SWR 550x Bias issue?

[kdawg]

Hello - I'm troubleshooting a SWR 550x, I've checked all power transistors, their 5W resistors, and their drivers - they all pass diode testing on the meter - but right now there is positive rail voltage on the speaker output. I have preamp disconnected, only power rails to poweramp. I'm assuming the bias circuit is gone? I measure full 100v rail voltage across all of Q4 B-C-E. I checked all diodes, I don't see any burnt resistors, I have checked all around the bias area and they measure OK. I only have not tested Q16 and Q4 since I have to disassemble further, I'm assuming one is bad? What should the voltages around Q4 be roughly, and what is the bias procedure? What should the C of Q16 be putting out also?

Thanks
-kdawg

550x-750x Power Amp Rev E.pdf

[Enzo]

Q4 is the bias. If Q4 shorts, the amp continues to work, but with greater crossover distortion. In fact shorting across Q4 is done as part of troubleshooting in various cases.

If Q4 opens, then both posisitve and negative sides of the output conduct HARD, and fuses blow.

All the bias does is maintain a voltage difference between oposing drivers (and thus outputs). If you have V+ on the output, then I bet you have close to V+ on the bases of the drivers and outputs. The output follows what the bias transistor does.

A dead Q3 or an open R9 or R15 would allow Q4 to snap up to V+, and the output would follow.

Of courwse an open Q1 would leave Q3 off, and the result the same.


Now then, notice the ground symbol for the outputs and the power supply. The three lines symbol. Now look at the ground symbol for the input, see how it is the triangle? If you do not have a grounded input cable connected, that input circuit may not have a ground reference, and that will result in DC on the output. SO at the very least, conect a clip wire from pin 2 of the signal input to the ground of the power amp.

[ne0]

Enzo, you're GOD!

[Enzo]

No, Clapton is God.