Weak Channel + Crosstalk

for the DIY ST35, the Dynakit and every other PP EL84

Weak Channel + Crosstalk

Postby SteveH » Wed Dec 24, 2014 6:34 am

All,
My faithful ST35 (Rev B) has given me nearly daily service since I completed it almost 8 years ago (banana) However last night the R channel was weaker than the left, and upon unplugging one of the inputs I noticed some strong cross talk, which was not occurring previously.
Considering the hours on the unit, I assume I may have a bad driver tube....and while Im at it I also ordered new power tubes as well. However, the crosstalk leads me to believe something more sinister may be afoot - perhaps a bad PSU cap ? I do have mine setup for tube rectification.

Thanks and congrats to Shannon - these boards were truly built to last !

-Steve
SteveH
KT88
 
Posts: 422
Joined: Thu Dec 14, 2006 5:38 pm
Location: Delaware

Re: Weak Channel + Crosstalk

Postby Shannon Parks » Wed Dec 24, 2014 9:56 am

If the disconnected input was unloaded, the crosstalk is probably due to the high impedance V1 stage and not really a concern. I'd just swap out the tubes when the new ones come and retest.

Shannon
User avatar
Shannon Parks
Site Admin
 
Posts: 3762
Joined: Tue Mar 18, 2003 5:40 pm
Location: Poulsbo, Washington

Re: Weak Channel + Crosstalk

Postby SteveH » Fri Jan 02, 2015 9:07 pm

Got a chance to play with the amp a bit this evening. Put new tubes in and biased it up. Unfortunately no change. I noticed that the right channel is not only weak, but has that crackling sound on the mids and lows...like when you turn the amp off and it's powering down. But the LH channel is clear. I replaced the coupling caps - but that didn't rectify the situation.
SteveH
KT88
 
Posts: 422
Joined: Thu Dec 14, 2006 5:38 pm
Location: Delaware

Re: Weak Channel + Crosstalk

Postby battradio » Fri Jan 02, 2015 10:10 pm

Check the bias on the bad channel
Mark
Image
User avatar
battradio
KT88
 
Posts: 927
Joined: Tue Mar 10, 2009 12:58 am
Location: near ST.Louis MO.

Re: Weak Channel + Crosstalk

Postby SteveH » Sat Jan 03, 2015 7:06 am

Mark,
I biased the amp when I installed the new tubes. Everything is steady at 350mV.
At this point Im thinking I may put a signal generator on the amp and trace the path with a scope. Hopefully this will show me where Im getting the issue ?

Thanks,
Steve
SteveH
KT88
 
Posts: 422
Joined: Thu Dec 14, 2006 5:38 pm
Location: Delaware

Re: Weak Channel + Crosstalk

Postby Shannon Parks » Sat Jan 03, 2015 10:52 am

SteveH wrote:Mark,
I biased the amp when I installed the new tubes. Everything is steady at 350mV.
At this point Im thinking I may put a signal generator on the amp and trace the path with a scope. Hopefully this will show me where Im getting the issue ?

Thanks,
Steve


I think the weakest link in a single channel issue would be the cathode bypass caps on the EL84s, particularly the Xicon ones from the old parts list. I'd change these out to 647-UVR1V471MPD - a cheap fix, but a little bit of a PITA to replace.

Shannon
User avatar
Shannon Parks
Site Admin
 
Posts: 3762
Joined: Tue Mar 18, 2003 5:40 pm
Location: Poulsbo, Washington

Re: Weak Channel + Crosstalk

Postby SteveH » Sat Jan 03, 2015 12:48 pm

Thanks ! I'll order some up and replace them when I replace the coupling caps w/the proper ones.
SteveH
KT88
 
Posts: 422
Joined: Thu Dec 14, 2006 5:38 pm
Location: Delaware

Re: Weak Channel + Crosstalk

Postby SteveH » Thu Jan 08, 2015 8:27 pm

Well - I was successful in finding the issue - unsuccessful in fixing it.
Shannon was right, the bias caps were shot. Unfortunately they ripped the trace off of the PCB when I removed them. I will do a closer exam tomorrow to see if the board can be salvaged- but right now its looking mighty tempting to just stuff a rev E.
SteveH
KT88
 
Posts: 422
Joined: Thu Dec 14, 2006 5:38 pm
Location: Delaware

Re: Weak Channel + Crosstalk

Postby Shannon Parks » Fri Jan 09, 2015 12:26 pm

Steve, I'd just leave 1" or so length leads on the new caps and connect from pin 3 of the EL84s (+) to a part of the ground plane (-). The Rev B has ground plane all over the place, so you can just scrape up the blue solder mask with a knife and then tin a spot for the cap (-) connection. Also, pin 3 isn't numbered on the EL84s, but Pin 1 is the square pad, so count two more clockwise.

Shannon
User avatar
Shannon Parks
Site Admin
 
Posts: 3762
Joined: Tue Mar 18, 2003 5:40 pm
Location: Poulsbo, Washington


Return to stereo 35

Who is online

Users browsing this forum: No registered users and 1 guest