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AO-43 amp problem in L122

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  • AO-43 amp problem in L122


  • #2
    Good evening,

    I have an L122 opened in home.

    Verify cinch colors are from left to right Red White Grey.
    Swap the 12AX7.
    Turn volume potentiometer Full CW.
    Pedal Full / Brillant down / Soft Up.
    Tap with your finger the connections to the bright and soft keys (No voltage).

    If still no sound the problem is probably in the output stage.

    J.P.

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    • #3
      Originally posted by ShadyTree View Post
      Having no electrical knowledge at all, I took the AO-43 amp down to my local electronics repair man with the schematics and relevant parts of the manual (including the pages showing the wiring to the tabs and the section dealing with the power and reverb amp in the “general operation” section) and it came back with a clean bill of health.
      if it has power, and a speaker connected, and you get nothing when plugging hi-fi audio into the white connector, then it is obviously not in good health at all.
      Current organs: AV, BC, A-100
      Current Leslies: 22H, 142, 147, 760
      Organs in the past: L-100 (several), M-100 (x2), T-100, E-100, CV
      Other keyboards: Roland FP-4, Yamaha DX7, Yamaha TX81Z, Yamaha Motif ES Rack, Korg Krome, Novation Mininova

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      • #4
        Thanks Enor - I agree that this amp seems to have returned in worse shape from the electronics shop than when it left! Thanks JP - the RCA plugs are ordered red white grey. I cannot get at the potentiometer without removing the amp because of how it now sits in the organ chassis but I will give your test a try when I have time to remove it again and will post the results.

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        • #5
          Well I managed to sort this out in the end. I took the amp to someone experienced in dealing with valve amps. It needed two new capacitors (C311 and C312). The electrician (who was very knowledgeable BTW) also discovered that someone had wired in a pot to control the reverb mix. I decided that it was not a modification that I wanted so when I got the amp back from repair, I removed this to return it to stock condition. There is only one thing that I am curious about now. In the parts layout diagram in the manual there is a resistor (R318) wired onto some tabs from one of the V6 pins (via a capacitor - C305) to the reverb switches This appears on the top graphic of the parts layout diagram (if i were clever enough I would attach a screen grab of it but I'm not sure how to do that!). However on my amp a resistor with the same value was wired between the same tab and one of the terminals to V8. There was also a capacitor (0.01) in precisely the position where R318 was supposed to be in the parts layout diagram. Therefore when removing the pot mentioned above I removed the small capacitor and soldered R318 back into the position indicated in the manual. All works well with the amp and the reverb I and II switches work properly (which they had not done when the pot was still in the amp). The only issue I can detect is a slight pop when reverb I switch is depressed and I am wondering if this has anything to do with that small capacitor. In trying to research this I have found photos of AO-43 amps with the resistor in place as per the parts layout diagram but I have also found a photo of an AO-43 with the small capacitor in place so i am now a bit confused!

          One final point. the only other modification that I can detect in the amp is that someone has removed R309 which the parts layout says is "factory selected to adjust volume". I am assuming this was done to boost the volume of the amp. I can see the values of the resistors that have been used in other AO-43s from the photos I was referring to above. Could anyone guide me on this - is is likely to be a problem to operate the amp without this resistor or should I put a new resistor to match one of the values indicated in the photos that I have to be on the safe side?

          Thanks folks.

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          • #6
            Originally posted by ShadyTree View Post
            One final point. the only other modification that I can detect in the amp is that someone has removed R309 which the parts layout says is "factory selected to adjust volume". I am assuming this was done to boost the volume of the amp. I can see the values of the resistors that have been used in other AO-43s from the photos I was referring to above. Could anyone guide me on this - is is likely to be a problem to operate the amp without this resistor or should I put a new resistor to match one of the values indicated in the photos that I have to be on the safe side?

            Thanks folks.
            No problems, sometimes this resistor is omited.

            If you want to recalibrate the amplifier, first you have to check all the resistor. Those out of the rated value are to be replaced.
            Check also tube for nominals.
            Then you can choose the right value of R309 to make the amplifier into the specs.

            It is a hard job for often a poor result.

            JP

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            • #7
              Thanks for clearing up that second issue JP - I am quite happy with the way the amp sounds now so i will leave as is.

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