Doorknob Capacitor advice needed

[ohohyodafarted]

I have finally finished rebuilding the chassis for the Midwest Radio and Television set, and was able to power it up on my variac last night.

I had to install a complete compliment of tubes as the set had none when I got it and after running down some issues due to marginal tubes I got a raster and video. Set looks like it will have a nice picture once I am able to narrow down this High Voltage issue.

At first I thought the flyback was taking a dump. I could hear arcing but could not see any corona or sparks. AS I increased the line voltage to 118, the arcing got more pronounced. I took the HV cage off and used a 2 foot peice of vinyl tubing with one end stuck in my ear and the other end as a sound sniffer to narrow down the location of the arcing sound. Much to my surprise it is comming from the bottom end of the doorknob caps. It appears they are breaking down internally because I cant see any arcing from them in total darkness.

One doorknob is from ground to the CRT HV lead, the other is from the filiment of the 1b3 to the plate of the damper. There is a 1meg resistor from the junction of the HV lead/doorknob, to the 1b3 filiment side of the second doorknob. When I approach 120vac on the variac, the 1meg resistor will start to smoke.

I removed the doorknob that goes from the HV lead to ground and that eliminates the arcing sound, but I get lots of corona generating ozone. Is it likely that the grounded doorknob is breaking down, even if I can't see the sparks that I hear?

Does anyone here have experience with bad doorknobs? If you could describe the typical symptoms that would be appreciated. If they are arcing internally can you still hear the sparks?

I don't have any doorknobs to sub or I would try that.

Thanks,
Bob

[radio nut]

I have a few 500pf doorknob caps if you need a couple, I can't test them do to the voltage needed, but they can be yours for free if you want. Doug

[7"estatdef]

Read this:
http://www.audiokarma.org/forums/sho...eferrerid=9580

I was getting a little blip on the screen and a tick noise about every 30sec or so. It turned out to be the HV filter. There is a link to some new replacement caps.
Terry

[Tube TV]

Could it be possibe that the room humidity could be effecting the hv ?
I have a set that has a hiss when it's cold and also when the humidity is high , once it warms up the hiss is gone .
If you cant see the corona , I would say the cap is leaky and is on it's way out .

[kx250rider]

I've never seen a doorknob arcing inside, but they do get leaky/shorted. I'd clean the whole HV area thoroughly with alcohol, etc, then after it's COMPLETELY dry, give it a light spray coat of HV corona dope. That should fix the corona leaks, and prevent humidity from getting at it.

Charles

[ohohyodafarted]

I swiped a couple of doorknobs from 2 other sets, and jury rigged them to test if the problem is the doorknobs. Sure enough it was. Problem is that all the doorknobs I have in my sets have unthreaded terminal posts.

I need doorknobs that have a threaded post on one end and a smothe post on the other end.

Does anyone have a pair like this with a threaded post on one end? I found some 500pf 20kv doorknobs on epay but the seller is in China, and it will take forever to get them, and then I will have to fabricate my own posts because the ones from china have threaded holes in each end and no posts sticking out.

Thanks for all your help. I am making progress slow but sure on this chassis. Next hurdle is to modify a NOS 2 watt 15 ohm wirewound control, to add a 4th center tap terminal at 7.5 ohms for the vertical centering control. the original one is toast.

Just in case I fail in my attempt to make the above miodification, does anyone know of someone that could rewind the resistance element in an existing wirewound control?

Thanks for all the help,
Bob

[Tube TV]

I have a box half full of old controls , I might have one hanging around ?
I know that the guy that used to own them had scraped a bunch to early
50's sets , I got a whole pile of channel selector knobs .

[bgadow]

I have a metal-tube Admiral that worked but had speckling in the picture. In the process of troubleshooting it I made it worse, with some arcing at the doorknob. I had the same problem of mounting the replacement, since none of them in my junkbox had the same design. I was able to make it work with a little ingenuity. Problem solved.

Was at a hamfest this summer and a guy had a cigar box full of doorknobs. I figured it would be a good time to stock up but he wanted about $8/each and wouldn't back down. He still has them, I suppose.

[Tube TV]

I wonder if there is a modern replacement that could be mounted under the chassis or inside the old cap . they are getting harded to find .
Has anyone bought any new caps with a 20 KV working voltage ?

[radio nut]

I know that I have one 500pf 20,000 v. cap you need . I should have more. I will check and let you know. Doug.