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?

[Adam]

After working on it all night, and fixing 3 bad resistors goofing up the horizontal sync and 1 bad solder joint messing with the AGC, it works.

Now I just have to replace some of these goofy temporary repairs I have in there, then I can put it back together. When you're working on a set late at night, you can't go to the store for parts, so you have to make do with what is around. I got one 1/2w resistor where it calls for a 2w, and for a .015 - 1000v cap I didn't have, I had to put two of these huge 35kv film caps in parallel. And that coil still is bypassed, but it doesn't seem to make much difference.

Another discrepency, this time between my set and both schematics, the horiz hold control is an adjustable coil with a tap in the middle. Between the middle of it and the side that hooks to ground, the schematics show a 2200mmfd capacitor. My set doesn't have it. Figuring that 2200mmf (or pfd) = .0022 mfd, I tried adding .002 mfd cap that I had, but with that cap in there the horiz sync is way off, without it it is fine.

And I'm thinking about recapping the remote reciever, it works good, but not perfectly, the channel changer does work perfectly, but the volume up/down on/off doesn't work every time you press it. And it is full of more of those notoriously bad 'black beauty' capacitors.

[jpdylon]

Glad you are making good progress on the space command. With all the receivers I have to work on and my other 4 sets, I don' think I was ever going to get to it.

I would love to see it in perfect working action when your all done.

[Phil Nelson]

This one sounds like a death-by-a-thousand-cuts sort of project. I applaud your determination.

Speaking of having to fix previous "repairs," my best/worst experience is the radio that I spent a few hours puzzling over. I recapped it and all the tubes tested like new, but the voltages were way off, and the darned thing just didn't work.

A previous owner had removed the tubes, mixed them up, and written the wrong tube numbers in pencil on the chassis. It took an embarrassingly long time to realize that those tube numbers couldn't possibly be right.

"Never assume," as the saying goes :-)

[Adam]

I replaced those resistors with the correct wattage, all I could find for that .015 - 1000v cap was a .012 - 1600v but it seems to work. I think I'm just going to have to leave that coil bypassed, it seems to work fine that way, anyway.

I don't know what it is with all these bad repairs I keep finding lately, my Hoffman 610 was the same way (previous repair guy had put in lots of resistors that were the wrong value, and then didn't even always hook them to the right place, he usually would go one pin up from where it should be on a tube socket.) I've never seen one where all the tubes were mixed up like that though.

*****
I spent another night working on this, recapped the remote receiver, put it all back together and everything works. My SC 600 remote works on this as well ("ch lower" changes the channel, while "ch higher" corresponds to the the on/off - volume control.) But I forgot to get a new light for the channel indicator, so I'm not going to be able to put the back on and be officially done with this one until next week.

[vintagecollect]

Glad to hear your are preserving this Adam, Early remote sets are rare, especially working. Should be a fun set once your can watch after a little work. As you know, many remotes discarded once remote functions failed.

[Adam]

I used to have a 1971 Zenith b/w space-command I found on the curb without the remote, (it worked great when I brought it home). I didn't know at the time how much of this stuff is available, and had I known I could probably have found a remote, I surely would have kept it.

I put this all back together with the lights and everything today. This set is great to watch, and it's the first early remote set I've gotten to work. So I've been flipping through all the channels just to see the dial turn, even though if I'm just watching the VCR on ch3. The first TV I ever had was a 17" Zenith b/w table top model quite a bit like this one (except it had uhf, and didn't have the remote).

[Adam]

This set recently lost its horizontal sync. I've been messing with it since 3 this morning. I recapped the whole set and checked all the tubes when I first repaired it last summer.

Today, I swapped the horiz osc tube for a new one, no difference. I had already replaced R77 and 79, now I went and rechecked all the resisistors I thought could have anything to do with this (by pulling up one end first) and found R75 to be at 120k (instead of 100k) and replaced it, I replaced the 15M R76 even though it tested really close to what it should be. I also replaced R49 which tested a bit high. I've replaced a number of ceramic capacitiors that I didn't replace before: C54, 57, 55, 51 A and B, 50 A and B (with 68pf instead of 51 because that's all I had), 41, 29, and 25. All this work has improved my situation a bit from no sync to just a very unstable sync, but I really think whatever my original major problem is is still there.

