N72 Fluttering output

[dcboucher]

I have 4 N72s and 2 A12s that have provided years of wonderful results. Unfortunately, two of the N72s have developed a fluttering sound at the output, similar to the sound of a bad tube. I am assuming this would indicate a dying transistor. Is this a safe assumption? I have a new NTE 130 that was rolling around in my spares kit for another piece. Is it safe to chuck that guy in there? NTE crosses the NTE 130 for the 2N3055.

These N72s are rev2, I think.

Thanks

d

[dcboucher]

Changing out the 2N3055 didn't fix it.

[dcboucher]

While I'm at it, any mods to newer revisions you'd recommend?

[tpryan]

Who assembled the N72s?

[dcboucher]

Me, circa 2004

[dcboucher]

Also, they may be my first two that I built. They are in the first two slots in the CH01, closest to the power supply and my guess is that's how I would have kept track of them.

[dcboucher]

All transistor voltages look good. Case of Q6 is 21.4 volts per original instructions. Unable to get 22.2 volts. Maybe that's part of rev3?

Thanks for your help in this matter.

d

[dcboucher]

Not a lot of love for BC184C these days at mouser and the like. Any suggestions if I need parts?

[tpryan]

The first thing to do is to re-flow the solder joints. If you have liquid flux or a flux pen, dab a bit on any and all suspicious looking connections and re-flow. Try exercising the rotary switch through its full range a dozen times or so. Clean the XLRs and check your cables. A loose connection is more likely than a bad transistor.

[dcboucher]

Ok. I'll give it a go and report back. Thanks again.

d

[dcboucher]

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