Today I disconnected everything from the 6AV6 socket and megger tested it, no problems here. I gave up on trying to solve this and to just reassemble the radio and live with the problem. Along the way I did three things which have greatly improved but not eliminated it.

I gave the volume control the same treatment as the 6AV6 socket and flooded it with cleaner, worked it back and forth a couple of hundred times and blasted it with the hair drier. I put a shield on the 6BA6 and removed the external wire aerial, relying on the loopstick only. This reduced the volume to an almost inaudible level when the volume is turned right down. With the external aerial connected the volume increases but no where near what it was. It's one of the most sensitive radios I've ever worked on, pulling in all Sydney stations with ease. Better than the Kriesler 11-99 which until now was my benchmark.

There's still something very strange going on in this set but for now I'll leave it alone.

Thanks everyone for your help but this one's beyond me.

Warren

As noted previously: One of the best & most unusual sets that I have ever worked on was a factory special AWA based on an R301(but not exactly the same as.. no circuit).(1930's) No PSU, or modulation hum & it had a 6A6 for an output tube. It had an RF stage ahead of the mixer. Despite a DX local switch it had delayed AGC.

A station over 60 miles away, in daylight, had to have the sensitivity cut back. 2CH at night (520Km) might as well have been in the back yard..... never had a set like it. Only domestic set that I have worked on with a bandpass filter as the first IF.

The only path for signal to continue with C16 removed would be via the tone and the wiper would need to to faulty & above ground, or a decoupling cap wired wrong, open or missing.

Marc

Marc

Hi Marc,

That would make sense except that C19 and C22 were still out of the circuit. The wire between R6 and the volume pot and the wire between the wiper and C16 both run through the same shielded cable. One of the last things I did was remove the volume pot end of the wire from R6 and was amazed when it went quiet. This is when I decided it was beyond me because C16 was still out. Maybe there's some sort of insulation breakdown inside the shielded cable or a strange inductance/aerial effect going on here but the plastic insulation I could see looked like new. Nothing showed up with the meter. I had so many components out of the circuit that it might have just been a coincidence. If I do get back to this one day I'll be starting with this cable!

Warren

"modulated IF when the probe was anywhere with a few centimetres of the 6BA6."

Is it possible the modulated IF is being detected and amplified again?

This is why I mentioned radiation from wires. I have had, over the years, seen and experienced a lot of issues in the area of the Det Audio on several sets.

When parts are changed this can result in induction between passing wires and on several sets I have found it necessary to add shielded wire to address the problem. The most unusual of late being a screen to plate & oscillator coil wire, coupling in an STC Autodyne mixer. And that was bad design layout.

It is not impossible for one stage to be oscillating, it is also not impossible for one of those new caps, or an old mica, that are supposed to be decoupling, to be a dud.

Marc

Dear Scraps.

Unless my memory is decreasing as fast as my sanity, I am fairly sure that this radio uses a technique known as reflexing.

This is where the received signal passes through the same valve twice.
A characteristic of this circuit is no zero volume possible. I seem to recall that distortion was also a problem. I do not think that the designers ever managed to achieve all that they wanted to with this circuit.

If hundreds of engineers, trained in valve technology, and working on this problem around sixty years ago, could not solve this, then I personally, would just increase the ambient noise level so as to not notice the no zero volume issue. Remember, you do not have minimum volume issue, just a zero volume issue.

If you confirm that this radio/wireless uses reflex circuitry for detection/A.F. amplification, then you can sleep well knowing that there is nothing wrong with your set and that it is working as its makers had intended. Especially as you did not mention any distortion.

Wayne Penrith

Reflexed radios are something I've only recently come across with a Kriesler and have been reading up on. I don't think the AWA 573 is reflexed, it uses a 6AV6