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Fender Champ AA764 I think '73

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  • #46
    Originally posted by SAG Electronics View Post
    LOL. not quite. So excited that it wasn't blowing fuses that I put it all back together only to find 0 output through the speaker. Put it back on the bench and started checking voltages. Holy cow! seeing way too high voltages everywhere. 480DC where it says I should be reading 205VDC. How can this be? Do I need to be checking with everything connected? Speaker and instrument, but that even makes little sense. Speaker measures 3.7 ohm and cable reads continuity on both signal and ground. At least its not blowing fuses.
    Not clear what exactly you did, but a tube amp must never be powered without a load connected to the speaker output. Otherwise it might oscillate and damage the output transformer (OT).
    When oscillating voltage readings will be off.
    - Own Opinions Only -

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    • #47
      Whenever you see really unexpected voltages, check the battery in your DMM.

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      • #48
        Yes, the heaters are glowing. I believe all the capacitors in that section are bi-polar. I am back to checking all the rest that I can.

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        • #49
          Originally posted by SAG Electronics View Post
          I believe all the capacitors in that section are bi-polar.
          Not clear what you mean. What is "that section"?
          A Fender Champ uses polarized (not bipolar) electrolytic caps in the power supply and as cathode bypass caps as well as non-polarized film/foil caps elsewhere.


          - Own Opinions Only -

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          • #50
            Well. the 6V6 may have been loose. Now the voltages are much better. 296-306 and 14-18. That's a relief. I also found that pin 5 on the 6V6 is not showing 205VDC or anything really. It is coming out of the 12Ax7 at 206VDC, but not getting into 5 on the 6V6....

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            • #51
              Originally posted by SAG Electronics View Post
              I also found that pin 5 on the 6V6 is not showing 205VDC or anything really. It is coming out of the 12Ax7 at 206VDC, but not getting into 5 on the 6V6....
              Reason is that there is a cap between which blocks DC. 6V6 pin5 DCV needs to be zero.
              Last edited by Helmholtz; 01-20-2021, 10:26 PM.
              - Own Opinions Only -

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              • #52
                Ha HA! Indeed the cap in between blocks the DC allowing only the signal (AC) to pass through. I swear that I just read that the other day and here I am trying to problem solve and wondering if I rewired it wrong because it didn't take. I definitely need to find some good books on this stuff, as my education was lacking in this pre-semiconductor technology.

                Sorry for the mis-speak, I meant non-polar caps.

                Any tips for troubleshooting the input section through the 12AX7 then, cuz I still have no output, now that the voltages are correct?

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                • #53
                  The way I see it, I need a way to trace a small AC signal through the inputs and into and out of the preamp tube. Unfortunately I haven't yet obtained an arbitrary function generator. Or am I wayy off and overthinking this?

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                  • #54
                    Most amp problems show in the DC voltages. Are you getting sound by now?

                    Please measure and post all power supply node and tube plate and cathode voltages (all tubes).
                    - Own Opinions Only -

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                    • #55
                      Originally posted by SAG Electronics View Post
                      I haven't yet obtained an arbitrary function generator. Or am I wayy off and overthinking this?
                      Yes, way overthinking. A good arbitrary function gen for you would be, plug a guitar cable into the input and send in a buzz by pressing the tip with your thumb or finger. But first, let's see the info Helmholtz asked for. You have come a long way in the last week. I have a feeling you're almost to the finish line.

                      Enjoy. Every. Sandwich.

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                      • #56
                        When your issue is no sound at all, then most any noise works as a test signal. There are places a nice signal generator is used, like checking specific parameters, viewing distortion etc. But in my shop, most times I just plugged in a line from my little shop stereo. And like Leo said, often as not, I just touch the inpt with a finger.
                        Education is what you're left with after you have forgotten what you have learned.

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                        • #57
                          Thanks Guys! Had to clear my temporary bench for the installation of my new bench-top. Cannot wait! Been waiting since March, when I troubleshot the dude's lazer cutter. I like the radio idea, but I do not have audio outs and/or 1/4 inch connectors in my shop. I love all the help and friendly advice I have been freely given so-far and look forward to our future meetings. Great advice all around!

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                          • #58
                            So, finally back to work. Voltages -- 6V6GT: Pin2 5VAC, Pin3 295VDC, Pin4 296VDC, Pin5 0.007VDC, Pin7 14VDC; 5Y3: Pin2 305VDC, Pin4 272VAC, Pin6 272VAC, Pin8 305VDC; 12AX7: Pin1 199VDC, Pin3 1.128VDC, Pins4 & 5 5.00VAC, Pin6 195VDC, Pin8 0.009VDC

                            Sorry that I didn't put it in tabular format. Still seems to be no signal passing through even when I tap the cable. Thinking to start scoping it.

                            Btw those are all voltages through the limiter.
                            Last edited by SAG Electronics; 02-14-2021, 12:14 AM.

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                            • #59
                              If the amp is not lighting up the limiter, remove it.

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                              • #60
                                Originally posted by SAG Electronics View Post
                                So, finally back to work. Voltages -- 6V6GT: Pin2 5VAC, Pin3 295VDC, Pin4 296VDC, Pin5 0.007VDC, Pin7 14VDC; 5Y3: Pin2 305VDC, Pin4 272VAC, Pin6 272VAC, Pin8 305VDC; 12AX7: Pin1 199VDC, Pin3 1.128VDC, Pins4 & 5 5.00VAC, Pin6 195VDC, Pin8 0.009VDC

                                Sorry that I didn't put it in tabular format. Still seems to be no signal passing through even when I tap the cable. Thinking to start scoping it.

                                Btw those are all voltages through the limiter.
                                Voltages on the LBL are always low and hard to interprete. Please re-measure without it.
                                Include the voltages on all 3 filter caps (power supply nodes) and don't forget the all important 6V6 cathode voltage, which shows if the tube is operating properly.
                                - Own Opinions Only -

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