Hi all,

You might remember my post from some time ago regarding a radio I was restoring. The I.D was, and is, a mystery and I still haven't been able to find a schematic.

I've recapped it (all except the mica's) and also had to replace the 6V6 (used a 6N6C) and the 5Y3, both of which were dead.
The cabinet is also largely complete.

Now I do get audio but the capacitor between (what I think is) the RF choke and the transformer gets hot, and pops if left running for more than 30 seconds.
The audio is quieter than I would expect and there is a decent hum as well.

The speaker seems OK and gives 1500 ohms across it, which is the labelled rating. (its an electrostatic unit)

Im hoping you guru's could help me out because i'm stuck.

There are some images here to show the layout - https://onedrive.live.com/redir?resid=DB1B08D6799...
Ignore the crappy radial cap, my nice new axial popped before I realised there was an issue!

Thanks
Ash

If you are calling that big Green thing an RF choke it's not & is liable to be broadside across the B supply. It will act as a voltage divider for screens & sometimes bias, depending on how it is set up.

Those are notoriously unreliable and will get hot. Crackling indicates overload an short.

What on earth is a 6N6 doing in place of a 6V6 which is still being made, as is 5Y3. 6V6 is a Beam Pentode and the other a pair of Triodes. What sort of uncontrolled result, you get with the full B+ potential on the wrong Triode, who knows?

Unless there are substantial circuit modifications for the 6N6 stop now before you cook something, if you have not already? The OP transformer is liable to be across both plates, if wired for 6V6 and that's not healthy.

Marc

Sorry, when I say "6N6" i'm talking about the Russian equivalent (or what ive been told is equivalent) 6V6. The "N" is cyrillic, I shouldn't have used it. to avoid confusion.
One of these - https://reverb.com/item/391660-pairs-of-russian-otk-military-6v6gt-6n6c-black-glass-power-tubes-tweed-blackface-clone-amps

Yes the "big green thing" is the term I should've used I guess, what is IS it called?
How is it tested?

The green thing is called a dropper, and is a wirewound resistor with tappings.

The cap that pops is the main filter capacitor, and should be rated at least 525 volts like the original. The negative end goes to the left (the transformer). Maybe you put it in backwards.

What is the voltage at the right-hand end? It should not exceed 400 volts. If it is over, perhaps the radio is meant for 120 volts.

Thanks for the info, I'll investigate the dropper and see what I find.

Regarding the polarity of that cap, that was my first thought too but I have triple checked my original photos and the original item definitely had positive end to the transformer. (The red end on a Ducon cap)

Definitely a 240V service, the transformer pins are marked and the cord uses common and 240.

The first caps positive should be on the cathode of the 5Y3, which is its filament. The negative either goes to the HV centre tap or chassis depending if the set is back biased or not..

Make sure that this is the case. reversing the cap destroys it and I have recently had a dud that let go before 500V. The aside being due to lack of interest, I have not bought from them since.

Marc

Regarding the polarity of that cap, that was my first thought too but I have triple checked my original photos and the original item definitely had positive end to the transformer.


Sounds like it was always wrong then. You can see that the transformer end connects to a wire which is earthed.

100% right you are, problem resolved.

Well that's a valuable lesson learned, I wonder how it came to be that way? Attempted repair back in the day? Poorly assembled kit?

It's alive anyway!!
The dropper tested fine by the way, the terminals, heading downwards were 5580 ohms, 14540, 23100 and 25700.

Thanks a lot for the assistance, I'm learning slowly.

I wonder how it came to be that way?

Not an unusual error. People tend to think that ('logically') the cathode should be negative, but in a rectifier that's in fact the positive output terminal.