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Fender Frontman 212R DC on output

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  • #16
    fm212r.pdf Ok I'll try to make this simple. Apparently, I have made some progress. When I dis-abled the mute circuit (quadrant A5), by cutting R73, The DC on the output went away. I changed Q6 and Q7 (which are 2N4403's not 2N4003). Now, when the mute circuit is connected, I get -2.5 volts DC on the output. As before, when R73 is dis-connected, there are 2 - 3 millivolts DC. Anybody?

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    • #17
      Are you still on the limiter lamp? And no load? Check that the +/- rails remain equal when you have the -2.5VDC on the output.
      "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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      • #18
        Originally posted by booj
        Apparently, I have made some progress ...
        - Check mute line contacts on jacks J1, J2, J4.
        FM212R power off. At jacks J1, J2, J4 nothing is connected. At mute point (R73/C46) should be resistance order 10 Ohm (R49).
        When plug is inserted into any jacks J1, J2, J4 at mute point (R73/C46) should be resistance order > kOhm.
        - Check RT2 (PRC100)
        It's All Over Now

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        • #19
          vintagekiki
          - Check mute line contacts on jacks J1, J2, J4.
          They're there
          FM212R power off. At jacks J1, J2, J4 nothing is connected.
          OK. Nothing plugged in
          At mute point (R73/C46) should be resistance order 10 Ohm (R49).
          That's there
          When plug is inserted into any jacks J1, J2, J4 at mute point (R73/C46) should be resistance order > kOhm.
          It is.
          - Check RT2 (PRC100)
          If that's that thing hidden by all the goop, it reads 200 ohms

          g1g1
          Are you still on the limiter lamp? And no load? Check that the +/- rails remain equal when you have the -2.5VDC on the output.
          They are

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          • #20
            Originally posted by vintagekiki
            - Check RT2 (PRC100)
            Correction. Sorry
            RT2 (PTC100)
            https://www.mouser.co.uk/Circuit-Protection/Thermistors/PTC-Thermistors/_/N-axfwr?P=1z0x8ah
            What is its resistance when it is out of circuit.
            It's All Over Now

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            • #21
              FM-65R schematic are very similar, only here you have some voltage test points.
              Check test point TP23 1)
              https://elektrotanya.com/fender_fm_65r.pdf/download.html

              1)

              https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/43795-fender-fm-65r-slight-noise-at-turn-on
              https://music-electronics-forum.com/filedata/fetch?id=837514

              https://music-electronics-forum.com/forum/schematic-requests/29176-fender-frontman-65r
              Last edited by vintagekiki; 08-27-2021, 12:06 PM. Reason: 1)
              It's All Over Now

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              • #22

                vintagekiki replied

                08-26-2021, 11:03 PM


                FM-65R schematic are very similar, only here you have some voltage test points.
                Check test point TP23 1)

                TP23 is supposed to be -31 VDC and its -30.3Volts. The zener diode seems to be working normally. This is with an input plugged in.

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                • #23
                  Originally posted by booj
                  TP23 is supposed to be -31 VDC and its -30.3Volts.
                  The measured voltage at TP23 is OK.
                  The difference between FM65R (112) and FM212R (212) is in number of built-in speakers, and FM212R has no mute Q5/Q20 at U7-A input.

                  Question.
                  1. If FM212R has no signal at inputs, and an empty jack plug is installed in J4, what is DC voltage on TP29.
                  2. Have any of semiconductors changed in power amp except Q6 and Q7.
                  It's All Over Now

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                  • #24

                    vintagekiki replied

                    Today, 02:31 AM



                    Originally posted by booj
                    TP23 is supposed to be -31 VDC and its -30.3Volts.



                    The measured voltage at TP23 is OK.
                    The difference between FM65R (112) and FM212R (212) is in number of built-in speakers, and FM212R has no mute Q5/Q20 at U7-A input.

                    Question.
                    1. If FM212R has no signal at inputs, and an empty jack plug is installed in J4, what is DC voltage on TP29.
                    2. Have any of semiconductors changed in power amp except Q6 and Q7.


                    Looking at the scat till my eyes are bleeding. What quadrant is TP29 in?

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                    • #25
                      Originally posted by booj
                      What quadrant is TP29 in?
                      TP29 is in quadrant B1
                      PDF zoom to 400
                      It's All Over Now

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                      • #26
                        TP29 reads 5mv with an empty plug in the input jack. With nothing plugged in it reads about 3 volts.

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                        • #27
                          vintagekiki replied
                          Question.
                          1. If FM212R has no signal at inputs, and an empty jack plug is installed in J4, what is DC voltage on TP29.
                          2. Have any of semiconductors changed in power amp except Q6 and Q7.

                          TP29 reads 5mv with an empty plug in the input jack. With nothing plugged in it reads about 3 volts.
                          2SD1857 (Q14) and 2SB1236 (Q15) and re-flowing all the connections in the output section. I also changed the drivers Q18 and Q19, the outputs, Q20 and Q21
                          and Q10 and Q11

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                          • #28
                            The amp is done. We sent it out with the mute circuit dis-connected.

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