Bad Coupling Capacitor

[rockpassion]

Hello all,

I had this problem about 6-8 months ago.  The problem is I keep getting crackling and thumping out of the right channel.  At first I would move the 300B around and it seemed to stop the issue.  But it got progressively worse.  Paul Birkeland told me to switch the 300B's to see if the problem was the tube.  When I switched the 300B's the issue did not follow the tubes.  I have tried changing the cables and that also did not have any affect. 

I have remeasured all the outputs per the build instructions and have found a some readings that do not appear to be right.  These are my findings:

Terminal 7 fluctuates between 3.30 to negative -.60.  Its rise and fall of voltage is not consistent.  Most of the time it fluctuates between .35 and -.60.  But if you measure it long enough it will rise for a short period then drop back.  The right coupling capacitor is connected to this terminal.  Is this an indication that the coupling capacitor is leaking and beginning to fail. 

Pin/Terminal D3 is 5.662
Pin/Terminal D7 is 180.8
Pin/Terminal D9 is 181.0

All the other readings were either right on or within the +/- 10% tolerance. 

I read on Audio Asylum  a similiar situation.  Mike Samra indicated that a coupling capacitor could be passing DC voltage which could be causing the Thump and Crackling sound.  Any comments

Hopefully, in the next couple of weeks I will receive some Russian caps I ordered from Bulgaria that I planned on replacing these 10uf just to see how they sound.  Should I wait for these and do the replacement before I put the BeePre back into my system.  By the way the thumping is not dependent on volume.  The thump is the same intensity no matter how high I turn up the volume.   I have a BH Foreplay II which I am currently using and there is no crackling or thumping.

Thanks

[Paul Birkeland]

If it was not the 300B tubes, I would try swapping EL84's.

If it's neither of those, it is almost certainly a solder joint.

A coupling capacitor that has failed will show a consistent positive voltage at the output that will not vary all that much.  In a BeePre, if you saw something like 5V DC at the output of the capacitor, then I would be concerned. 

[rockpassion]

Paul, 

Thanks, I did swap the EL-84's and no change.  I will try resoldering everything again.  On the one hand I am glad it is not the capacitor but it would have been so much easier if it were. 

Regards