On Sat, 19 Apr 2008 14:36:36 -0700 (PDT), EADGBE
<hwbosshoss@[EMAIL PROTECTED]
> wrote:
>On Apr 19, 5:04 pm, "DaveM" <masondg4...@[EMAIL PROTECTED]
> wrote:
>>
>> In your tape circuit, the big clue to the problem is lack of any
voltage at the
>> collector of Q205. Q205 must be turned on, at least partially, in
order for any
>> voltage to appear at its collector (which is showing no voltage). It
is the
>> only path that can supply the current to Q203 is through the E-C
circuit of
>> Q205. If Q205's B-E is properly forward biased, current should flow
through the
>> C-E circuit.
>
>Could you repeat this part, please?
>
>There is no Q203 in this circuit.
>
>Thanks for your help!
Q205 is biased just fine. It has nearly 5V emitter to collector, There
is a problem around Q206, though. This transistor provides thermal
regulation for the quiescent current in the output stage. If you look
at the base and emitter voltages, you will see that it has become
reverse biased by 0.4V and there is possibly no voltage difference
between collector and emitter. It is not easy to see why this might
be. What is the voltage on the collector of Q208? It is misbiased
right now, possible by a bad Q206.
Just looking at the basic bias conditions, I would say
Q204 ok
Q205 ok
Q206 problem - maybe C-E short
Q207 ok
Q208 reverse biased B/E
So whatever is wrong, I think it is happening down around that bottom
right hand corner.
Perhaps more to the point, how do all these points measure on the good
channel?
--
Pearce Consulting
http://www.pearce.uk.com
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