SWR California Blonde Power Amp Troubleshooting

[TieRBuoY]

The issue with this amp when it first came to me is it kept blowing the 8A output fuse. A quick check identified VCC+ on the output. After reading the number of posts on this forum regarding this amp, I decided to replace all transistors, diodes and electrolytics. The problem remains.

There is a very informative thread initiated by Gonzo78 at this link:

http://music-electronics-forum.com/t44034/

The following are voltage readings I am seeing throughout the power amp:

VCC+ 48.4v
VCC- -48.4v
Output ( measured at R19 and R20 junction ) - 48.2v
Q4 - 48.2v at each of BCE
Q3 - B= -45.4v, E= -45.6v, C= 48.2v
Q10 - C= 48.2V
Q5 - B= 48.2v, E= 48v, C= -48.4v,
Q6 - B= 48.2v, E=48.2v, C= 48.4v

I should also note that Q7 and Q8 at removed.

In the attached thread, there is a very informative post by Enzo that outlines how the voltage space between the bases of Q5 and Q6 are maintained, how Q4 does this and is controlled by Q3. In my scenario, it appears to me that Q3 is conducting. VR1 has no affect at all. He also mentions that Q10 acts as a switch that is operated by the mute circuit. As stated above I am seeing close to full VCC+ on the collector of Q10. which is reflected at the base of Q4. Is this correct? What should I be seeing at Q10.

Thank-you for any and all replies.

[nevetslab]

What you SHOULD be seeing at the two bases of your driver transistors Q6 and Q5 is about 1V on both sides of 0V (1V for Q5 base, -1V for Q6 base). That would have a pre-bias of 400mV at the base of Q8, about 400mV B-E of Q7. The output stage is Quasi-Comp, so the negative driver looks different, but it works non-the-less.

Now, since you have the output latched up at the positive rail, the output xstrs are both removed, and it's pulling current, you probably have a shorted driver Q6. You may also have open emitter resistors for the neg driver Q5 R17 & R18 and perhaps the collector resistor for the neg output R20.

Once you have restored the driver circuit, you can deal with the bias xstr Q4. If Q4 is open circuit, it would turn on the output stage. But, your output is latched up, so look for a shorted driver Q6.

As for Q10, it will turn on, pulling up the common emitter resistor R4 to power on the front end. I don't recall if it goes into full saturation, or just enough to get the front end turned on.. We can find out that once you've found the fault in teh otput stage/driver stage.