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Old 01-07-2018, 11:42 PM   #1
ratdude747
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Default Manually Degaussing a CRT

My HP A7217A (Rebadged FD trinitron) needs degaussed badly. The lower right corner's blues are non-existent and auto-degaussing isn't doing shit. It was in Topcat's attic for years, stored next to goodness what, then hauled in the back of my truck for 5 hours. Ever since that it's been screwed up and I'm now getting around to fixing it.

Question is what's the best kind of degaussing tool? Even on fleabay they're a bit pricey, so before I plunk down money, I'd like to know if there are any gotcha's to look for. New Chinese coils better than old used coils? What about wand-type degaussers?

I'm a noob at CRT work, but I do know there are a few former TV techs around here...
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Old 01-08-2018, 12:10 AM   #2
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Default Re: Manually Degaussing a CRT

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Originally Posted by ratdude747 View Post
My HP A7217A (Rebadged FD trinitron) needs degaussed badly. The lower right corner's blues are non-existent and auto-degaussing isn't doing shit. It was in Topcat's attic for years, stored next to goodness what, then hauled in the back of my truck for 5 hours. Ever since that it's been screwed up and I'm now getting around to fixing it.
Aperture wires "stuck" together?

Seen this in a Mistu diamondtron computer monitor. Ironically, gentle tapping on the cabinet near the faceplate of the CRT in the affected area helped. Begin tapping, then engage the degauss coil. You'll probably have to repeat this several times.

If that doesn't help, get a small box or stand fan, with an open C-frame motor, and pass this over the face of the CRT. Try various placement to find where the field from the motor is strong enough. Enclosed motors don't seem to work very well.



A Weller soldering gun also works.

Look inside for any magnets on the PCB that've fallen off the CRT; these (and the ring magnets) are for setup.
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Old 01-08-2018, 12:34 AM   #3
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Default Re: Manually Degaussing a CRT

Forgot to mention, the issue goes away for 1 second after auto-degaussing, but then I hear a click and it goes back to normal. Still a screen magnetism issue?
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Old 01-08-2018, 12:43 AM   #4
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Default Re: Manually Degaussing a CRT

In a TV repair shop, we'd use degaussing coils stripped out of old TVs. They can run for 10s of seconds on 110VAC before getting too hot.
Or use a cassette tape head demagnetizer, the 120VAC wand types.
Or use an AC solenoid. A Variac can throttle it down.

For really bad stuff, where the CRT frame or mounting hardware is stubbornly magnetized, I have actually used a magnet first.
Yes, a magnet and you try guess where to move it around to null the original magnetic field. Like pulling dents from a car, guess how it was hit and do the opposite. But waving a magnet around. Not super close unless the corners are stuck. Not near Trinitron neck convergence magnets.
Then use the usual AC field to degauss.
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Old 01-08-2018, 12:46 AM   #5
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Default Re: Manually Degaussing a CRT

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Originally Posted by ratdude747 View Post
Forgot to mention, the issue goes away for 1 second after auto-degaussing, but then I hear a click and it goes back to normal. Still a screen magnetism issue?
Back to normal means wonky?
Unless the NTC is keeping the coil on?
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Old 01-08-2018, 12:56 AM   #6
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Default Re: Manually Degaussing a CRT

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Back to normal means wonky?
Yes.

See attached... it's worse than this, but this is the best picture of the issue I have.
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Old 01-08-2018, 05:40 AM   #7
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Default Re: Manually Degaussing a CRT

Glue a small chip of permanent magnet on to the area where the impurity occur. You may have to move the magnet around to locate the G-spot, but that works for me.....
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Old 01-08-2018, 11:35 AM   #8
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Default Re: Manually Degaussing a CRT

so is the internal degausing working??

if yes, then a screw has been slightly magnetised - it will slowly clear.
if no, lets fix the degausing.
it's a posistor and sometimes a relay.
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Old 01-08-2018, 06:08 PM   #9
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Default Re: Manually Degaussing a CRT

If the degaussing is stuck on, the screen would be wiggling at mains frequency.

Common on the old Zenith flat-screen monitors, the coil would stay weakly energized (relay stayed on but NTC hot).

It was really visible if it hetrodyned with your vertical sweep rate i.e. 60Hz mains and 59Hz scan gave a 1Hz wiggle.
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Old 01-08-2018, 07:36 PM   #10
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Default Re: Manually Degaussing a CRT

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Originally Posted by ratdude747 View Post
Yes.

