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Dead amplifier circuit

Ok,
As this is a push pull o/p stage,is this transistors bias excessive compared to it's series transistor 208,so the o/p stage is assymetrically biased?
I would check voltage drop across 2r15,this drop should be the same as 2r14,or very near.
If voltage drop across 2r15 is far greater then this would show t209 to be sinking far more current,hence greater heating.

If this is the case then biasing circuitry needs investigating.

Paul.
 
I did download the console schematic and have been looking at the output circuits....which transistor exactly is hot?
I really don't like these complimentary transistor push/pull circuits...way too touchy on bias and not forgiving enough
for any accidental misconnections and/or improper loads. If this were my console I'd be building some 6SN7 tube-based
output drivers and be done with it.
 
Have you measured the voltage on the emitter of 2Q7? Is it correct? You might check diode 2CR8 and transistor 2Q6.
Measuring the voltages and comparing them to the schematic will point you in the right direction.
Good luck.
 
Looks to me like the 2C9 capacitor bypassing the 270 ohms on the emitter side of 2Q9 is a prime suspect. If it has too much leakage, or has failed into a shorted condition, 2Q9 will be biased too high ( and run hot). Do you still have 22 volts on lines 3 and 4 or is it a bit low?
 
WOW!!! Thank you for your support.

The hot one is the 2N4905

I have a problem... I don't have an extender card. Doing measures under those conditions is difficult. Perhaps I'll start by replacing 2C9.

Thank you again!
 
Is this problem resolved, or are you still working on the problem?
 
Due to personal problems I've been "away" from my faulty gear... Making a flashback I can't be sure if this can be related with the issue:

This happened while connecting pc speakers to the speaker terminals of the console. One of them died then. Coincidence?...

In the early days of having the vintage gear I had pc speakers as monitors without any issue:
http://www.youtube.com/watch?v=c814F1VjE_4

This makes me scratch my head...
 
Does the speaker still work (on a computer, for example)? Maybe the coil in the speaker shorted? This might have been caused by or caused the output transistors to short.
 
Back to one or the earlier posts.
If the original transistor was "hot" and the replacement transistor gets hot too, the problem lies somewhere else.
You don't need an extender card to check the amplifier module.
One-by-one, solder a wire from each point on the schematic where the voltage is indicated on the schematic.
Measure the voltage at each of these points with a multimeter. That should show where the fault lies.
 
Measure the voltage at the speaker output with your multimeter set to DC. If there's DC there, that'd explain the dead speaker and should tell you that the amp circuit is imbalanced.
 
Due to personal issues I've been away from my hobby. Yesterday I followed a suggestion that a workmate gave me a few months ago. I've wired checkpoints 1,2 and 3 of the faulty card and took some measures. 1 and 2 gave me voltages around 40v. Point 1 gave me null. Having this info could you please help me to trace what components should I replace? Some of you gave me hints a while back ago. Useful but they were given with no complementary info like a gave this time.

here's the schematic:
http://i215.photobucket.com/albums/cc118/SFM-PTGAL/Broadcast console/amplifier.jpg?1280254923

The faulty (hot) transistor is the signed one.

New hints? :)

Regards
 
That transistor is not the problem. The problem lies earilier in the circuit. Test each transistor with a Multimeter.
I'll bet that one or more of them is shorted or open.
Compare your measurements on this bad card with a known good card and you can easily resolve the problem.
 
Yes, I also believe that the transistor is not the problem. Testing each one of them and comparing with a good card is a pain in the *** cause I don't have an extender card. anyway thanks for your help. Can the zeners be bypassing the tension?
 
You really don't need an extender. Use a VOM or Multimeter and compare the transistor junctions on the bad card against the good one. You will probably find a shorted or open transistor. It should be a simple repair.
 
Yes it was a simple repair, after almost 2 years.
2 complementary transistor circuits (push-pull), I changed them all. pita to find the same ones, most of them are replacements. It's working although the repaired output module heatsink warms a little, compaired to the other modules.
 
SFM-Ptgal said:
Yes it was a simple repair, after almost 2 years.
2 complementary transistor circuits (push-pull), I changed them all. pita to find the same ones, most of them are replacements. It's working although the repaired output module heatsink warms a little, compaired to the other modules.
Click to expand...

Excessive heat is common when changing parts with generic replacements because the hfe of the replacements is probably slightly different, resulting in them wanting to be biased differently. Measurements of distortion will probably also show a difference between the original and the replacement. It's possible that the crossover distortion may actually be lower than before due to the transistors being biased closer to class A. If the heat is not destructive, you can probably live with it for many years. I fixed a friend's beloved 1972 Pioneer receiver last year and ran into the same situation.
 
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