SX550 Right Channel Clipping

[wjc]

I have a positive right channel clipping on an SX550. Idle Current on the Right is only .3mV, Left is 19mV. Any hints on where to start would be appreciated. All the test point voltages except the right idle current are fine and signals coming off the muting assemble are fine.

 

[PE9ZZ]

The output amp if this nice SX-550 receiver is very simple. I think the driver transistor Q6 has gone phut. Confirm by measuring the DC conditions. Its base voltage is omitted in the schematic but should be 1.2 V. The absence of bias current is a clue for this symptom. The output transistor Q10 is the second suspect. Since there is a bit of voltage over zero I expect a B-C junction to be open. R68 open also possible.

 

[wjc]

The output amp if this nice SX-550 receiver is very simple. I think the driver transistor Q6 has gone phut. Confirm by measuring the DC conditions. Its base voltage is omitted in the schematic but should be 1.2 V. The absence of bias current is a clue for this symptom. The output transistor Q10 is the second suspect. Since there is a bit of voltage over zero I expect a B-C junction to be open. R68 open also possible.

I'm on it, thank you

 

[wjc]

I'm on it, thank you

I have noticed the clipping starts about 1 minute after I turn it on. I placed a fan blowing directly on the AF amp portion, cranked the vol and it hasn't clipped in the last 10 minutes.

 

[mbz]

as @PE9ZZ measure Q6base voltage, Q6emitter voltage and resistance check R68(5.6oms, with amp powered off)

 

[PE9ZZ]

I have noticed the clipping starts about 1 minute after I turn it on.

This is a new development. Does the clipping start suddenly or is the clipping level slowly dropping? Not really relevant though, a bad transistor can cause both. Sudden drop is loss of Q6 collector voltage due to whatever reason. R68 can be checked easily with the amp powered by measuring Q6's collector voltage. R68 phut -> no voltage. Also check PCB for cracks and dry joints. Test DC voltages with no load or signal. I can't think of anything else (famous last words...).

 

[wjc]

This is a new development. Does the clipping start suddenly or is the clipping level slowly dropping? Not really relevant though, a bad transistor can cause both. Sudden drop is loss of Q6 collector voltage due to whatever reason. R68 can be checked easily with the amp powered by measuring Q6's collector voltage. R68 phut -> no voltage. Also check PCB for cracks and dry joints. Test DC voltages with no load or signal. I can't think of anything else (famous last words...).

The unit has been sitting for a long time not used. I've built a few tube amps so I decided to hookup this sx550 to my scope just for fun. If I blow a fan on the amp it doesn't clip out. I suspect Q6 since both the base and emitter show 9 volts. The collector is 27 v.

as @PE9ZZ measure Q6base voltage, Q6emitter voltage and resistance check R68(5.6oms, with amp powered off)

Thanks, I'm going to replace Q6 C26 R68 and R26.

 

[wjc]

I was poking around checking voltages and tapping on transistors - tapped Q10 and bam, the full sine wave popped up. Replaced Q10 and everything seems fine. Thanks for any all all suggestions.