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AO-28 / B3 Vibrato Volume issues

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  • AO-28 / B3 Vibrato Volume issues


    I'm on the homestretch of re-casing an A100 into a B-2 case. The B2 keybed/etc was in pretty rough shape, but the case cleaned up nicely. The A100 was in a ruined case, but the keybed is unbelievable. The instrument feels brand new and it was sounding amazing! Until last night (ugh).

    While playing with the vibrato "on", I heard a "pop" through my Leslie then I lost most of the volume in the vibrato channel in both manuals.

    First, some more salient info on the instrument:
    • All electrolytics in the AO-28 were changed out in the last year.
    • Vibrato scanner rebuilt within the last year.

    Here's what I've done to troubleshoot so far:
    • Pulled the Leslie kit B+ tube socket extender from the rectifier for now & removed the tube socket extender for the Trek II reverb for now as well (no other add-ons/modifications in the mix)
    • Checked the wiper in the AO-28 housing. This is fine and working as it should (verified on the non-vibrato channel).
    • Wiggled tubes & verified they are properly seated. They were a bit wobbly so I got them tightened up.
    • Verified solder connections on the vibrato line box.
    • Swap V1/V2 in the AO-28. The issues don't follow the tubes. (I also tried other known good tubes for the rectifier / output tubes)
    • Verified the scanner IS properly turning
    • Metering on the C terminal while playing a note with vibrato on, I get under 3 volts. Tested with a high quality multimeter, as I don't trust my old 'scope so I'm not sure that this is a good test.
    • Pulled the amp & test R6/R16. Both are within spec @ 47.35k / 48.2k, respectively.

    I'm just not sure how to rule out either the pre-amp or something going on between the scanner/line box/etc.

    Ideas on where to go next would be appreciated.

    Thanks in advance,

    Last edited by gaker; 06-19-2018, 09:19 PM.

  • #2
    With amps, I know enough to *not* be dangerous and kill myself, but I am still pretty new this. So please stay with me if any of this information is not needed. If I'm going down the wrong path, information as to why is appreciated so I can learn for the next time :)

    For more information in the event someone asks, here are some values of resistors around V1/2

    R7 - 2.3 Meg
    R19 - 2.5 Meg

    R8 - 475k
    R20 - 490k

    There is some upwards drift on the vibrato resistors, but this shouldn't really be enough to care about, right? For voltages, I think they're good. On both tubes, I read ~150VDC on the plate / pin 5. However on V1, I'm reading a bit low on grid 2 / Pin 6 at 52.13 VDC.

    At this point, I think the amp is likely just fine? Perhaps I need to go onto the switches?


    • #3
      Holding a note, I've read over 5 VAC on Terminal C. Have you looked at the 12BH7?



      • #4
        Thanks for the help, Jim!

        On the vibrato half of the 12BH7, I measure the following:

        Pin 1: 309 VDC
        Pin 2: 70.2 VDC
        Pin 3: 130 VDC

        C13 was changed out last year.

        Edit to add: swapping for another 12BH7 yields the same results.


        • #5
          Oh how frustrating. I decided to look at this again over my lunch hour today, and to start with, I flipped the amplifier back over off its side. When I turned it over, the rectifier tube had become loose, so I re-seated it (again). I turn the organ on, and had my meter on the C terminal, & I got 5-8VAC depending on what I was playing. At this point, I think I'm going to stop trying to save the V8 tube socket and just replace it with a new one.

          In an effort to learn the "why" on this, can anyone explain to me how the seating of the rectifier tube could affect only the AC signal of the vibrato channel output of the 12BH7? I had verified the B+ voltage coming out of V6 both before & after the filter caps.



          • #6
            There is no power supply rail that is unique to the vibrato channel. 280V and 290V rails are used in multiple places.

            What are the voltage readings of V2??

            Meter reading on terminal C aside, is vibrato now working? Your last post wasn't clear on that. That much variable AC voltage sounds like it does work. No audio means your problem is downstream. Line box, scanner, or connection in between. Also check that the shielded wires are not making contact with the top terminals of your vibrato line box. That would create a short to ground in that part of the delay line.


            • #7
              Sorry for not being clear. Yes, vibrato is now working & sounds great...I'm going to say it was an issue with the rectifier seating. I'm planning on ordering new tube sockets and switching them out.

              That's a good tip on watching what the shielded wires are touching. Appreciate it!


              • #8
                Originally posted by gaker View Post
                I'm planning on ordering new tube sockets and switching them out.
                Hi Gaker -
                While some folks may disagree, in my experience, changing tube sockets is a lot of labor intensive effort that will gain practically nothing. Look at all of those wires and components to keep up with, unsolder, re-solder. You can clean the insides of the pin sockets with a tiny wire brush or similar, and then squeeze the sides (very slightly and gently) with needle-nose pliers to tighten them. Don't over-do it. I would re-consider.
                Cheers !
                Roger Memphis
                C-3 with O-M, 145, 122RV, 2 PR-40's, PSR-36
                CV with HR-40, 2 B-40's