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Vintage fender sidekick tube 20r 1986 model burnt resistors! R57 R58

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  • #16
    If it specifies 100V operation, it must have been originally from Japan.
    Did you try measuring the resistance of those resistors you removed?
    "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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    • #17
      ...And if you sketch out the circuit(s) from which those resistors are from it might help to determine their component values. If not exactly, then at least a close-enough values.

      Sometimes you simply have to live with the fact that no technical documentation is available. It's a basic mid 1980's amp design, how hard can it be?

      They seem to be in close vicinity to power supply circuit and what looks like either power amp or a regulator IC. Part of the regulator or voltage dropping circuit of the supply perhaps? What is the power amp or regulator IC, and could its datasheet perhaps provide some clues about involved circuitry? Where are the circuit board tracks running from those resistors?
      Do you -really- need a schematic for this? I understand it can be a convenience but for fixing this thing... do you really need it?

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      • #18
        It's good that Teemu mentioned these being from the same origin as the R.A.D. & H.O.T. Those schematics are available.

        neilemborgo7, are there numbers on the power transformer that can help us match this up to another schematic?
        "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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        • #19
          Originally posted by nosaj View Post
          Well a search turns up no schematic for the 20r but we get the 15, 25, 35, 65 here https://music-electronics-forum.com/...ad.php?t=33010
          nosaj
          It's good that Teemu mentioned these being from the same origin as the R.A.D. & H.O.T. Those schematics are available.
          Sadly none of those applies.
          Those are all transistor amps, while this one is the very odd TUBE one.

          So unusual that I didnīt know they even *existed* , go figure.

          Did anybody actually read thread title and look at amp picture?
          Juan Manuel Fahey

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          • #20
            Can you give us a pic of the whole PCB and the back side of the amp please?

            Thanks,
            nosaj
            Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

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            • #21
              Originally posted by J M Fahey View Post
              Sadly none of those applies.
              Those are all transistor amps, while this one is the very odd TUBE one.

              So unusual that I didnīt know they even *existed* , go figure.

              Did anybody actually read thread title and look at amp picture?
              I looked at it, and the board pics. I could only see a preamp tube socket so I figured maybe it just has a single preamp tube. Regardless, the burnt resistors are around a couple zeners and typical +/-15V rail size caps, so I figured the LV supplies might be the same.
              But you have a good point, the transformer won't match any of the others because it will have a HV winding.
              "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

              Comment


              • #22
                Originally posted by g1 View Post
                I looked at it, and the board pics. I could only see a preamp tube socket so I figured maybe it just has a single preamp tube. Regardless, the burnt resistors are around a couple zeners and typical +/-15V rail size caps, so I figured the LV supplies might be the same.
                But you have a good point, the transformer won't match any of the others because it will have a HV winding.
                only close thing I can find and no schematic. is the sidekick tube 30r 2 preamp tubes and a solidstate rearend.https://www.radiomuseum.org/r/fender..._tube_30r.html
                nosaj
                Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

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                • #23
                  Originally posted by nosaj View Post
                  only close thing I can find and no schematic. is the sidekick tube 30r 2 preamp tubes and a solidstate rearend.https://www.radiomuseum.org/r/fender..._tube_30r.html
                  nosaj
                  Interesting.
                  Letīs wait for a few closeups covering the full amp to start guessing with at least some data.

                  What I find REAL weird is that power IC shown not only is not attached to a heat sink, but it works as heat sink for a TO220 transistor
                  Juan Manuel Fahey

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                  • #24
                    That output IC may be the HA1388.
                    HA1388.pdf

                    Here is a crop of the 25R power supply.
                    Sidekick 25 Reverb Pwr Supply.pdf
                    Particularly the 15 Vdc supply.
                    It shouldn't be that different.

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                    • #25
                      https://drive.google.com/drive/folde...GtWTc95ImPo0az

                      Sorry for this very late reply. Here some more pics of the amp. I hope you can help me.

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                      • #26
                        Originally posted by g1 View Post
                        ......Did you try measuring the resistance of those resistors you removed?
                        ^^^^^^ I'll go back to this. Often resistors darken, but don't change value much. Measurements might at least get us in the ballpark.
                        "Yeah, well, you know, that's just, like, your opinion, man."

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