Bias Problems!

[Mike Borish]

Can anybody figure this circuit out?

I'm having problems with heavy clipping distortion in this circuit. I can't seem to bias this circuit correctly. It is an old circuit and I have two of the same boards because it runs in stereo. One works with the power transistors in each channel and the other one distorts. Unfortunately, it is very hard to take measurements in circuit. I checked every component on the boards to see if they're within tolerance of each other and they seem to check out okay. The only way that I can keep the circuit from clipping is if I set the bias to a point where Q7 (TIP30) literally cooks. I'm literally scorching the board it is running so hot but no clipping. I know the circuit isn't going to last very long in such an unstable state and I would really like to avoid blowing the power transistors. Any ideas what could be wrong? I know that the design is sound.

**broken link removed**

 

[Nigel Goodwin]

If Q7 is cooking, and Q6 isn't, I would suggest Q9 is O/C - Q7 is just the first half of a compound darlington, with Q9 the output half. As Q7 should only be providing drive for Q9 it shouldn't be getting overly hot - but if Q9 is faulty it could be trying to drive the load directly - resulting in distortion and over-heating.

It's a pretty standard looking old circuit, quasi-complementry, probably mid 1970's or so.

 

[Mike Borish]

Thanks! I appreciate your input. Any ideas for a cheap cross reference of Q9? It's a TO-3

 

[Nigel Goodwin]

Thanks! I appreciate your input. Any ideas for a cheap cross reference of Q9? It's a TO-3

I would suggest a 2N3055, an incredibly common and popular transistor, I would also change both output transistors, to keep them the same.