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[SOLVED] Loud POP sound when swicthing audio filters i a vintage audio amplifier

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hafrse

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Hello,

I am trying to repair a nagra DSM audio amplifier, it pops when switching the audio filter selector switch S2. Checked for coupling caps on the filter board, no short circuit there. Any clue what the reason can be?
Uploaded the schemtics of the amplifier, filter board is on the upper right side. Not sure if the problem from this board or somewhare else.
Thanks
 

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Probably leak current of one of the electrolytic capacitors in filter board. You'll check the DC voltage at selector switch inputs, there should be zero difference.
 
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    hafrse

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Hello,
Thanks, I replaced all the electrolytic caps on the filter board: C3, C10 and C4. Now I only have a loud pop when switching between FLAT postion and Max. Loudness position, switching between Max. Loudness and Speech no or external speeker no pop sound. Checked for DC, there 0 voltage on all the 2 outputs of the filter board: #2 is for the Max. Loudness correction and #3 is for the FLAT correction.

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If I leave the amplifier for 2 min, the POP sound goes away gradually until no pop is heared.
 

Checked for DC, there 0 voltage on all the 2 outputs of the filter board
Click to expand...
According to schematic, -0.6V (equal to TP8) is expected.
 
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    hafrse

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Correct, that is the input #4 to the filter board, checked, it is -0.6V , the outputs #2 and #3 is 0V after the coupling caps. But why this issue clears after about 2 min ? I
 

Problem solved: the new electrolytic cap to replace the C10 on the filter board was bad!replaced it with another one, all ok now.
 

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