Re: Amplifier Problem - Revised Schematic

Richard Crowley wrote:
> "EADGBE"  wrote ...
>> In the interest of clarification, I have added some information to the
>> schematic of the tape deck headphone amp that I'm having problems
>> with.
>>
>> Some circuit paths are now labeled:
>>
>> http://i240.photobucket.com/albums/ff58/eadgbe123/amplifier-1.jpg
>>
>> As always, the points at which voltage readings are marked with a star
>> are NOT producing the voltage indicated.  The +7V location is
>> measuring 131mV instead.  The other two points where a star is
>> indicated are measuring between 20mV and 30mV (fluctuating slightly).
>>
>> I am starting to think that a circuit OUTSIDE of this circuit might be
>> at fault, because NOTHING inside of this circuit seems to be
>> faulty...but the darn thing still WILL NOT work.
> 
> Can't see what parts external to the circuit shown could
> cause any of your symptoms. The input and output are
> decoupled with capacitors.  Furthermore, you confirm
> that the internal, DC side of the input and output are
> at the prescribed voltage. You have also confirmed that
> the supply voltage is OK. There is nothing else externally
> that could affect the circuit.  I am assuming that the
> node at the lower left corner finds its way back to
> ground somewhere.
> 
> The question remains why the collector of Q205 is at
> 131mV rather than the predicted 7V.  Q205 is simply
> not conducting and that is causing all the rest of the
> symptoms.
> 
> One of the marvelous things about stereo is that you've
> got two identical channels. So you have a good one to
> compare it to.
> 
> What is the base voltage of Q205 on the working side?
> And what is it on your broken side?

All points to Q205 collector being open apart from one thing. On the 
revised schematic, the output voltage at the junction of the two 3R3 
resistors is measured as being correct. That doesn't add up at all. I 
could do with seeing this voltage checked.

I've put the circuit into Pspice and played around with various fault 
conditions to see what the volts do. Q205 is the only thing that seems 
to reproduce the symptoms.

d