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Fender Champ AA764 I think '73

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  • #61
    Okay, off the limiter and was a bit scary, but I persevered.

    Voltages: 5Y3 -- Pin1 NC, Pin2 4.8VAC, Pin 3 NC, Pin 4 348VAC, Pin5 NC, Pin 6 347VAC, Pin 7 NC, Pin8 387 VDC; 6V6GT -- Pin1 NC, Pin 2 (heater) 6.35VAC, Pin3 372VDC, Pin 4 382 VDC, Pin 5 --, Pin 6 NC, Pin7 Gnd., Pin 8 (cathode) 23.3VDC; 12AX7 -- Pin 1 245VDC, Pin 2 --, Pin 3 1.75VDC, Pins 4 + 5 (heaters) 6.35VAC, Pin 6 239VDC, Pin7 Gnd., Pin 8 1.73VDC; Filter Caps -- 1 387VDC, 2 382VDC, 3 361VDC.

    Everything seems O.K. to me. Just no output! What do Y'all think?

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    • #62
      Ok, now test the OT: https://music-electronics-forum.com/...7&d=1590953469
      - Own Opinions Only -

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      • #63
        O.K. so i built the tester, and got bright flashes on the amp side, but dim flashes on the speaker side. Is this a sign that the O.T. is bad, or just that the resistances are higher on the output (speaker) side? I tried this test on both the original and the replaced O.T. and they both react the same.

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        • #64
          Originally posted by SAG Electronics View Post
          O.K. so i built the tester, and got bright flashes on the amp side, but dim flashes on the speaker side. Is this a sign that the O.T. is bad, or just that the resistances are higher on the output (speaker) side? I tried this test on both the original and the replaced O.T. and they both react the same.
          So the OT has no shorted turns. An other OT fault could be a short between primary and core or secondary. Use an Ohmmeter to check.
          No difference with replacement OT?

          Are speaker, speaker cable and speaker jack (contacts) good? Do you get any sound from the speaker like hum or hiss or when switching on and off?

          Do you have a scope?
          Last edited by Helmholtz; 02-17-2021, 10:06 PM.
          - Own Opinions Only -

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          • #65
            Lol. I was afraid that i had blown them both with my inexperienced fumblings. No hiss at on and off. Resistance at speaker 2.9 ohms. continuity through the cable in both leads. Yes, I have a scope that I have never used, but back to the signal generation problem.

            Old OT no shorts between primary and secondary. Primary resistance 274 ohms secondary .2 ohms

            New Ot no shorts between primary and secondary. Primary resistance 263 ohms secondary .2 ohms.

            I will see if I can scrounge up connections from a audio out on my P.C. to a cable to the amp.

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            • #66
              Now let's check the power stage.
              Find the 6V6 heater pin (#2 or #7) which carries the 6.3V heater voltage wrt ground.
              Now connect one lead of your ACV meter to the heater pin with the 6.3VAC and briefly touch the grid pin #5 with the other meter tip.
              You should hear a strong mains frequency hum from the speaker.
              - Own Opinions Only -

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              • #67
                60Hz right? I have not managed to put it back together yet to test. On a whim I decided to try using my power supply to test the OTs. I was surprised to see no induced current on the speaker side while i was putting 13.5VDC through the primary. Then I tried going from the speaker side (supply attached there) and voltages came up registering in AC on the primary. Weird. about 33.5VAC actually.
                Going to put it together and try this new test soon.

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                • #68
                  Originally posted by SAG Electronics View Post
                  60Hz right? I have not managed to put it back together yet to test. On a whim I decided to try using my power supply to test the OTs. I was surprised to see no induced current on the speaker side while i was putting 13.5VDC through the primary. Then I tried going from the speaker side (supply attached there) and voltages came up registering in AC on the primary. Weird. about 33.5VAC actually.
                  Going to put it together and try this new test soon.
                  Not sure what you did. But transfomers can't transfer/transform continuous DCV. It can transfer pulses, though.
                  Last edited by Helmholtz; 02-19-2021, 09:47 PM.
                  - Own Opinions Only -

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                  • #69
                    Did the 6v6 pins thing. no sound at all. Just warm tubes. Sorry about my forgetting how transformers work. Yes, the voltage spikes were as i dialed up and down the voltages from my supply. I did actually manage to prove that the speaker can produce sound, though. And to top it all off, my primary p.c. just died and I have to do all this with a raspberry pi now.

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                    • #70
                      So, I have this thing hooked up to my 4 channel o-scope running a 1000 hz test tone though the headphone out on my P.C. speakers. Ch 1 is on 12AX7 pin 2 showing my input signal, channel 2 is on 12AX7 pin 1 showing an inverted and amplified sine wave. Channel 3 and 4 I am not sure where to put. I currently have Ch 3 on the resistor going in to Pin 8 on the 6V6GT shoving nearly input level and about 60 degrees out of phase, Ch 4 is on the output to the speaker showing virtually nothing. Please advise.

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                      • #71
                        Originally posted by SAG Electronics View Post
                        So, I have this thing hooked up to my 4 channel o-scope running a 1000 hz test tone though the headphone out on my P.C. speakers. Ch 1 is on 12AX7 pin 2 showing my input signal, channel 2 is on 12AX7 pin 1 showing an inverted and amplified sine wave. Channel 3 and 4 I am not sure where to put. I currently have Ch 3 on the resistor going in to Pin 8 on the 6V6GT shoving nearly input level and about 60 degrees out of phase, Ch 4 is on the output to the speaker showing virtually nothing. Please advise.
                        No need for 4 channels. Do you see signal at the 6V6 grid (pin 5) with volume fully on? Signal at 6V6 pin 8 indicates a bad cathode cap (25F).
                        - Own Opinions Only -

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                        • #72
                          I have moved channel 4 to 6v6 pin 5 and see a nice sine wave In phase with the signal. Channel 4 is now at pin 3 and also shows a sine wave about twice the amplitude. No sound from the speaker. Wait, volume now at full there is a quiet sound from the speaker 1000 Hz. Tinny and far away sounding.

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                          • #73
                            Originally posted by SAG Electronics View Post
                            I have moved channel 4 to 6v6 pin 5 and see a nice sine wave In phase with the signal. Channel 4 is now at pin 3 and also shows a sine wave about twice the amplitude. No sound from the speaker. Wait, volume now at full there is a quiet sound from the speaker 1000 Hz. Tinny and far away sounding.
                            Single channel is all you need for this simple troubleshooting.
                            Replace 6V6 cathode cap (observe polarity).
                            Then scope 6V6 plate (pin 3) signal ( AC coupling, 10:1 probe).
                            - Own Opinions Only -

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                            • #74
                              Originally posted by SAG Electronics View Post
                              I have moved channel 4 to 6v6 pin 5 and see a nice sine wave In phase with the signal. Channel 4 is now at pin 3 and also shows a sine wave about twice the amplitude.
                              What are the signal levels?
                              6V6 pin 3 should have 20 or more times the AC voltage of pin 5. The voltages should be 180* out of phase.
                              You didn't by any chance have a x!0 probe on pin 3 and a x1 probe on pin 5 did you?
                              Check the cathode resistor measures 470 ohms.

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                              • #75
                                Sorry if I missed it, but I didn't see that you had tried this speaker with another amp, or this amp with a different speaker?
                                "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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