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Distortion Issue solved... I hope


Andrew Wilson (awilson40)
Username: awilson40

Registered: 05-2006
Posted on Thursday, November 23, 2006 - 07:54 am:   

Well...I replaced a couple of opamps, cleaned and retensioned tube sockets. Retubed and replaced all the electrolytic caps. and it would still start to distort at low levels.
Then I lifted the preamp board...thinking the distortion was coming from the preamp, and reflowed all the solder joints.
I thought it was OK...then it started again. I had to drop the volume to 2 or below to keep it from distorting.
A scope is vital at this point. I checked the signal at the exit of the preamp section and it was clean all the way up to 7 on the volume. Hum...now what ??
I started checking signals as it flowed through the PI driver section and low and behold...the PI section was where the distortion was coming from.

Ed Goforth was really helping me a lot with this and endured my constant emails, His latest suggestions was to lift legs of the resistors on the driver board and check to see if one is out of specs. So while I'm doing this...much to my surprise, R50, 100k resistor feeding 340v to pin 6 of the PI tube lifted right out...with no heat applied !!!!. Man...it sure looked OK visually.
So I cleaned the legs, resolder it in place and reflowed every joint on the driver board and it
seems OK for now. Hopefully that was the problem all along.
Big Thanks to the guys here and especially to Ed for all the help. I sure hope this fixes it. I'm sick of hauling this heavy sucker home to work on it :-)
Edward Solberg (edward_solberg)
Username: edward_solberg

Registered: 05-2006
Posted on Thursday, November 23, 2006 - 10:53 am:   

thanks for the update, andrew. I don't know if you have read about my recent misadventures with my 210 sixty-five after a pwr-xfmr short in the secondary a/c windings last august. I've replaced every capacitor, diode and many resistors on the driver and rectifier boards since then in a vain effort to "cure" a mysterious overcurrent problem (read "short") that (an assumption) originally put the amp on my bench.

after months of dogged efforts to find the source of my overcurrent, I just needed to put that project aside for a while (i've currently got a 1947 Magnatone, Model 197 to rebuild on my bench...this one is FUN; an ancient class A using tubes I'd never heard of and with parts I'd only read about or seen as a boy on my pop's bench...he was a "tube jockey" until he decided to early-retire in the early '70s because those "damned transistor-things").

the other day, while half-heartedly poking and probing things with power on, I discovered an electrolytic cap (C32 in the bias circuit) that was still a part of the circuit but the solder joint of which had failed. I've belatedly come to the conclusion that I will need to lift all of my boards again (excepting the rectifier board on which EVERYTHING is new) and go through it component by component.... perhaps I will finally discover the source of my overcurrent in this fashion. (I now have the parts in hand to completely rebuild each of the three tube sockets from the ground up with new materials--just in case of need. I'll probably have a brand new 1975/2006 music man amp when I've finished this "repair").

the oddest part of all this is that when it originally occurred, I was glad that the problem wasn't signal but rather power.... I supposed it to be easier to diagnose and cure! this elusive short has magical abilities to "move" once I've homed-in on a target area I suppose to be the fault location. I think my grey hair has silvered in some areas as a result of this project. (and maybe I'm only imagining that my bald spot has grown... )

again, thanks for the report.

ed s.
Jeremy Shaw (jemshaw)
Username: jemshaw

Registered: 09-2006
Posted on Thursday, November 23, 2006 - 03:51 pm:   

Im a reader but infrequent posters of this site and got to say its very encoraging getting these tech tips and hopefully happy ending stories. I am one of those people with a "bit" of knowledge but careful with it. Armed with advise and tips from this site I have fixed up my old 210 65 . After a recapping and cleanout of substances ejected from erupting capacitors (it was still working with same!(very strange distortion, made for a unique recording)) and going over components tapping with a chopstick and finding hidden loose connections it has come back to life and sounds great. Just posting to say lack of postings doesnt mean lack of interest. Thanks to steve and all

RD100 unwanted distortion, help!

I have my RD100 chassis 2100RD on the bench. It is the model with 4 op-amps. It has new filter caps, good tubes and new op-amps.
When I turn it on, the clean channel sounds normal and plenty of headroom. After 5 minutes, It becomes distorted. I tried using freeze spray on all the parts to see if it was a thermal issue with anything. Nothing seemed affected by the freezing. Voltages on the power tubes look good. I'm stumped. Any suggestions?



Try injecting signal into either of the the loop jacks. This will tell you where the distortion may be appearing. Do you have a scope?

Lars Verholt


Yes, I have a function

Yes, I have a function generator and a scope. I will try injecting in the loops. Tried earlier into the input but couldn't tell where the problem is. Thanks!


You'll find it

If you have a scope, you will most likely find the issue. The RD and RP series with the two insert points are great to troubleshoot as they will allow easy access to inject and extract signal at different points of the amp.

Lars Verholt


Problem appears to be solved... we'll see.

Well, earlier I had replaced the 4-1458 op-amps with TI OPA-2134 as I had done in another MM amp. So, I went back and pulled all of them one-by-one and IC4 seemed to be the culprit. After putting the 1458 back in, it cleaned up. So, I'm going to burn it in and test it several times more before reassembling into the cabinet. I'll follow up later.


Problem appears to be solved... we'll see.

I let it burn in overnight with a signal generator, still working normally. Reassembled it into the cabinet. Now no sound. More later.


Problem appears to be solved!

Another problem that I introduced: I made a box as an adapter to be able to unplug the footswitch. Found out that I had inadvertently created a short which muted the amp. Fixed the short and all is well.

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