance.

full line current begins to flow through the coil of wire creating a magnetic field. the resistor (in line) being heated by the other resistor (across the line) begins to increase in resistance, lowering the current flowing through the coil down until a very negligible amount flows and the field is very small.

tremble in fear of my spectacular mspaint skills!!

[shess]

I found this schematic, http://technical-department.servegame.com/monitor_schematic_diagrams/wg_k4800.jpg , and in the lower left corner is the degaussing coil.  I'm not really sure what to make of that diagram.

[BobA]

Looks like the degaussing coil has a thermister inline with it to control it.  

Thermister

[MonMotha]

Have you checked that the degauss coil is in working order?  If it's failed open, some designs will hit AC input voltage across the coil and never go down since the PTC will have no load and never heat up.  Since there's no load, there's nothing getting hot or blowing fuses, either.

Hit the coil with a meter on resistance check (disconnect from the board for this test).  Should be finite and low but non-zero resistance (ohms).

[shess]

I figured since I tried two different coils at least one should be in proper working order, but I'll check them anyways.  Thanks for the suggestion.

[Kevin Mullins]

Looks like the degaussing coil has a thermister inline with it to control it.  

Thermister

Marked TH501 on the schematics.
It will look like a ceramic disc of sorts about the size of a dime. (two legs)

I think lilshawn was describing the black box style. (three legs)

[lilshawn]

I think lilshawn was describing the black box style. (three legs)

correct.

they used to use just a thermistor to do it, not exactly the intended purpose for it, but it did the job.  now there is a dedicated item for it.

 

[shess]

So that resistor possibly needs to be replaced then?  Is it tested the same way as a regular resistor?

[lilshawn]

Is it tested the same way as a regular resistor?

yes.

if i't failed likely it would be cooked right off the board. thyristors aren't the most gracious fail mode devices ever. (next to backwards installed caps)

if it reads continuity, it's probably good. For what it's worth, test it... say you get 1000 ohms. tun on the monitor let it run for 30 seconds, then shut it down and test it again immediately. it will likely read in the megaohms. this would be good. the resistance will go down slowly as it cools back to room temp.

[shess]

Looks like my coils give me about 5 or so ohms and the thermistor gives me 7 ohms at rest.  I probably didn't leave it on long enough but when I tested it after being powered up and then off, it did seem to spike.  I also do have continuity on the resistor when powered off so it's probably fine? 

[qrz]

what Zenith used on several chassis was keyed off the 12v run line.
 the + end of a 1000uF 'lytic was connected to 12v run , neg to a 12v relay coil and then to gnd . (series)

upon 12v activating , inrush current would close relay contacts to engage deguass coil.
 which , would then be active for a few seconds until cap charged enough to allow relay to drop out .

simple circuit !

of course, cap degradation will affect performance 

[shess]

So I did some more testing.  I ensured I had continuity on my degauss coil and then plugged it into the board.  The chassis had been off for a while so I stuck the multimeter leads in from the top so I could measure the voltage across the leads of the coil.  When I turned it on, I got no voltage.  However with the coil unplugged, I get 120V across the pins.  Does this help?  In the meantime I'll probably use the coil from another monitor and do something like this, http://mirrors.arcadecontrols.com/OscarControls/degauss/index.shtml .  I looked on ebay at the degaussing wands and they all require 220V which doesn't work for me.

[MonMotha]

Immediately upon power-up, the voltage across the coil should spike to 120V (line voltage).  It will then decline rapidly to near zero as the PTC thermistor shuts off the coil.

If the voltage never spikes and always reads near zero, the PTC thermistor is defective (failed open) and should be replaced..

[shess]

What are the specs on the thermistor?  I'm having a hard time finding it at Jameco.  I did a search for ptc-070 (from the schematic) and I got this, http://parts.digikey.com/1/parts/954658-thermistor-ptc-100ohm-70deg-rad-b59100m1070a070.html .  Is that right?

[MonMotha]

PTC-070 or PTH631-138E is probably a Wells Gardner internal part number.  Their parts department can probably sell you one (at an outrageous markup), or someone else may know what it is.  Is there any writing on the part itself on the board?  Can you get a close-up picture?

[Kevin Mullins]

Ken Layton has referenced these two before.

MCM Electronics - http://www.mcmelectronics.com/
32-7120 Square box style
33-650   Round disc style

Can be found on page 3 of this catalog -
http://www.mcmelectronics.com/content/ProductData/Catalog51/section07.pdf

I didn't see them on their website, so might want to give them a call if part numbers have changed or something.

[shess]

One last thing on this.  I had the machine powered up for just a short time and after powering it off, I noticed that the thermistor was redhot to the touch.  Is that normal? 

Presuming the thermistor is bad and I use an external degaussing coil to degauss the monitor, is there a trick to doing it?  I tried doing a side to side sweep and a circular motion while pulling back but it doesn't fix the discoloration.  I pulled the tube away from the power supply and speakers so I don't think there's any magnets close by.  What's the trick to getting this back to normal?

[Rickn]

A lot of good comments from our friends on the forum!

But I will try to explain a little more.

yes on the input to the thermistor (some times call a positor) there will be 120 volts as it is directly connected. the reason it is called a Thermistor is that when it gets hot it esssentially shuts down it's output so there is no power going to the degaussing coil on the tube.
On power up the thermistor will see about 8 cycles of th 120 VAC (@60 hz) and then be hot enough to shut down the output. BTW yes it is normal for it to be 'Hot to Touch". If you shut the monitor down for abount 15~30 minutes it will fire again. If it stayed on you would almost definitely blow the fuse and the Degaussing coil would get "pistol hot and melt down" So unlikely it is staying on. If you see a 'waving in the picture it may not be entirely shutting down.
My guess is the problem is somewhat self inflicted... picture tubes are very much affected by the earth's magnetic field and when you rotated it to vertical mount it does not "like it" when manufacturing a vertical mount monitor we use magnetic chambers to make the horizontal monitor think it is sitting vertical and the align the picture tube. If I am right about your issue you will need to break the seal or loosen the lock rings on the purity/convergence ring assembly... before you do this mark them with a white pencil crayon or china marher so you can reset them if needed. Look closely at these rings, normall the first 2 rings will have a "P" embossed on them this means purity. Sometimes on smaller tubes they are the last 2. Now carefully rotate them as a pair, normally when you switch to vertical mount they will need to be rotated clockwise (as viewe from the back). Idealy you should have hand degaussed the picture tube and have the monitor facing east/west. This should take care of your problems.
Good Luck Rick

[shess]

Thanks for that info!  I'll try that with the purity rings and see if I can get it looking normal.  That thermistor got hot to touch after less then a minute of being on.  Also there is a slight waving in the screen.  It's not quite the hula but just a slight trembling.  What do I do to get it to fully shut down?

[shess]

I just re-read your post and missed something.  It was previously vertical but I rotated it to horizontal.  I'm assuming I rotate the purity rings counter-clockwise then?

[shess]

Give Rick a gold star.  After rotating the purity rings about 90 degrees or so counter clockwise, the picture has no more discoloration.  Since I degaussed it manually I still don't know if the automatic degauss is working, but right now I don't care....thanks to everyone for the help!

[Rickn]

Thanks Shess, that is why I spend time on these forums.... sometimes too much! it is just great when I can help someone!

Rick

[Kevin Mullins]

Wow, thanks Rick.... I never would have thought to rotate the purity rings like that.

I wonder if this is part of the problem with some of the Vision Pro monitors I have seen that just do not like to degauss very well?
Seen a few where the degauss circuit works fine, but still looks like crap. Manual degauss not much help either.