Yes I thought it could be poling in the speaker, from your description. The fact that it has a 75 means it has a diode detector and an "audio" volume control, so that pretty much rules out any scenario that would result in distortion on low volume in the audio amplifier stages. Injecting a known-good signal as I described will prove it one way or the other.

But I should have a little talk to you about how to fault-find, based on a lifetime spent doing it......

1. Don't pull anything apart or replace anything unless you have proof it's the cause of the problem.
2. Disturb as little as possible - collect evidence instead BEFORE you dive in.
3. If you have a hypothesis, test it first. Doing so saves a lot of time and frustration.
4. Think before you act! Use your head, not your hands.

I agree with that Ian! A good scientific approach makes good sense, I have a Bakelite Astor Mickey that I made the mistake of "diving in" and don't know where I am with it, but I've been lucky with the Lekmek, it had issues, the candohm originally was open between all sections, well and truly cooked and the cathode resistor 425 ohm, which serves both the 6D6 and 6A7 was open, real evidence that too much juice had been going through carving a path of destruction. What I should have realised sooner is that the power cord was put on to the wrong ie. 22 volt tap (by myself) but the original cord was cut and I soldered a new one to where the old one went... Derrrr! I should have just checked all taps with my ohmmeter and not assumed it was previously correct, especially with the faulty candohm!

Photo uploaded to Post 17.

What I like to call it is methodology. Develop a plan of attack which never starts with powering it. As pointed out with the current pair here. one had several errors and it would have never run and the other had a patently obvious, by the contents leaking, dodgy cap. All achieved by looking before you leap. Neither fit to be powered.

I align with an Oscilloscope as the measuring device. It often picks up distortion and is used to track its source down. It is amazing at times how easy & formidable an Oscilloscope & Signal generator can make finding lost signal & distortion at its source.

Anything that eliminates speculation & guesswork is good & often a time saver.

Marc

I don't have a signal generator, but I do have an oscilloscope, I just don't know what to do with it?

You don't really need either to fix old radios but they can make tricky jobs easier.
A smartphone makes an excellent audio signal generator. The example I set out earlier
in this post is recommended.

Ok I had never considered that, but a brilliant idea, I shall have to do that and get my head around it as I also have a couple of good books.