Now all the voltage readings on the horiz osc tube 6GH8 are all good except for pin 1 and pin 2 which both read a bit on the low side. Also all the pins on the sync seperator tube 6BU8 read a bit lower than they should (but the tube itself tests good).

The power supply reads a strong 270v, so that isn't a problem.

Now when you look at the 3 waveforms i posted down below, W 12, 13, and 14 on the schematic (posted in that order from left to right), only W13 is weird. Now when I turn the AGC all the way up to where the picture is all white W13 becomes as it should be and all the voltages and everything look fine (except now the picture is all white).

I also looked at those 2 waveforms taken at pin 6 and 7 of the 6BU8 (w3 and 4 on the schematic, posted in that order on the 2nd line of pics) and they look like absolute gibberish, not even close to what they should be, while the one at pin 3 of the 6BU8 (w6 on the schematic, posted 3rd on the 2nd line) looks just fine.

Now as I mentioned a long time ago in this thread, I bypassed this bad coil (L7) with a wire. (I circled the coil in that 2nd section of schematic I posted.) Now the horiz sync has never been that great on this set, could that have anything to do with it? But it is still a lot worse now than it was when I first fixed it last summer so I don't know if that could be it.

Oh, and I also tried replacing that 2 part diode (M1) with another one I knew to be good from a junk chassis, and it made no difference, and after I removed the old one it tested good with the ohm meter.

[Adam]

Update: I had fixed it and I didn't even know it. I replaced that diode back with the original one and now the horiz will sync through 3/4 of the span of the hold control, much more stable than it ever was. My guess is that those diodes aren't interchangeable between the set I pulled it from and this one. Now I'm dissapointed that I don't know which of the many pieces I changed actually was bad.

But if you look that one scope image taken between the 2 diodes (w13 - the one that only looked as it should if the AGC was cranked all the way up so there was no picture - top middle in the last post) is better, but there's still some of that junk in the background (see pic below). And those ones from pin 6 and 7 of the 6BU8 still don't look anything like they should. Oh well, I guess it doesn't really matter as the set now works.
I still have to find 51pf caps to replace those 68s that I used to replace those 2 51s. I wonder if that'll do anything. Also, anyone know what a modern equivalent for that 2 part diode is, in case I do need to change it.
This set was really the exception to the rule of these 60s Zeniths being nearly problem free, that one I just worked on with the broken crt mask had been sitting out in a barn for decades and used so much prior to that that the crt is a bit weak worked as is, this one that was packed in it's original box and hardly saw any use during its life, has had loads of problems. Either way, as much as I like space command, I'm taking this set out of my daily watcher rotation, so I don't have to keep fixing it.

[Adam]

I drove all around town looking for 51pf capacitors, the closest I got is 56pf, put them in and they work fine. No more sync troubles, even though I'm not sure what did it. I know I could order the caps, but I want to get this set off the bench and get a few more on, and I don't think 5pf makes all that much difference. I have a few spare days right now and I want to fix as many sets as I can, and get at least one color set going, I've had nothing but b/w since the beginning of January.

[Adam]

ugh... I put it all back together with the cover on and everything and no horiz sync again...

Turned it on 5 min later and now it works again, this is just weird.

Now, I turn it on, off, mess with the horizontal, it continues to work with no problems. Maybe it will just stay like this.

[Adam]

It works this morning.

Tried it again it took about 5sec to sync horizontally, then was fine. On the up side I changed some weak tubes in the remote receiver, and now the remote works at a much greater distance. As a side note, the original SC 300 remote works on this set at about twice the range of the spare SC 600 remote I have.

[jpdylon]

Perhaps there is some intermittent failure in the sync section? I would think that would throw off the vertical too, but you just can never tell with oddball problems like this.

[dtuomi]

Originally Posted by Adam

ugh... I put it all back together with the cover on and everything and no horiz sync again...

Turned it on 5 min later and now it works again, this is just weird.

Now, I turn it on, off, mess with the horizontal, it continues to work with no problems. Maybe it will just stay like this.

I feel for you, my conrac is treating me similarly.

David

[jpdylon]

could also be an intermittant connection on the hoirz hold pot or oscillator coil. If you give it a thwack does it sync up again?