help trouble shooting amp distortion

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I have an SWR bass amp. 200 watts per channel/ 2 channels that uses 4 2sd424 power transistors in each channel. i'm trying to
trouble shoot a problem. One channel sound great. The other channel has distorted audio. I've looked at the board for obviously burnt parts. I check the 2d242 transistors with a VOM. I have gone over the board tryed to check diodes and some resistors on the board. I'm at a loss as what to look for now. It has 7 ceramic disk caps and 4 electrolitics. Could caps be the problem or is it more likely the small transistors on the board. Any trouble shooting tips or ideas would be greatly appreciated. I have a VOM and a cap checker but I don't Have a scope :(.


Thanks
 
mikebarg said:
I have an SWR bass amp. 200 watts per channel/ 2 channels that uses 4 2sd424 power transistors in each channel. i'm trying to
trouble shoot a problem. One channel sound great. The other channel has distorted audio. I've looked at the board for obviously burnt parts. I check the 2d242 transistors with a VOM. I have gone over the board tryed to check diodes and some resistors on the board. I'm at a loss as what to look for now. It has 7 ceramic disk caps and 4 electrolitics. Could caps be the problem or is it more likely the small transistors on the board. Any trouble shooting tips or ideas would be greatly appreciated. I have a VOM and a cap checker but I don't Have a scope :(.

Thanks


Did you switch inputs to see if it is the source? Did you switch speakers to see if it is the speaker?
Can you measure supply voltages, to see if they are the same at each channel? Any fuses?

Jan Didden
 
what I've checked

I have checked the supply voltage to each of the boards and it reads the same. I have used the same speaker on both channels and there is a fuse in the output to the speaker. I have swithced the fuse from the good channel to the distorted one.
I checked the pre-amp output on both channels and it is not distorted.
 
mikebarg said:
I have an SWR bass amp. 200 watts per channel/ 2 channels that uses 4 2sd424 power transistors in each channel. i'm trying to
trouble shoot a problem. One channel sound great. The other channel has distorted audio. I've looked at the board for obviously burnt parts. I check the 2d242 transistors with a VOM. I have gone over the board tryed to check diodes and some resistors on the board. I'm at a loss as what to look for now. It has 7 ceramic disk caps and 4 electrolitics. Could caps be the problem or is it more likely the small transistors on the board. Any trouble shooting tips or ideas would be greatly appreciated. I have a VOM and a cap checker but I don't Have a scope :(.


Thanks

Hi you should check all diodes and any offset voltage. A large offset is usually an indication of a problem.
2SD424 by Toshiba may also have shorts by aluminium of the case peeling over the gap to the emitter and base of the TO-3. The gap is rather small and the Alu alloy rather soft of these types. This his plagued Ampzilla amplifiers, long ago. :idea:
 
schematic

I am attaching a jpg of the power amp schematic. I just got it today.
 

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Fine, I have no time now to check, will be back tomorrow but most probably others will jump in before that.

Just for a quick check, measure with no signal, and compare among units:

-Voltage across emitter and collector of Q4

If different, **very slowly** adjust the bias preset in the failing unit and check sound.

-Base to emitter voltage of Q5 and Q6

Rodolfo
 
Re: Voltage across emitter and collector of Q4

mikebarg said:
I checked the E-C voltage on Q4. On the good board it's 2.09v and on the bad board it's 0.0.


The distortion you experienced is crossover, the amplifier does not work at all with low level signals.

If you know how to do it, verify that Q4 is good, or replace it with a known good device. Before turning on after replacement, turn the bias potentiometer wiper all the way *towards* the point of connection to the base of Q4 and after turning on, check de voltage and adjust *slowly* to 2V, but take your time and verify the heatsink temperature in comparison with the good amplifier, all this with no signal.

If Q4 is good, we must look for other failed components, please come back with results from the above procedure before.

Rodolfo
 
May I suggest monitoring the output current directly when adjusting the bias, just measure the voltage across the output 'emitter' resistors - say from Q9 emitter to Q11 collector.

Oh - measure the good channel first. Then adjust other to match, then cook for a time and monitor for drift, readjust, and monitor - many amps drift some until warmed up.

Sounds like Q4 is shorted, but why??? Better look a little
alternatively protection is shorted (or turned on), but that would have to include shorted diodes as well as Q4 and Q5.

Something about the way that schematic is drawn makes it hard to follow.
 
B Cullingford said:
May I suggest monitoring the output current directly when adjusting the bias, just measure the voltage across the output 'emitter' resistors - say from Q9 emitter to Q11 collector.

Oh - measure the good channel first. Then adjust other to match, then cook for a time and monitor for drift, readjust, and monitor - many amps drift some until warmed up.
.....

It is a good advise.

It should also be worth visualy checking connections, for the short may be elsewhere though less likely.

Rodolfo
 
Re: Q4 tests OK

mikebarg said:
I pulled Q4 and tested with a VOM. It looks OK.
I checked the diodes in circuit. Nothing immediately obvious.


Just in case, did you verify there is no continuity between emitter and collector? Often you measure the b-e and b-c junctions as OK but e-c is in short.

Next thing is, with the amplifier unplugged, measure continuity between emitter and collector pads of Q4. If there is a short, and Q4 is not yet soldered in place, you must follow both visually and with the VOM, the path from collector to emitter along all paralell branches carefully cutting traces until the short or failed component is located. Then resolder the cut traces.

You must end with a polarity dependent resistance between collector and emitter, similar to the one measured in the other board (unpowered)

Rodolfo
 
Re: Q4 test -EC

mikebarg said:
I checked E-C in both direction, no short.


Good.

Go on as suggested in the previous post, before replacing Q4. You may want to compare resistances (unpowered) along the various branches going from Q4 collector and emitter, with the ones measured in the good board.

If you cut traces, be careful when reconnecting to scrap the solder mask and lay bare copper before resoldering.
(sorry if all this is obvious to you, I do not know your experience)

By the way, did this amplifier work before or were they just assembled?

Rodolfo
 
anatech said:
Hi mikebarg,
Check Q5 and Q6 for shorts and the two diodes in series with these.

-Chris


You are right, but don't you think highly it unlikely to have all four parts shorted at the same time to yield a 0V result?

It may happen though.

I will post more things to verify once the previous checks are done and without positive results.

Rodolfo
 
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