I'll try it and see. Thanks.

I remember adjusting the trimmer in October of 2014, and it didn't change the output level.

Thanks, Mooger5. I'll take it out and have a look at C61, R108, and the vactrol. I was thinking that the fault might be with the vactrol, and that I should try a different one, as there are several similar modern types available. The original one isn't made anymore, and there's no exact equivalent. ...

Hi, Unfortunately, I never did find the exact location the problem, so I stopped working on it soon after the October 29, 2014 post. I had to return the multimeter I borrowed. Maybe one of these days I'll get hold of another multimeter, and have another go at it. But I wish you good luck in your end...

One thing I noticed with the trimpot R101 (for adjustment of output) -- adjusting it doesn't seem to change the output any, no matter what position I turn it to. From what the manual says (p. 10), I assume this pot adjusts the amount of modulated signal to unmodulated signal, but I can't hear any ef...

Also, I examined the Modulation board for broken tracks and joints but didn't find anything.

Yes, the soft volume switches are working normally.

I did audio testing with Q22 and Q23 today, and both are showing a strong, modulated signal at the base and emitter (the left and right legs), but nothing at the collector (the middle leg). I had replaced these two transistors about a month ago with NTE199's, so they're both brand new. I looked onli...

Oops, sorry, Dr. Phibes, I thought I was quoting Yekuku in my response a few minutes ago.
I'll also check for broken tracks/joints in the paths of Q22 and Q23.

So the opto-coupler was not the issue? Bummer. Sorry for dragging you down that blind alley. But did you say the base on Q22 is not showing much? Did you replace Q22 along with Q23? That's okay, Yekuku, your help (and Dr. Phibes') is very valuable to me. I never would have gotten this far without y...

I tested the output of transistor Q23 at the base, it's strong and modulated.

Out of curiosity, I also tested the base of transistor Q22; there's no signal there.

Two of the pink dots are not giving the same output: the leftmost one and the rightmost one. The leftmost one gives a strong, unmodulated signal (a clean signal with no ensemble effect), and the rightmost one gives a strong, modulated signal. I haven't checked the signal at the center pink dot (the ...

I replaced the photocoupler with a VTL5C4 vactrol. It works, but I'm still getting the same weak outputs at the same points as before. The problem must be in some other component. I also replaced the 100K trimpot (R101) that adjusts the modulation output. Again, same results as before. Could the pro...

Yes, I'll look for a new opto-coupler. I tested all the resistors in the preamp section and they all have readings very close to their original specifications. And last month I replaced that capacitor you mentioned, when I replaced all the electrolytics on the Modulation PCB). I've been comparing th...

https://www.flickr.com/photos/128279615@N04/15311899297/in/photostream/ Good news! Using the audio tester you described, I've narrowed down the problem to the preamp section of the Modulation PCB. I've attached an image of the schematic, showing all the points I tested. I think it's either a transis...