Zenith 24h20 Porthole

[VintagePC]

Originally Posted by leadlike

Thank you. It really does look pitiful, doesn't it?

What makes it look so bad is just the browned, aged paper. My audio output transformers looked the same, and until I tested the continuity, my main thought was "gee, I hope those are still OK".

Rewinding coils is one of those things regarded as a black art by those who don't know the secrets... (or have a machine) but once you know them, it's really not that difficult at all!

[leadlike]

Can't really restuff these cans, as this set uses several sections that are 475v, so I had to jump up to caps that are just too physically large to do so. I'm just about done with the basic capping (new lytics and paper/bumblebees replaced) so I'm working towards the initial powerup now.

[leadlike]

More problems with the audio amp: I found a wiring mistake that messed up the cathode bias on the output tube, causing the amp to remain silent.

After that was cleared up, I finally had a roughly recapped chassis:



But more audio issues were evident: I wasn't getting any audio still, although the amp was sound. I found an IF tube was dead (open filament) but still nothing. Something wasn't quite right with my measurements on the oscillator, and when I pulled the tube, the plate pin remained in the socket. Luckily, I have another 6C4, and when I dropped that in, I finally was rewarded with the sweet sounds of static.

The fly is also good! After capping, HV came up right away! Now, to hook up the crt (really wish I had a mini test crt right now) and see if I am getting a picture!

[leadlike]

I had the picture tube hooked up, and was rewarded with an out of sync image, but audio and video were of reasonable quality. I removed the crt and flipped the chassis again, changed a few resistors and micas, flipped it again, reinstalled the crt and power up, this time, the 5u4 flamed out. I quickly ID'd a short in the horizontal section, so I dreaded that the flyback was out.

Checking a little more closely, I found that the width coil was missing the inner former tube, making it flop around loosely on its leads. The threaded steel wire that slides in and out of the coil was shorting to it, the adjuster being grounded to the chassis. Essentially, the 5u4 was shorted dead to ground.

With the problem identified, I slipped a cocktail straw into the coil as a new form, and glued the coil back into place. Now the width coil is electrially sound, and still adjustable.

[charokeeroad]

Originally Posted by VintagePC

What makes it look so bad is just the browned, aged paper. My audio output transformers looked the same, and until I tested the continuity, my main thought was "gee, I hope those are still OK".

Rewinding coils is one of those things regarded as a black art by those who don't know the secrets... (or have a machine) but once you know them, it's really not that difficult at all!

Ok I'll bite. What's the secret? sounds like a possible new thread. "The Black Art of Coil Rewinding Exposed.."

[leadlike]

Well, I found a resistor that had gone low, and replacing that one bumped the voltage up to about 60 volts. But just then, the width coil flashed over and caused the flaming 5U4 effect again.

So....completely disassembled the width coil this time. It is toast. The coil is in two sections, with a center tap, I believe, and one section is just destroyed. This may have been what took this set out of service so long ago, as I found a few other attempts at trying to shore up this coil before I used that cocktail straw.

I'll try to give more details later, but now I have to see if I have anything close to this coil on my parts sets...

[VintagePC]

Originally Posted by charokeeroad

Ok I'll bite. What's the secret? sounds like a possible new thread. "The Black Art of Coil Rewinding Exposed.."

Not much really - go slow, be careful, and have a fair amount of wax handy to save your progress every so often...

It worked for my oscillator coil (my Fleetwood thread). The big thing is the number of turns and the direction in which you wind that need to match. In the case of my Fleetwood, I couldn't get the number of turns, so I opted to get the total length of wire, and wound it in as close to the same winding pattern as I could - and it worked!

[leadlike]

FINALLY had some time tonight and rewound the width coil. From the closeup picture earlier in the thread, you can see that it is two spools. When I pulled the coil, the top spool crumbled in my hands. The center shaft on the spool burned up, exposing bare coil wires to the adjustable inductor, shorting the windings to ground, causing the 5u4 to flash over.

As the spool was gone, I could only make note of the winding direction of the coil, and strip it. The spool ends were still intact, if a little cooked (no conductive carbon, thought). So I made a properly sized cardboard tube for a new center shaft, and began winding magnet wire onto it (#36, seemed pretty close to the original) by hand. Once I got a good base laid down, I chucked the coil into my drill, and spun the rest on. The whole operation took about an hour and a half.

I hooked it up, and the set powered up. The coil didn't heat up, like the old one did, so I hooked up a video source and the crt, and was rewarded with a decent picture. Vertical hold is still twitchy, but it you leave it alone, it stays put. Sound is extremely poor. I suspect low bandwidth getting through, but adjusting the various sound and tuning controls still produces the same result with fuzzy highs and lows.

BTW, the old width coil measured 15 ohms, and the newly wound one checks out at 15.6 ohms! Not bad!

Otherwise, the picture looks decent, and the set is much easier to operate. I suspect that the bad width coil was heating up and loading things down a bit. Will take some measurements next.