help trouble shooting amp distortion

mikebarg · 2nd May 2005, 04:56 PM

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

janneman · 2nd May 2005, 05:10 PM

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?

Jan Didden

mikebarg · 2nd May 2005, 07:44 PM

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.

janneman · 2nd May 2005, 08:38 PM

Is there a DC output voltage on the distorting channel, even with no input signal?

Jan Didden

Elso Kwak · 2nd May 2005, 09:09 PM

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.

mikebarg · 3rd May 2005, 02:10 PM

The DC output voltage on the distorting channel with no input signal is .083V. I cheched the other channel for comparision - it reads .039V.

ingrast · 3rd May 2005, 07:53 PM

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

darkfenriz · 3rd May 2005, 08:39 PM

maybe distorting chanel is simply underbiased?

mikebarg · 3rd May 2005, 10:02 PM

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

ingrast · 3rd May 2005, 10:20 PM

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

2nd May 2005, 04:56 PM	#1
mikebarg diyAudio Member Join Date: Apr 2005 Location: texas	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

2nd May 2005, 07:44 PM	#3
mikebarg diyAudio Member Join Date: Apr 2005 Location: texas	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.

2nd May 2005, 08:38 PM	#4
janneman diyAudio Member Join Date: May 2002 Location: Where Germany, The Netherlands and Belgium meet Blog Entries: 6	Is there a DC output voltage on the distorting channel, even with no input signal? Jan Didden __________________ /Another new issue: Linear Audio Volume 3!

3rd May 2005, 02:10 PM	#6
mikebarg diyAudio Member Join Date: Apr 2005 Location: texas	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.

3rd May 2005, 07:53 PM	#7
ingrast diyAudio Member Join Date: Sep 2004 Location: Montevideo	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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3rd May 2005, 08:39 PM	#8
darkfenriz diyAudio Member Join Date: Jun 2004 Location: Warsaw	maybe distorting chanel is simply underbiased?

3rd May 2005, 10:20 PM	#10
ingrast diyAudio Member Join Date: Sep 2004 Location: Montevideo	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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