grundig vcr - gv9000 - erase problem

[b]

This VCR was not fully erasing tapes, symptom being incomplete erasure
of chroma and hifi audio on most tapes. No voltage was found at the
erase head pins when in REC.

Opened it up and found R662 , 4R7 burned open. Replacement did the
same, but strangely it did so from switch on - the record circuit was
not energized. Not sure how significant this is - presumably this
means there may be a problem with the 12v coming from the BA7755
(pin1) into choke L652? Would I be right in suspecting the amplifiier
transistor Q654 (2sc3203)

here are some scans of the circuit.

this one is a close up of the oscillator stage, http://www.mediafire.com/?5qmy6qj6y060p2w

this one is the general area, with visible lines to heads and supplies
http://www.mediafire.com/?6na5n8dhgroee4c

....any thoughts appreciated.
B.

 

[Meat Plow]

On Fri, 29 Oct 2010 15:46:16 -0700, b wrote:

This VCR was not fully erasing tapes, symptom being incomplete erasure
of chroma and hifi audio on most tapes. No voltage was found at the
erase head pins when in REC.

Opened it up and found R662 , 4R7 burned open. Replacement did the same,
but strangely it did so from switch on - the record circuit was not
energized. Not sure how significant this is - presumably this means
there may be a problem with the 12v coming from the BA7755 (pin1) into
choke L652? Would I be right in suspecting the amplifiier transistor
Q654 (2sc3203)

here are some scans of the circuit.

this one is a close up of the oscillator stage,
http://www.mediafire.com/?5qmy6qj6y060p2w

this one is the general area, with visible lines to heads and supplies
http://www.mediafire.com/?6na5n8dhgroee4c

...any thoughts appreciated.
B.

Pin one of the BA7755 is the Vcc of that chip and is supplied 12.9v via
the choke as is T652. Look for a way for that voltage to get back through
Q654 and R662 to erase ground to provide enough current flow to destroy
the resistor.



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