ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
This Beocenter really is the gift that keeps on giving.
However, I've now encountered a problem to which I can see a solution, but it's a bodge. I don't know if the problem pre-existed because the tape player was non-functional.
What happens is that on cassette play only, there is significant mains hum in the right channel. The problem appears to be provoked by the metal cover of the pre-amplifier that effectively bonds the internal floating signal and chassis grounds with the external casework of the unit. When the metal cover is removed, the hum disappears. There is continuity (0.5R) between the internal and external cover mounting points without the metal cover in place. When the tape section is energized, I can measure 2.8 mV between internal metal and external case, cover off.
I could refit the cover using insulating fasteners and disrupt the ground loop (if that is what it is) but if anyone has any other ideas, that would be great. The cassette section does not use the same modular mounting on an insulated base but is bolted direct to the metal case - as is the tape power supply.
My guess is that some stray voltage is quite likely with this arrangement. The signal ground from tape section (Dolby board) to pre-amp is good.
Picture shows a couple of clips in place to replicate the hum.
Grateful for any guidance, as always.
I find this is neatly solved with a strip of electrical tape.
Onto the next problem (the unreliable auto-stop).
In fact, the ground loop (?) fault is not solved with electrical tape, though it helped up to a point.
The problem turned out to be fractured solder on the Play Back amplifier header pins and elsewhere.
Replacing all the elkos (some tants were borderline or worse) and replacing the playback level trimmers and the associated relay in the signal path seems to have helped.
The playback/record quality is now very good indeed, without any major work on the transport apart from spool belt replacement.