Bad Tube In My '66 Admiral ? - Videokarma.org TV - Video

ihmeyers · #1 10-29-2008, 09:10 AM

I fear a tube is going bad in the recently set-up (with the help of the board) Admiral.

Shortly after I turn it on, maybe 2-3 minutes it the video flickers, not jumps but kind of flickers. I'm not doing a great job of describing it but it would be analagous to the noise a gassy audio output tube makes, almost like a sputtering. It will do that for a few seconds and then settle down, but might do it again another 2-3 times in a half hour period. What is (are) the likely suspects? I'm hoping its simply a tube going bad but without knowing which one it is there isn't much I can do other than replace a bunch of them. My wife made me discard my old tube tester a while back (no room).

Any help greatly appreciated!

bgadow · #2 10-29-2008, 12:06 PM

First I would see if I could replicate it by tapping on various points on the chassis. Can you tap on, say, the video output tube and get this effect? Applying pressure to circuit boards, so that they slightly flex, can help locate intermittents caused by bad solder joints. My impression is that the problem is minor, but might be hard to track down.

freakaftr8 · #3 10-29-2008, 12:59 PM

Yes I agree with Bryan, check the video output tube. I have a Magnaxox that had that problem. With the set on, set a miror in front. Tap the video output tube with something like a wooden dowel. Watch the screen. Could be a bad solddor joint on the socket or just a bad connection in the tube. Try to pull the tube and reseat it. Good luck.

ihmeyers · #4 10-29-2008, 01:54 PM

Video output tube makes sense. I will check. Do you know what the tube number would be? I have attached a picture...

Thanks!

freakaftr8 · #5 10-29-2008, 05:10 PM

Not sure as sets vary. Possibly a 12BY7 or something along those lines. It might be the tube behind that cardboard piece sticking up on the back of the chassis left of the RGB bias knobs. It's most likely one of the skinny tubes.

ihmeyers · #6 10-29-2008, 06:00 PM

Good call guys. When I wiggle the 6BN11 (listed as a 1st video and sound i.f) I can recreate the problem. Does that sound like it should be the right one?

I'll look online for one now...

*zenithfan1* · #7 10-29-2008, 08:45 PM

Just wiggling it may not indicate a bad tube, sounds like a bad socket connection. Try spraying a very small amount of de-oxit in the socket and replace the tube, wiggle it again and if it stopped, try tapping on the tube with your fingernail or a popsicle stick. If the problem continues, the tube might be shorted. If it goes away altogether, it was just a bad connection. I had a problem like this with the '63 Zenith I recently restored, drove me nuts for a few days. It was bad socket connections at several I.F tubes and the sound detector IIRC. Good luck with it, she's a nice set. I have a similar chassis in my Admiral.

marty59 · #8 10-30-2008, 10:36 AM

Follow Zenithfan1's approach as he lists, and if the socket proves to be the source there may be one other thing to do. If the socket is in good condition you could try to retension the contacts. Caution!! Skill and patience are important here! Using a small precision scribe tip, you can pry the contacts against each other to reduce the circumference of the pin hole. If the socket is brittle, this will only bust it up as you are prying the contact away from the socket body. Sometimes the metal itself becomes brittle too and if that's the case the socket was not too good to begin with.
Octal sockets are easier to deal with also and in some cases the pins can be removed if necessary, and wafer style sockets with exposed contacts can respond to tightening with the gentle work of the needle nose pliers.
Sometimes you can save a socket over replacing it but like I mentioned you have to be careful!

*zenithfan1* · #9 10-30-2008, 11:27 AM

Good advise Marty, one other thing, when working on sets with circuit boards, always be careful when wiggling or removing tubes. If one is really tight or seems stuck, spray a LITTLE de-oxit at the base of the tube first. This will penetrate and make it easier to remove. The more hours on a set the more brittle those boards become from the heat. The board can crack and break without notice. You can check for "tube scorch" by looking at the surrounding area of the socket. Most boards are green on the bottom and brown on the top in vintage sets, if you see these areas turning dark brown or black, use extreme caution. Most of the time it happens around shielded tubes or horizontal/vertical output tubes as these get really hot. If you plan to use the set regularly, it might be advisable to point a fan at the back to keep it running cooler. This is one more reason I am a big fan of the "Handcrafted chassis" point to point method used by Zenith. No boards at all, only solid steel!

zenith2134 · #10 10-30-2008, 10:03 PM

Point to Point is really the only way to design a tube chassis IMO! The boards always always get brittle or develop tons of bad connections due to the tube heat. On my portacolor and GE SF chassis sets, the boards show slight discoloration but are functioning okay. I have seen a couple of burnt up sets though---one was a circa 1973 GE color set. I found it in the trash a few years back and it had no HV. I didn't pursue the repair since I saw a LOT of scorching all around the boards when i opened it up. So bad that thew caps were mostly all wiggly and dried out. Mess like that is rarely worth the effort.