1960 Zenith SC 300 - ongoing repairs

[Adam]

I moved this over from the swap board, seeing I was fixing the set now, and not trying to buy/sell it. Here's the thread in the swap board which talks about the earlier work I did on it: http://www.audiokarma.org/forums/sho...ghlight=zenith

Last weeks situation:
I've discovered the source of many of the problems with this set, all the electrolytics in the set were already replaced. Two capacitors were used to replace each one for the power supply filters, but they were in series rather than parallel. Which I believe (unlike resistors) makes less capacitance than either capacitor would have alone, rather than adding together that of the two capacitors. It's really rather a messy job too. There's also a number of those 'black beauty' capacitors in there, one of them even has a crack down one side of the plastic case (probably not a good sign).

After recapping this last night (and I checked them after I pulled them, lots of them were bad), checking the resistances at the tube sockets, and changing a few bad resistors in the horiz sync circuit, I powered it up. The raster fills the screen, I get sound good, and the space command works. First, I thought there was no video at all, but I can just see just enought to tell it won't sync horizontally when I move the fine tuning. What is odd is how I can have good clear sound and at the same time really a weak video signal. All the IF tubes are good, and so is the vid out.

update:
First I checked to see if all the IF tubes and the video amp were actually lighting up, everything was good. But I noticed the 1st IF tube was a 4BZ6, not a 6BZ6, I put in a 6BZ6, it made no difference. I doublechecked all the IF tubes and the video amp on the tube tester, they were all good. I tried a few replacement 6AW8 vid. amp tubes anyway, it made no difference. My next thought was to check the video detector diode, it seemed ok. Then I just started checking for the video signal with my scope, I started where it enters the 3rd IF tube (pin 1), it looked ok, then right before the vid detector, it looked ok, after the video detector, still ok. But right before the vid output tube (pin 7) it wasn't there at all! At this point I decided it was just too weird and went to go eat. Then I checked this coil inbetween the video detector and the video output tube (L6 on the Zenith schematic, L7 on the Sams, it calls it a 6mh choke coil) with the ohm meter, it reads 5k. So I bypass it with a bit of wire, and I have video! I suppose I should find a replacement though.

But now, I have to deal with the horiz not being able to sync. There are quite a few off-value resistors down there, and I'm hoping that's all it is.

This is the first Zenith I've ever had that needed this much work, I wonder if it's the result of running it for too long without enough filtering in the power supply. This set was full of bad resistors, most every resistor connected directly to the 275v supply needed to be changed.

Now there is also a difference inbetween the Sams and the Zenith schematic following that bad coil: the factory schematic has a 330 ohm resistor inbetween the coil and the video output tube, the sams does not. And neither does my set, and it looks like it never did. Any reasons why I should put one in?