Technics m205 tape deck-No board I/O

[JR North]

Struggling newbie help please...
This is a 1980's Tape deck. Simple RC&Q board with ICs for
Dolby.
The decks power supply seems ok. The tape drive works, but
no meter operation in play and no line out, both channels.
I scoped the play/record head board input-good both
channels. The board input is to 4 BC168 amp transistors, 2
in line for each channel. Signal to Q1 and Q2 base OK. No
apparent output from Q1-2.
Input was tested using signal ground for ref, and 5MA at 5X
mag.
Q1-4 supply tested with board powered, no tape run, using
motor ground for ref:
DC:
B C E
Q1 5 2 5 right channel 1st stage
Q2 5 2 5 left channel 1st stage
Q3 2 5 2 right channel 2nd stage
Q4 2 5 2 left channel 2nd stage
I looked for A/C component(signal amp) at C and E riding on
DC at 5 MA on Q1-Q2- no signal.
Are the trans supply voltages correct? I have no schematic.
Am I testing for signal amp properly? Any opinions/advice
appreciated.
JR

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[Asimov]

"JR North" bravely wrote to "All" (26 Mar 04 18:19:18)
--- on the heady topic of "Technics m205 tape deck-No board I/O"

From your admitted inexperience, I'm not too surprised in being a
little confused. However, if indeed you did it right, it would seem
from your voltage measurements that the supply voltage at the
collectors is way too low. Ideally the collector voltage should
typically be in the ballpark range of 10 to 20 volts for the preamp to
operate properly.

To be sure you must check the supply voltage and that going to the
preamp board. Since there should be far more voltage on the collectors
than the emitters, that is why you have no signal output.

I can only guess what may be the problem but it might be a very leaky
preamp filter electro on the preamp board or on the main supply.
Perhaps the resistor in series with that electro has gone up in value.

In any case, the leaky electro may be the result of the equipment not
having been used in a very long time. Electro's tend to lose their
internal polarization if left unused for a long time and need to be
properly reformed before being put back into operation (or simply
replaced).


JN> From: JR North <jasonrnorth@bigfoot.com>

JN> Struggling newbie help please...
JN> This is a 1980's Tape deck. Simple RC&Q board with ICs for
JN> Dolby.
JN> The decks power supply seems ok. The tape drive works, but
JN> no meter operation in play and no line out, both channels.
JN> I scoped the play/record head board input-good both
JN> channels. The board input is to 4 BC168 amp transistors, 2
JN> in line for each channel. Signal to Q1 and Q2 base OK. No
JN> apparent output from Q1-2.
JN> Input was tested using signal ground for ref, and 5MA at 5X
JN> mag.
JN> Q1-4 supply tested with board powered, no tape run, using
JN> motor ground for ref:
JN> DC:
JN> B C E
JN> Q1 5 2 5 right channel 1st stage
JN> Q2 5 2 5 left channel 1st stage
JN> Q3 2 5 2 right channel 2nd stage
JN> Q4 2 5 2 left channel 2nd stage
JN> I looked for A/C component(signal amp) at C and E riding on
JN> DC at 5 MA on Q1-Q2- no signal.
JN> Are the trans supply voltages correct? I have no schematic.
JN> Am I testing for signal amp properly? Any opinions/advice
JN> appreciated.
JN> JR

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