See attached... it's worse than this, but this is the best picture of the issue I have.
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Old 01-08-2018, 08:02 PM   #11
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Default Re: Manually Degaussing a CRT

That "click" sound should be the relay that shuts off the degausser, but apparently it's not "done" when it shut off...
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Old 01-09-2018, 04:51 PM   #12
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Default Re: Manually Degaussing a CRT

Quote:
Originally Posted by stj View Post
so is the internal degausing working??

if yes, then a screw has been slightly magnetised - it will slowly clear.
if no, lets fix the degausing.
it's a posistor and sometimes a relay.
Yes degaussing seemed to be working just fine. However, the display has been distorted for months.

I did try using the supermagnet trick. Made it worse at first but then it got better?

I'm out for a few weeks, so until then I can't really test anything
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Old 02-04-2018, 10:42 PM   #13
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Default Re: Manually Degaussing a CRT

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Originally Posted by ratdude747 View Post
then hauled in the back of my truck for 5 hours.
Why would you do that?
It's like bagging your groceries with your eggs on the bottom below everything - i.e. asking for them to get smashed. The monitor should have been on a car seat or at least on several thick blankets if in the bed.

Quote:
Originally Posted by kaboom View Post
A Weller soldering gun also works.
You mean one of those with low-voltage high-current AC passing through the tip (which is what happens to be the heating element)?

I never thought about using one like that. Genius!
Mine's been sitting in the closet for over 10 years with no use. It's such a poor device for soldering.

Quote:
Originally Posted by kaboom View Post
Look inside for any magnets on the PCB that've fallen off the CRT; these (and the ring magnets) are for setup.
+100

That would be the first thing I suggest doing. Some of these Sony tubes will have lots of magnets and strips inside to "correct" the tube's image, while others have almost none.

A small piece of weak flexible fridge magnet can also be used sometimes to correct the issue... but it's usually too strong or doesn't correct everything the way you want it to.

If you have a really strong VARIAC (capable of 10+ amps), you can hook the CRT monitor's internal degauss coils straight to that and slowly ramp the voltage up and down. Depending on the resistance/inductance of the degauss coil, though, you might have to put a low-resistance load in series with the coil to avoid tripping the breakers in your house or burning out your VARIAC. Something like a 13-15 Amp hair drier or toaster should work.

In any case, DO NOT run the degauss coil straight to your mains without any means to slowly tapper off the AC current/voltage - any sharp cut-off in the degauss coil's current may result in a picture worse than you have now. I had the degauss relay go out on one of my 21" CRT monitors right in the middle of degauss during power-up. The screen colors looked terrible afterwards - basically like your corner, but all over the screen. It's funny, because you could still read stuff and make out what's what in the picture, but the colors were just so off. Dare I even say "trippy"
I managed to get my picture back to normal only after 2-3 proper degauss cycles through the degauss circuit of a Dell CRT monitor (had both my 21" and the Dell CRT opened, and the Dell's degauss circuit hooked to the 21" CRT's degauss coils). Also, I noticed that doing a degauss on a monitor that is turned off doesn't work too well. You will want to have your monitor powered up and displaying a picture before attempting a degauss. On that note, when you tried degaussing your monitor, did you do it by cycling power On and Off or through the menu while the monitor is turned On. If you haven't tried the latter, try it and see if that helps.

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Old 10-27-2018, 09:23 PM   #14
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Default Re: Manually Degaussing a CRT

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Originally Posted by ratdude747 View Post
My HP A7217A (Rebadged FD trinitron) needs degaussed badly. The lower right corner's blues are non-existent
The colour in the problem area looks to be magneta in colour, suggesting the blue and red pixels are being affected.

I would think with a degaussing issue that you'd see colour images around print characters in that area. Can you get gray scale printed characters in the area at a low resolution?

The manual for your HP suggests there are dynamic colour and convergence adjustments (page 14, 15) that affect different regions of the screen. Seems to me that may be the issue rather than an overall convergence problem related to degaussing.

I don't know if it's possible on these displays to set purity with red, blue, or green screens only. If I remember correctly, on older colour TVs, you set the purity for mid-screen, adjusted the screen for a white raster, then used the dynamic convergence panel to adjust the extremities of the beam movement for each colour. I recall using a static pattern on the screen to do that but it was a long time ago.

I know this unit uses the Trintron, single beam scan and it seems there may be electronic deviations in the dynamics control system at the bottom LHS. There may even be components out of spec.
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