Zenith porthole 12" console

[Adam]

Any ideas what I can replace this bad coil with? (circled in the schematic) All I have is the Zenith part number: S-15126 and 6ohms.

[old_coot88]

Originally Posted by Adam

Any ideas what I can replace this bad coil with? (circled in the schematic) All I have is the Zenith part number: S-15126 and 6ohms.

You could simply jumper it temporarily till you can find a replacement. It's a peaking coil whose sole function is to 'tailor' fine detail in the pic.
I've tried jumpering peaking coils just to see if there's any difference, and usually there's no discernable difference whether the coil is jumpered or not.

[Adam]

I spent some time cleaning the chassis. That power supply chassis was so rusty that even after cleaning it with the naval jelly, sandpaper, etc... I still decided it would look better if I painted it. (I'm going to dig up a junk can to replace that missing one)

(With that coil jumpered) I'm now getting a nice full blank raster - no sound. The sound comes off before the 1st video amp tube, so I'm thinking the problem has to be back in the tuner to be affecting both sound and picture.

[old_coot88]

So now does the brightness control work correctly?

[Adam]

Originally Posted by old_coot88

So now does the brightness control work correctly?

Yes. (It's all the way up in that picture)

[Phil Nelson]

Originally Posted by Adam

The sound comes off before the 1st video amp tube, so I'm thinking the problem has to be back in the tuner to be affecting both sound and picture.

Have you checked anything in the audio section downstream from that point? If you turn on your soldering iron and touch it to the wiper (center terminal) of the volume control, do you hear a buzz?

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[Adam]

I haven't gone through it systematically yet, to absolutely eliminate anything in the sound portion of the set, but I was getting static by tapping the AF amp tube.

[Phil Nelson]

I wouldn't mess with the tuner until you have eliminated the sound section and other downstream circuits to confirm that they're all hunky-dory.

Phil Nelson

[Adam]

Now that I finished with that newer Zenith, I'm back trying to fix this one...

I poked around with the scope and didn't see any video or audio signal anywhere. Then what I tried was to set just a regular signal generator (with the AF modulation on) to the IF frequency, then hook it into the grid of the 1st audio and 1st video IF tubes, just to see if a signal would get through. And it did, I got buzzing in the audio, and those lines on the screen.

But the problem still turned out to be not in the tuner, but in getting power to the tuner. That choke coil I circled, L19 is open. What can I replace this with? All I have is the Zenith part number S-15128 and 1.5ohms, it doesn't give me the inductance, or any equivalent part numbers.

[Adam]

I swapped in a coil (of unknown value) and got: weak sound, crappy video, and sync that won't stay put (H or V). I tried several different coils and got the same results, so even though I'm not quite sure what to put for that coil, I'm fairly sure the problem I'm looking at now lies elsewhere.

I'm going to poke around the AGC amp tube and check the voltages, but I have a suspicion, with all the other goofy stuff that was done to this set, that someone got in there long ago and twiddled every adjustment they could find and goofed up the IF alignment, video and audio.

[old_coot88]

Originally Posted by Adam

I swapped in a coil (of unknown value) and got: weak sound, crappy video, and sync that won't stay put (H or V). I tried several different coils and got the same results, so even though I'm not quite sure what to put for that coil, I'm fairly sure the problem I'm looking at now lies elsewhere.

That's correct. The inductance value is not critical, since the coil serves no other purpose than as a choke. In fact you could probably jumper it completely and never know the difference.

...I have a suspicion, with all the other goofy stuff that was done to this set, that someone got in there long ago and twiddled every adjustment they could find and goofed up the IF alignment, video and audio.

Actually it does not show symptoms typical of video IF misalignment, namely strong
smearing-out and ringing of fine detail in the pic. Fine detail looks pretty crisp. That's not to say the audio IFs haven't been messed with, though.

[Adam]

Well, this one stumped me for a long time. I even went and replaced loads of resistors that tested good. But there turned out to be 2 problems with the picture:

1. Looking in the 1st schematic I posted, that C7B I put in backwards, this caused the "crappy video", strangely it didn't explode, but I could see it was bulged out and I put in a new one.

2. And this was the really tricky one, what was wrong with the sync, looking in the 2nd schematic pic I posted, the cathode of that sync clipper tube needs to go to ground, not to the heater, otherwise the 6V AC on the heaters seems to get mixed in with the sync pulses. Connecting it to ground fixed the problem.

This didn't trip me up like the other problem, but I also pointed out in that schematic on the 6SH7, pin 5 is the cathode and pin 7 is the heater, the schematic has it the other way around. This tv had so many resistors wired to the wrong place, that I basically rewired it following the schematic almost like putting together a kit. The cathode of the 1st sync clipper tube was probably wired correctly to ground to begin with.

Next pic is the picture as it stands. The horiz still takes about a minute before it's really stable, but then it locks in good. No issues with the vert at all.

------

Now for the sound, first I thought to put the signal generator on the grid of the 1st audio IF tube, and just tweak all the IF transformers for loudest volume, but it was still pretty weak.

Then I hooked up the full blown early 1950s RCA sweep generator alignment set up I have and was able the get the picture on the scope for the discriminator transformer alignment just like it should be (ignoring the fact that for some reason the marker-adder decided it only wanted work set to reverse polarity) But, when I moved back to the 3rd IF transformer I could see the peak was set to twice the IF frequency, and I couldn't get it down much below 31MC. The sound section is in the 3rd schematic I posted...

[Ampico-kid]

Originally Posted by Adam

Now for the sound, first I thought to put the signal generator on the grid of the 1st audio IF tube, and just tweak all the IF transformers for loudest volume, but it was still pretty weak.

Then I hooked up the full blown early 1950s RCA sweep generator alignment set up I have and was able the get the picture on the scope for the discriminator transformer alignment just like it should be (ignoring the fact that for some reason the marker-adder decided it only wanted work set to reverse polarity) But, when I moved back to the 3rd IF transformer I could see the peak was set to twice the IF frequency, and I couldn't get it down much below 31MC. The sound section is in the 3rd schematic I posted...

Don't know if you've checked this yet, but while I was working on my 12" porthole I discovered the same issue in the sound IF's that I had already found in Zenith's table model sets of that vintage. They used silver mica wafer caps in the IF cans that started to break down and leak(silver mica migration). I had to disassemble two of them and remove the damaged mica sheets and then add external mica caps to the base of the IF cans. That solved the problem in my case.

Bob