A dying cap in the sound path will not react within a couple of seconds... dead is dead. Only some heat can change this fact for some time.

No, this behaviour is typical for the muting relay. Clean the contacts either with contact cleaner or very fine sanding paper or a special contact file.

You can test this fault with the second unmuted pair of loudspeaker plugs. If there is no problem, it's the relais. If the problem consists also on the unmuted path, it's a cap problem.

A more conservative approach would be to test the resistance of the relay contacts as a first step. Look for another source of the problem, like a dirty potentiometer, if the contacts test OK.

Cleaning the contacts as part of the debugging process seems counterproductive. The contact plating on most relays is very delicate and improper cleaning will destroy it. Once the plating is damaged the relay will be much more susceptible to contact failure. Put another way, improper cleaning may provide a short term improvement, but in the long term, the relay will fail. Attempting to clean a good relay simply adds the possibility of damaging it.

IMHO, improper cleaning is using any method that damages the plating or leaves a residue. The proper cleaning method depends on the contact material. Abrasives will remove oxidation or arcing marks, but also remove plating. Contact cleaners that clean by acid (or base) can work. The tricky part isn't getting the chemical reaction started, but getting it to stop. Contacts are designed to be clean and dry. Any cleaner that leaves a oily film is asking for trouble.

I typically don't use anything more abrasive than a piece of smooth white paper. I'll start with dry paper. I retest the contact resistance between steps. Next paper soaked in red Deoxit, followed by paper soaked in clean solvent (i.e. acetone), then dry paper. Next step up would be fine abrasives. However, at this point I'm just trying to get the relay working well enough to continue with other debugging while I'm waiting for its new replacement to arrive ;-).

Sorry for long winded reply. I'm a bit too passionate about this topic. ;-)

Glitch

 

I agree on the cleaning. A piece of paper soaked in contact cleaner pulled through the contacts - nothing more, or the very delicate platinum coating on the relay contacts will be damaged.
- And you may find that oxidation from airs moisture already did just that.
So - replace the relay.

Martin

Thanks everyone - very helpful. I swapped the speakers over to #2 and the problem goes away, so will focus on the muting relay.

Firstly, would I be right that the power relay is by the large transformers, and the muting relay is close to the speaker outputs? (See attached image).

I have never got involved with such transformers - might I ask for a bit more detail for “cleaning the contacts” and a view on the resistances shown attached. Some of them are very approximate as they were slowly falling during my measuring them. I have not yet removed the transformer, so it was measured in situ.

 

You may have gathered that I am a beginner at such things, but I do have a multimeter, and ESG meter, soldering equipment, IPA & some Dioxit.

link

Firstly, would I be right that the power relay is by the large transformers, and the muting relay is close to the speaker outputs?

Yes.

(See attached image). I have never got involved with such transformers – might I ask for a bit more detail for “cleaning the contacts” and a view on the resistances shown attached. Some of them are very approximate as they were slowly falling during my measuring them.

Replace the relay. Cleaning will not be a long lasting solution.

I have not yet removed the transformer...

You don't have to. Martin

I have just fitted the new muting relay and the Beomaster is sounding great again. Thank you to all for your input.

PS I meant to say relay and not transformer in my previous post - sorry. But I did take the opportunity to replace the 8 transformer mounting bushes which had failed.