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Are my Coupling Caps leaking DC?


 :
5/24/2000 2:05 PM
Mark
Are my Coupling Caps leaking DC?
When I measure the voltages after the first .022 coupling cap* (feeding the 3.3M/10pf reverb splitter network) in my Princeton Reverb, I get some DC fluctuating between 4mv and 18mv with the amp idling. When I play a note, DCV shoots up to several volts. I thought that there should only be AC at this stage and no more than 5mv DC. But after replacing this cap several times with new O'drops I get the same results. This is either:  
 
a) normal and operating within spec  
b) Sprague is having a QC problem  
c) there's something else going on to cause DC here  
 
The distortion in my PR is ugly and I'm hoping the answer is b) cause I have no other leads right now.  
 
Can anyone clue me in on this?  
 
 
 
* When measuring, the cap is in the circuit as opposed to pulling out one end. I get 0-2mv readings after the tone caps.
 
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5/25/2000 6:08 PM
Gil Ayan

quote:
"When I measure the voltages after the first .022 coupling cap* (feeding the 3.3M/10pf reverb splitter network) in my Princeton Reverb, I get some DC fluctuating between 4mv and 18mv with the amp idling. When I play a note, DCV shoots up to several volts. I thought that there should only be AC at this stage and no more than 5mv DC. But after replacing this cap several times with new O'drops I get the same results. This is either:  
 
a) normal and operating within spec  
b) Sprague is having a QC problem  
c) there's something else going on to cause DC here  
 
The distortion in my PR is ugly and I'm hoping the answer is b) cause I have no other leads right now.  
 
Can anyone clue me in on this?  
 
* When measuring, the cap is in the circuit as opposed to pulling out one end. I get 0-2mv readings after the tone caps.  
"
 
 
Mark, remeber that DC could be traveling backwards as well. The .003uF coupling disc, coming off the reverb return stage plate, could be leaking DC, which could tarvel backwards through the 3.3Meg resistor until you finally see it on the .022uF coupling cap...  
 
Gil
 
5/25/2000 6:21 PM
Mark

Thanx Gill for your comments. That .003 cap is about the only one I didn't replace in my amp. I'll definitely check it out. I didn't realize DC could travel the other way!  
 
In order to rule out the coupling cap, I guess then that should disconnect the leg going to the next stage and measure DC there instead of leaving in the circuit.
 
5/25/2000 8:18 PM
Gil Ayan

quote:
"Thanx Gill for your comments. That .003 cap is about the only one I didn't replace in my amp. I'll definitely check it out. I didn't realize DC could travel the other way!"
 
 
Hmm... DC likes if both ways. :)  
 
Furthermore, if that cap is leaking, then the dry/reverb mixing stage could be biased real funky, and that may explain the odd sounds you're getting out of that amp.  
 
Gil
 

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