Zenith 24H20 help please - Videokarma.org TV - Video

PaulOF · #1 09-23-2010, 10:14 AM

Kind people, I'm out of my depth.
I finally had my Zenith porthole (chassis 24H20) working just dandy on the bench with a little 5" test CRT. Nice picture, good hold, not too much buzz.

So I reinstalled the big tube, slid it into the cabinet, popped on the ion trap that banderson so kindly provided me with and fired it up.

As soon as the tubes warmed, I saw sparks dancing in the neck of the CRT and heard a loud whine at what I would guess is around horizonal retrace frequency (yeah, I can still hear 15 Kcps if it's loud enough). Turned it quickly off but still got the smell of hot resistor. As I say, I'm out of my depth. My experience prior to this project is all radios and phonos.

So, any idea where I should look? Could this be a symptom of a dead 16GP4?
Inquiring minds want to know.
Thanks,
Paul

Big Dave · #2 09-23-2010, 09:35 PM

Sparks in the neck are a sign that the CRT has gone to air. As far as cooking resistor, is any part of the bell touching the chassis? Try it again with the 5AXP4. No part of the metal bell can touch the chassis. I think it may be possible that the an insulator may be bad. If it still works with the test tube, I would presume the 16GP4 bad.

Big Dave · #3 09-23-2010, 09:38 PM

Sparks in the neck are not a good sign. I think your 16GP4 may have gone to air. Are you sure no part of the metal bell is touching the chassis? Try it again with the test tube. If it works, I would presume the 16G bad until you can confirm its condition. It's not uncommon for insulators to fail. Recheck those as well.

PaulOF · #4 09-26-2010, 12:21 PM

Thanks Big Dave. That is pretty much what I was afraid of. Some confirmation is that the filament resistance was reasonable when I took the tube out. Now its open, so the tube is dead. Drat... guess its time to start a frustrating search for a replacement. The set was my mother in law's so at least I'm not out any serious investment beyond the price of the new caps.

Meantime I can get started on getting the CobraMatic rehabilitated.

I think I sent you a pdf of a service manual for a 1957 Zenith a couple years ago. Did you ever finish that renovation?
Paul

Big Dave · #5 09-26-2010, 11:07 PM

I did get the bugeye done. Sadly, not long after you sent me that scan, I had a crash and had to reload my hard drive, so the file was lost.

If you can find a 16LP4 (glass tube), I think that is almost a direct replacement for the metal tube.