Hi! New in this forum, found in google when facing the same issue in my 540A. First thought the problem was with my newly bought used refurbished B&O Beovox CX100 speakers and was about to call the speaker seller back and complain about a badly distorted speaker... but a quick check turned out that bridging the speaker output relay made the sound crystal clear.
For making the amp a tiny bit more "hi-fi" I decided to simply bridge the relay bypassing the contact paths completely with tiny bits of wire. I understood I would be hearing a loud "thump" in the speakers upon every push of the power button, but that was according to me a cheap price to pay for never having a relay issue again. But how wrong I was! After only a couple of power-ups with the relay bypassed (yes, with audible "thumps"), there was a small "pop" from the power board - two tiny fuses blew. So the amp was definitely not designed to work with a bypassed relay due to the apparent big power surge in the speaker paths.
It appears my 540A has a single, 2-pole relay controlling both channels with one relay coil. The left channel path shows ca. 6 - 7 ohms on the multimeter when the relay is closed, indicating a bad relay contact surface.
Side note:
This page shows that the 640A has two relays in the speaker path(s), either it is one for each channel or it is one for A and B speaker pairs respectively. The 540A has a hardwired front panel pushbutton switch for "Speaker B". I have not seen a 640A myself but I bet the B pair has its own relay and a "soft button" to it.
Side note 2:Could decide to go with connecting my 2 speakers only to the "B" outputs, another way to bypass the relay issue temporarily. Anyone has opinions about that? The B should have identical output ratings - or not?
For making the amp a tiny bit more "hi-fi" I decided to simply bridge the relay bypassing the contact paths completely with tiny bits of wire. I understood I would be hearing a loud "thump" in the speakers upon every push of the power button, but that was according to me a cheap price to pay for never having a relay issue again. But how wrong I was! After only a couple of power-ups with the relay bypassed (yes, with audible "thumps"), there was a small "pop" from the power board - two tiny fuses blew. So the amp was definitely not designed to work with a bypassed relay due to the apparent big power surge in the speaker paths.
It appears my 540A has a single, 2-pole relay controlling both channels with one relay coil. The left channel path shows ca. 6 - 7 ohms on the multimeter when the relay is closed, indicating a bad relay contact surface.
Side note:
This page shows that the 640A has two relays in the speaker path(s), either it is one for each channel or it is one for A and B speaker pairs respectively. The 540A has a hardwired front panel pushbutton switch for "Speaker B". I have not seen a 640A myself but I bet the B pair has its own relay and a "soft button" to it.
Side note 2:Could decide to go with connecting my 2 speakers only to the "B" outputs, another way to bypass the relay issue temporarily. Anyone has opinions about that? The B should have identical output ratings - or not?