by dcboucher » Wed Apr 09, 2014 3:38 pm
So I had to put the N72 problem on the back burner to get back to making records, so it's been a while. With the current break in my schedule I decided to try to get the bad modules back into service and what did I find?
Now all 4 N72s have the same fluttering sound on their outputs. I reflowed all the joints and checked again - no change. I desoldered with my hakko 880 and resoldered the entire board - no change. With the scope I went through the gain stages and the fluttering begins to appear at the collector of Q2 and appears on all pins of Q3. I replaced some electrolytic caps - still no change. All my voltages are slightly low but good in relationship to each other, I believe.
The A12s in the same CH01 (with PS02) are clean.
My transistor voltages are:
Q---C-----B------E---
1--3.75--2.3----1.8
2--12.0--3.75---3.15
3--21.9--12.0---11.4
4--3.4----0.9----0.3
5--22.3--3.4----2.85
6--22.3--2.8----2.3
output of U1 is 23.7
I believe all my boards are rev 2. I built them in 2004-2005.
They worked flawlessly for a number of years, so I am not sure how much the build quality (meaning my work) can really play into it. It seems like they would have had some problem at the point I built them. Anybody got any ideas? Also, I could really use a schem that matches my board. The current one clearly doesn't.
Thanks,
d