1951 Zenith 16" Porthole

[Radiotronman]

It's got obvious sync issues that I'll have to solve, but what is strange, is if I turn the volume all of the way down, the left side of the screen will get brighter, further over and the sync will somewhat stabilize. Any thoughts?

[Electronic M]

Originally Posted by Radiotronman

It's got obvious sync issues that I'll have to solve, but what is strange, is if I turn the volume all of the way down, the left side of the screen will get brighter, further over and the sync will somewhat stabilize. Any thoughts?

Now this tells us something. Some sets used the audio output tube as part of the B+ voltage divider (instead of a dropping resistor they used a tube to put that energy to work). On sets with that form of stacked B+ if the audio output stage is miss-biased or otherwise not working correctly it will reduce B+ to other stages that it is the B+ dropper for. I recommend voltage checks in the audio out stage with the volume low and high...You may also want to scope the lower B+ rail that comes from the audio output cathode to see if it is not filtering well.

[Radiotronman]

Ok great thanks! I'll start doing resistance checks on that chassis too where the audio output tube is.

[dieseljeep]

Originally Posted by Electronic M

Now this tells us something. Some sets used the audio output tube as part of the B+ voltage divider (instead of a dropping resistor they used a tube to put that energy to work). On sets with that form of stacked B+ if the audio output stage is miss-biased or otherwise not working correctly it will reduce B+ to other stages that it is the B+ dropper for. I recommend voltage checks in the audio out stage with the volume low and high...You may also want to scope the lower B+ rail that comes from the audio output cathode to see if it is not filtering well.

I honestly, don't remember Zenith using stacked B+. I'll have to consult my Rider's CD on the older models.
RCA used it on their lower-end models in the mid-50's, but not as much as some of the other makers.

[Electronic M]

Originally Posted by dieseljeep

I honestly, don't remember Zenith using stacked B+. I'll have to consult my Rider's CD on the older models.
RCA used it on their lower-end models in the mid-50's, but not as much as some of the other makers.

I'm honestly guessing it uses that circuit based on the symptoms...If it is not using stacked B+ I can't imagine the audio pulling enough power to affect other circuits short of the output redplating (which I think he'd notice).

[Radiotronman]

Checked all resistors in the small chassis and could not find any problems. Is this a horizontal issue or a problem with my yoke?

[Kevin Kuehn]

I think if you rotate your yoke upside down it should verify if it's one side of your CRT(set up) related. If the problem moves to the other side of the CRT, it's likely something modulating your video or CRT brightness. It's a weird problem.

[Radiotronman]

Ok, with a little help from my friend Sean, I found out why I was getting the symptoms I had. I neglected to reattach a black wire with an eyelet to the main chassis. It fastens with a quarter inch screw on the chassis, right next to the main power socket; an octal connector. I thought this was a mere ground, and had no idea it was connected to the audio output providing the stacked B+ that was mentioned earlier. I hooked this back up and got a perfect raster! I seem to remember making this same mistake on the last two-chassis porthole I worked on. Probably need to write this down...anyway, another porthole restored. Now I can put full ficus in my Dumont 15" tv.

[Radiotronman]

Thought I'd take a picture with the set all back together! This is my favorite of the portholes and I'm excited to have finally finished it!

[Paul Knaack]

Looks Great, excellent work.

[Electronic M]

Nice!