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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! 

[cam.in.head]

i think were onto something here !
after the connector on the board for the mute switch (the green wire switched to gnd) there is an inline resistor and then its straight to that first transistor .i have removed the resistor effectively taking the mute switch trigger out of the equasion .this appears to have cleaned the signal up considerably . so maybee that transistor had some leakage and was partly switching . i  beleive it might /shouldhave a resistor somewhere to keep it pulled off (or not?) but it seems like weve got it .no noticeable difference to how the rest of the unit works . was affected more in damp or cold weather so may have been partly switching that transistor on ? will try the one in the car on a damp cold day and then remove the resistor in situ and see how it sounds .
if this is the fix then you have solved me a problem that has been frustrating me for years and i spent a hell of a long time replacing stuff /swapping stuff i didnt need to ! learning curve or what !!

[LC0112G]

If I'm looking at the correct circuits - the resistor is R1310, a 1Kohm, connected between the base of V1310 (a PNP BC808) and pin 6 of connector P1530/N1530. When pin 6 of the connector is shorted to ground (by a microswitch on the tape deck) the transistor will conduct causing the mute circuits to activiate. The microswitch should be open circuit in normal tape operation. There is no resistor shown to pull the base high or keep the transistor "off", so the base is indeed floating during normal operation.

Lifting R1310, or disconnecting the wire into pin 6 of P1350, will have the same effect - and will stop the mute circuit from working. However, if that works it's not a leaky transistor - it's a problem with the wiring to the connector or the microswitch. However, it might be worth wiring a high value resistor (10K or more) between pins 3 and 6 on the connector. Pin 3 should be a stable 5V supply, and 10K should be more than enough to ensure the transistor (V1310) stays off when it should be off.

PS - I'm dying with man-flu at the moment, so might not be able to add any more unless I make a miraculous recovery.

[Andy B]

this conversation went way way over my head pages ago! 

[cam.in.head]

beleive me this has been screwing my head up for ages too . i started with this a few years back as one of my stereos went distorted .i bought another off ebay and the problem SEEMED sorted but i was curious what was actually wrong with the original unit .didnt realy find the faulty component then so gave up temporarily .THEN the same issue started happening to my other "same " stereos so i thought its a obviously a component that goes faulty with age .makes sense because they used to work and wouldnt be designed to not work right in winter !!
so now (if the problem is found ) il find out over the next few weeks, then although i still dont know which component is actually failing this workaround may just solve the issue anyway and doesnt seem to affect its functioning .radio plays fine ,cd changer and cassette and the display shows what it should and the selector switch does what it should .il try the suggested pull up resistor as the next stage if any issues present themselves .
a great big thankyou to anyone who helped especially mr LCO112G whom i am extremely gratefull to .