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[cam.in.head]

its certainly more fun than christmas telly !

[LC0112G]

None of the circuits seem to show a 'second stage of preamp' as such. The rcr87_essen circuits show the tape preamp (an RC4558) feeding a couple of transistors (V1120 and V1130 both BC549C's), and from there into the TDA7348.

This is also where the tape mute circuit comes in - transistors V1310 (BC808), V1311(DTC1432KA), V1312(BC848C) and V1313(BC848C). This is where it gets sort of interesting. The Mute circuit appears to be fed from a couple of microswitches mounted on the cassette mechanism, via connector P1350/N1950. It looks like pin 2 of this connector should go to +5V when the cassette is inserted. Pin 6 of the connector should be open circuit when not-muted, and shorted to ground by a second microswitch when muted.

It's not obvious from the circuits what controls the microswitches, but I'd guess it's loading and unloading the cassette tape. I'd be much more suspicious of these microswitches than any semiconductor device. Given the 'single fault affecting both L&R channels' issue, it's most likely the feed to the base of V1310 is dicky - from pin 6 of P1350/N1950.

[cam.in.head]

hi and thankyou for all this .
i  mistook the other transistors as another amp stage !
yes the 4558 is the preamp ic and it all goes eventually into the 7348 .on one unit i have fed the head into a seperate preamp board robbed from a philips 752 and wired it straight down to the 7348  coupling caps.all works well so thats one unit sorted and proves im in the right area .
as for the other units i will now check the muting switches for any resistance leakage (or could i eliminate them completely ?
and yeah that 1350 plug contains the 12v motor supply and also two microswitches .one looks like it tells the motor to turn on and the other looks like it tells the system wether you are in play or fast forward/rewind and not play so yes i agree i this is the mute command ?
will have a play with this wire .thankyou again .will report back

[cam.in.head]

this also sort of ties in with the fact that even on a good unit if you get your finger close to that gren wire from the mute microswitch you can make it sound distorted but never saw the connection .!

[cam.in.head]

BUT  it also isnt a problem with the microswitch as such because ive swapped complete plug and play decks between units so more a case of an issue with the muting circuit itself .funnily enough of all the smt components that i swapped none of those transistors (or nearby resistors/caps )were touched!