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AO28 preamp vibrato channel issues

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  • AO28 preamp vibrato channel issues

    This AO28 is acting up.The vibrato channel has gone fuzzy and lost volume.The scanner is fine,so are the switches.Somewhere between R13 and R20 and I suspect C9 no longer blocking enough AC to keep that channel loud and clean.Tested tubes with EICO635 and ears,everything good there.
    Is the replacement for C9 a larger voltage now? Can I clip on a test cap at C9 to give it a listen? Can R14/R16/R18 stack off spec reduce the volume and make it fuzzy?
    C9 at 30uf/25V became a larger voltage rating in this instance that solved this before? Maybe I used a 33uf/50V ?
    Will check for 130VDC on pin 5 of V2? 12BH7 also checks out good.Any advice on where to look first?
    Thanks in advance
    cheers
    Peter
    A100/251 A100/147 A102/222 B2/142 BV/147 BCV/145 M3/145 M102/145 M111/770 L101/760 T222/HL722 M111/770 no B3/C3!

  • #2
    Also check for screen voltage (60 vdc) on pin 6. I pull V1 and compare the two.
    The cathode circuit C9 & R17 is common to V1,2 & 3.

    Jim

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    • #3
      How old is this AO28? What vintage/era?

      I'm sure you know always to check the 6AU6 plate and screen resistors and replace if suspect.

      C9 became 100uF in late production, but 25V is more than enough. C9 does not block AC. It bypasses R17 at audio frequencies and blocks DC. Like Jaim says, it's common to both channels.
      I'm David. 'Dave' is someone else's name.

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      • #4
        David,
        Preamp is 1961 silver transformer in an A102,not mine.Not mine,studio organ.
        Jaim,
        Will verify those 60V and 130V pins.Looks like I should see 1.7V at the B tab?

        Thanks both Jaim and Dave who are great techs!

        AO29 in an M3 also has a C9 in the same location that I had issues with a few years back.
        Looks like the yellow wire with 'triangle' mark off the multi section can caps behind the doghouse is where I should tack a 'tester' on and listen,after verifying voltages and resistances?
        The problem M3 worked fine until it didn't one day. Reflowing solder got it going briefly,then I replaced C9 with a .33uf/50V and it worked.
        This AO28 in an A102 mimics the low distorted vibrato/chorus channel of that incident exactly!
        Last edited by Sweet Pete; 09-29-2022, 06:29 PM.
        A100/251 A100/147 A102/222 B2/142 BV/147 BCV/145 M3/145 M102/145 M111/770 L101/760 T222/HL722 M111/770 no B3/C3!

        Comment


        • #5
          Hello gentlemen,

          I have HAMMOND model B preamplifier (possible with serial number S4294) and looking for technical help. Can anyone tell me how big the output voltage (Vrms, Vpp, dBV, etc) is from the matching transformer, or what is the input signal range of preamplifier.

          Thank you in advance.
          Sincerely,
          Vitaly.

          Comment


          • #6
            Hi, Vitaly -- please make a new topic with your question, or you likely will not get any help.

            Wes

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            • #7
              Thank you Wes!

              Hello gentlemen,

              I have HAMMOND model B preamplifier (possible with serial number S4294) and looking for technical help. Can anyone tell me how big the output voltage (Vrms, Vpp, dBV, etc) is from the matching transformer, or what is the input signal range of preamplifier.

              Thank you in advance.
              Sincerely,
              Vitaly.‚Äč

              Comment

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