Mighty Monarch of The Air

[vts1134]

Got a good supplier for a 500pf 20,000v door knob capacitor?

[VintagePC]

Digi-key has a few... for example:
http://search.digikey.com/scripts/Dk...me=445-3890-ND
http://search.digikey.com/scripts/Dk...me=445-2235-ND


... but they won't be cheap :S

[old_coot88]

Well sunnava gun, so it was the doorknob all along, and now the filament winding is popping through. Some kind soul oughta send him a hunk of HV wire to replace that with, along with a doorknob.

[vts1134]

So as I sit here and ponder I can't help but think that the door knob cap ISN'T shorted to ground. If that cap was shorted to ground it would certainly account for our problem, but could just a leaky cap cause our problem? If not could it be the ground side of the cap some how isn't grounded? Or some other problem other than the cap itself being bad. I just know that some times in my work when I think I've found the answer from trouble shooting I just end up "fixing" things that aren't broken and still not finding the real problem.

[old_coot88]

OK, did the CRT light up with a raster after you removed the doorknob? If so, it would pretty well prove the doorknob was at fault. Probably arcing internally since it measured open circuit on your ohms test.

Earlier you mentioned a "new arc" that's developed. I (probably erroneously) interpreted it to mean there's a hole in the insulation of the filament winding that's allowing a DC arc. That would be evinced by a loud, sharp snapping sound. If instead the "arc" is a bluish, lightly hissing corona discharge from the 'tire' to the filament loop, you may be able to insulate it with some silicone. Is it possible to move the filament loop further down on the frame, away from the 'tire'?

[vts1134]

I have a new theory. The old 1X2 tube was fine. The new 1X2 is bad. The "new" arc I described from the "tire" to the filament loop was always there and I never noticed it. Every time there was a discharge from the tire to the filament loop it was echoed inside the 1X2, making me think the tube itself was bad. I've removed the flyback and there is no insulation missing from the filament loop at all. The "tire" has an area where it looks slightly melty, but only slightly. I'm going to get it nice and insulated with the afore mentioned sensor safe silicone, attempt to get the filament loop as far away from it as possible and reinstall with the old 1X2.

One thing that struck me as odd was there was already three or four pieces of electrical tape squares stuck between the "tire" and the filament loop. Looks like the problem could have been there before when the set was in use.
This theory makes absolutely the most sense to me so far.

[vts1134]

Originally Posted by vts1134

I have a new theory. The old 1X2 tube was fine. The new 1X2 is bad. The "new" arc I described from the "tire" to the filament loop was always there and I never noticed it. Every time there was a discharge from the tire to the filament loop it was echoed inside the 1X2, making me think the tube itself was bad. I've removed the flyback and there is no insulation missing from the filament loop at all. The "tire" has an area where it looks slightly melty, but only slightly. I'm going to get it nice and insulated with the afore mentioned sensor safe silicone, attempt to get the filament loop as far away from it as possible and reinstall with the old 1X2.

One thing that struck me as odd was there was already three or four pieces of electrical tape squares stuck between the "tire" and the filament loop. Looks like the problem could have been there before when the set was in use.
This theory makes absolutely the most sense to me so far.

FAIL!
No more discharge from the fly to the filament winding but problem is not solved. Every time I get over 95v I get sparks on the inside of the old 1X2, and sparks plus a blue glow on the new 1X2.
Time for a new $35 capacitor.

[old_coot88]

Originally Posted by vts1134

FAIL!
No more discharge from the fly to the filament winding but problem is not solved. Every time I get over 95v I get sparks on the inside of the old 1X2, and sparks plus a blue glow on the new 1X2.

This is with the doorknob removed, right? What's happening to R89, the 470K resistor? Is it getting hot, or remaining cold? If R89 remains cold, there may be a leakage path from the 1X2 socket itself to ground that's dragging down the HV. If R89 is getting hot, it may be that the 1X2 (in this case either one) is gassy and passing unrectified pulse that's hammering the resistor.

Time for a new $35 capacitor.

A new cap isn't going to help until you get the current problem sorted out. All this cap does is smooth the rectified pulse coming out of the 1X2.

One other thing- when you re-installed the flyback, where you soldered the filament loop back to the 1X2 socket, are there any whiskers or sharp points to the joints? In soldering any HV connections it's very important to build a nice 'ball' of solder on each joint to eliminate any points.

[vts1134]

Originally Posted by old_coot88

OK, did the CRT light up with a raster after you removed the doorknob? If so, it would pretty well prove the doorknob was at fault. Probably arcing internally since it measured open circuit on your ohms test.

The CRT lit up with a raster before I removed the doorknob, and after as well.

[Phil Nelson]

A capacitor can leak internally when voltage is applied, yet test as "not shorted" using an ohmmeter. The voltage applied by a general-purpose multimeter is negligible compared to the actual working voltage. That's why they made dedicated capacitor testers. Mine can check a cap at any voltage up to 500v. I haven't heard of any cap checkers that can test in the KV range, however. For a doorknob like this, the test is substituting a known-good cap and observing what happens. Which was no problem in Ye Olden Dayes when the repairman would just fish one out of a bin on the shelf and pop it in.

If it looks like you need a new doorknob, Surplus Sales of Nebraska has one at 500pf/20KV for a low, low price of $35:

http://www.surplussales.com/Capacito...cap_trans.html

Click on the "Available in these styles" link to see whether that type would replace what you have.

It's not a cheap part, so I'd proceed with whatever testing is needed to isolate that as your trouble source.

TVs can have more than problem at a time, of course. If you also want to replace the 1X2 filament loop, I believe that needs to have HV-rated insulation.

The photo shows some sensor-safe RTV that I used on a Philco flyback.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[vts1134]

Originally Posted by Phil Nelson

If you also want to replace the 1X2 filament loop, I believe that needs to have HV-rated insulation.

Is this a locally available material? I tried the local electrical supply house that we have an account with but they don't have anything like this. I'm not sure what to call this and therefore google it and get online results. Any thoughts?