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Old 2nd May 2005, 04:56 PM   #1
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Default help trouble shooting amp distortion

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
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Old 2nd May 2005, 05:10 PM   #2
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Default Re: help trouble shooting amp distortion

Quote:
Originally posted by mikebarg
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?

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Old 2nd May 2005, 07:44 PM   #3
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Default 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.
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Old 2nd May 2005, 08:38 PM   #4
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Is there a DC output voltage on the distorting channel, even with no input signal?

Jan Didden
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Old 2nd May 2005, 09:09 PM   #5
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Default Re: help trouble shooting amp distortion

Quote:
Originally posted by mikebarg
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.
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Old 3rd May 2005, 02:10 PM   #6
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Default DC output voltage readings

The DC output voltage on the distorting channel with no input signal is .083V. I cheched the other channel for comparision - it reads .039V.
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Old 3rd May 2005, 07:53 PM   #7
ingrast is offline ingrast  Uruguay
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Default Schematic

Can you draw and post a schematic (even rough)?

We can help direct you to make some measurements and comparisons for the most likely points to show telltale differences in the good and the bad units.

Rodolfo
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Old 3rd May 2005, 08:39 PM   #8
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maybe distorting chanel is simply underbiased?
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Old 3rd May 2005, 10:02 PM   #9
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Default schematic

I am attaching a jpg of the power amp schematic. I just got it today.
Attached Images
File Type: jpg sm400schem2.jpg (75.8 KB, 262 views)
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Old 3rd May 2005, 10:20 PM   #10
ingrast is offline ingrast  Uruguay
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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
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