[MrFixr55]

DOPF!!!! I thought of that, but I forgot to suggest that. Zenith did not use metal tubes. Zenith tubes at the time were made by Sylvania, and they had an aquadag coating. They referred them to Metal-Glass. For the 6K7 and 6A8, the RCA Receiving Tube manual shows the tube shell connected to Pin 1 for the metal version of the tube. for the G and GT versions of these tubes, there is no connection for Pin 1. For the 6H6, there is a connection for Pin 1 to a shield on both metal and glass versions.

It may not fix your problem, but make sure that Pin 1 of the 6A8, 6K7 and 6H6 was not used as an interconnect terminal and connect them to chassis ground.

For the heck of it, connect the antenna to the 6A8 grid and let it play. One of your problems is the intermittent nature of the issue. If the problem does not appear, it may be the primary of the antenna transformer intermittently opening.

[DaleHCook]

... make sure that Pin 1 of the 6A8, 6K7 and 6H6 was not used as an interconnect terminal and connect them to chassis ground.

And that is something which I forgot to mention. In chassis which were made with glass octal tubes pin 1 was sometimes used as a tie point. If you want to use a metal tube in such a socket, you will have to provide a new tie point to which you can move that connection, in order to ground pin 1.

The first octal tubes were introduced by RCA in 1935, and all of RCA's octals were initially metal tubes. Other tube manufacturers, such as Sylvania, started building octals in glass envelopes, such as the 6A8G and 6K7G, to avoid paying RCA to license their metal patents. The "G" designated the glass envelope. When RCA started making glass versions of metal tubes they used the designation "-G." I'm not sure when RCA introduced their glass versions of octal tubes, but it was before 1940, as they are in RC-14.

[dconant]

Pin 1 is grounded.

[dconant]

Played for about 3 hours yesterday and then did its thing. Hooking the antenna directly to 6A8 did not bring in reception. Removing the ground bought back reception. Next step is to rewind the antenna coils.

[RodB]

I can tell by the sound of your voice that you're frustrated.

[morzh]

Rewinding coils is a very soothing thing to do.

[MrFixr55]

You can try subbing an antenna from an AA5 for the coil to see what happens

[dconant]

Yeah I'm a tad frustrated. From what I am seeing on the schematic there appears to be six coils on that core. Guess I will start with the one off the antenna connections. I'd rather be outside puttering around.

[RodB]

Well, to pick up on Mike's theme you could hum a little tune while winding;

The wire on the coil goes round and round, round and round, round and round.
The wire on the coil goes round and round, and then it's Grain Belt time.

[DaleHCook]

From what I am seeing on the schematic there appears to be six coils on that core.

??? There are only three windings. The secondary winding has three taps.

BTW, what are you using for an antenna? If it is a single wire, are you sure that the "Z" terminal is firmly connected to the "G" terminal?

Also, are you sure that the band switch is working properly? One winding is, IIRC, supposed to be shorted out for SW, and used only for Broadcast and Police.

[Eliot Ness]

With a problem this intermittent it will be difficult to find, but not impossible. When the problem rears its ugly head use a non-metallic object like a chop stick to poke every wire and connection you can reach (above and below the chassis). That set uses pretty common tubes and I'd just replace them all to rule them out. If that isn't it put them in your parts stash for later use. I'd have to do a lot of wiggling and poking before I started rewinding coils

[RodB]

Dan has done all the wiggling and poking and jostling. We've been outside the box for some time now.

Dan, have you tried a heat gun or, the opposite, component freezing with freon or electronics spray cleaner?

[morzh]

If it is heat and contact combo, taking 3 hrs to develop, then

1) a good whack should restore reception, 2) having the chassis on its side or upside down gives a convinient and quick access to all soldering, and hitting it with freeze spray (in multiple points) might reveal something.

[DaleHCook]

2) having the chassis on its side or upside down gives a convinient and quick access to all soldering, and hitting it with freeze spray (in multiple points) might reveal something.

A chassis stand is immensely helpful for under-chassis troubleshooting like this.

[morzh]

Yes, and I have two from Rich, the smaller and the bigger.
I just allowed for the possibility of having not acquired one yet.