SWR SM 400 left channel distorted

[lowell]

The right speaker out is good, left bad. This unit has 2sd424 power transistors in it. I am getting +58v on collectors and -58v on emitters. No DC on output. Not sure where to start w/ this one.

[lowell]

here's power supply.

[olddawg]

First of all have you swapped the input signal to make sure it isn't the source and swapped speakers to make sure it isn't the load? Have the output transistors been replaced? I'm not saying to replace them, but previously shorted outputs can cause problems in the driver circuit, etc. The first thing that I would do is meter out the low ohm resistors in that channel. All of the 100 ohm, 120, the emmiter resistors, etc. The great thing is that you have a working channel. Start at the begining and compare the voltages on the active components until you find the difference between the working channel and the distorted one. That should tell you a lot.

[lowell]

I am using the same speaker for all tests. It wouldn't be the speaker or input signal if one channel works and the other doesn't. I will check some of those voltages/components. thanks.

[lowell]

Ok found that R6 2.7k 1w was fried on left power amp board. All other components LOOK ok. I replaced it and it's still distorted. What might be the someone's educated guess at why that resistor fried? R5, C10, and D3 measure fine so I wouldn't think the differential was the issue. C7 does not seem to be open either.

[Enzo]

Power comes in through Q4, so if R6 burnt open, I'd be more suspicious of a shorted C10, and check the health of R15, Q7, Q8, Q9, and R29,30,31.

Are you sure this is the right schematic? 2SD424 are TO3, and the schematic shows the 2SC3264 flat types.

[lowell]

The only other schem I find online is http://www.schematicheaven.com/newam...sm400_bass.pdf and it still doesn't have the 2SD424's in it. In any case these in fact ARE TO-3 types. I'll check those things out.

[Enzo]

I was unclear. Yes 2SD424 ARE TO3, but your schematic uses transistors that are not, which makes me think it is not the correct one. Try this older one instead:


This older drawing doesn;t number all the parts, but the circuit is similar enougjh, use the part numbers from the wrong drawing to find the equivalent parts in this old one.

[lowell]

Enzo,
All checks out good except the drivers 2n3440 and 2n5416. thanks for that schem it's the right one. The drivers tested good except for the b/e junction of 2n3440 and the b/c junction of 2n5416. i'm ordering them now.

[lowell]

should i replace the outputs too? maybe they were stressed? they test fine as well as the ballast resistors.

[Enzo]

Did you pull them and test them? There are parallel semiconductors in the circuit.

[lowell]

you mean the outputs or the drivers? I pulled the outputs, not drivers.

[Enzo]

I am sorry, I was unclear. You said you got funny readings on the two drifver transistors. I asked if you removed them to test, because while in circuit they have parallel parts that can easily confuse your meter.

For example, the 2N3440 Q7 has across its emotter to collector, the base to collector junction of the output transistor Q9.

I am not suggesting you are wrong, just wanting to be sure.

[lowell]

i'll double check and pull them. would they test "open" IN circuit?

[Enzo]

No, in circuit, they should test like any transistor, but once you get funny readings, then you should pull it to be sure there was nothing in the circuit messing with your readings. Things like parallel paths, or stored small residual charge voltages in filter caps, etc.