63 Zenith Roundie - Videokarma.org TV - Video

Kalamazoo-DJ · #1 11-21-2009, 01:50 PM

I purchased another Zenith roundie 25mc33, it has a strong CRT but has tuner problems it will recieve UHF fine but no VHF input from DVD/vcr/Direct tv
has a snowy picture, the picture fluctuates but no images/ program sound.
Where do I start on this?

julianburke · #2 11-21-2009, 07:14 PM

Try replacing the 6HA5 first in the tuner then the 6GJ7. See if that works. I used to see failures of the 6HA5's that would cause this problem.

Kalamazoo-DJ · #3 11-21-2009, 10:48 PM

Ok thanks I have a twin to it so I will switch tubes

jpdylon · #4 11-22-2009, 12:04 AM

Great set. I have the same chassis in a slightly different cabinet and no UHF.

drh4683 · #5 11-22-2009, 09:41 AM

The 6GJ7 is the mixer oscillator in the tuner. Whenever I find an old zenith, most of the time both 6HA5 and 6GJ7 are toast. That TV is actually a 1965 model.

Kalamazoo-DJ · #6 11-22-2009, 09:58 AM

OK thanks for that year clarification I was only going by a repair tag I found under the cover probably from another nearby Zenith

Kalamazoo-DJ · #7 11-22-2009, 01:37 PM

Quote:

Originally Posted by drh4683

The 6GJ7 is the mixer oscillator in the tuner. Whenever I find an old zenith, most of the time both 6HA5 and 6GJ7 are toast. That TV is actually a 1965 model.

Both tubes check good, any other ideas for this problem??

drh4683 · #8 11-22-2009, 01:57 PM

There are instances in which the 6GJ7 may check fine, but its characteristics changed enough to the point where it won't oscillate on any frequency, or sometimes it will only oscillate on the high or low end of the VHF section. Try subbing the tube out. How is that tuner? Dirty contacts? Also, very very common for the fine tuning gear that turns the slug on the VHF tuner strip develops a crack split down the middle, where it just spins on the shaft. 9 times out of 10 they are cracked making fine tuning impossible. I take it apart, and drop a touch of super glue on the shaft and spin the gear around really quick to distribute the glue before it sets up. It works perfect and the trouble won't come back.

Kalamazoo-DJ · #9 11-22-2009, 02:06 PM

Quote:

Originally Posted by drh4683

There are instances in which the 6GJ7 may check fine, but its characteristics changed enough to the point where it won't oscillate on any frequency, or sometimes it will only oscillate on the high or low end of the VHF section. Try subbing the tube out. How is that tuner? Dirty contacts? Also, very very common for the fine tuning gear that turns the slug on the VHF tuner strip develops a crack split down the middle, where it just spins on the shaft. 9 times out of 10 they are cracked making fine tuning impossible. I take it apart, and drop a touch of super glue on the shaft and spin the gear around really quick to distribute the glue before it sets up. It works perfect and the trouble won't come back.

I subbed the tube and same problem, I will remove the tuner and see is there anything to worry about during removal?

drh4683 · #10 11-22-2009, 02:10 PM

Should be nothing to it. Zenith tuners are the easiest to remove and service because all the wires are plugged in, so the tuner can be removed independently of the chassis. Only four wires to unhook. Brown is the 6.3V for the filaments and dial light, Red is the B+ (I think its 250V on that chassis) and yellow is the AGC and the last being the IF lead in. You might have an AGC problem in that tuner. The green B+ dropping resistor may be suspect.

Robert Grant · #11 11-23-2009, 09:12 AM

If worse comes to worse, hunt down one of those cheap plastic "cable TV convetors" from the early 1980's, that upconvert VHF channels to UHF.

p.s.: I'm in MI, let me know if you want rid of it :-)

oldtvman · #12 11-23-2009, 12:36 PM

If you can't fix it I'll drive up from South Bend and buy it from you. I need a Zenith for my collection.

Kalamazoo-DJ · #13 11-23-2009, 05:27 PM

I will get it fixed, I am working on a 3rd Zenith and an RCA purchase also you might be intersted in that one

Kalamazoo-DJ · #14 11-23-2009, 07:06 PM

Something doesnt look uniform inside the tuner connections, it appears solder is broken and the white wire near the brown antenna lead, how should this really look? there is one soldered and one hanging loose.

compucat · #15 11-23-2009, 08:39 PM

Those are great sets. I have had mine for two years now and I love it. I took the tuner apart on mine to clean the contacts. It is not that complicated, just be careful where everything goes.

I noticed you are missing the arched wooden piece that is supposed to go between the two front legs.