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1971 Randall RG-90 head, smoking speaker!

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  • #46
    One question, what are the yellow .22uf/200v caps? [pic] They look polarized so are they like 'speaker divider' type capacitors? There's 4 of them in this amp.
    The yellow .22µ cap is a (non-polarized) film cap. The black ring/band identifies the terminal which connects to the outside foil. Probably used as coupling cap or in the tone stack.
    - Own Opinions Only -

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    • #47
      My advise is to NOT do anything until else to the amplifier you get the amp working.

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      • #48
        https://soundcloud.com/damalistik/ro...ival-dubplatessigpic

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        • #49
          If you start mucking around replacing caps & other parts with the amp nonfunctional at this point, how will you know if you got it correct.

          That is why I strongly advise that you troubleshoot the amp & repair it so it works?

          Then have at it with replacing parts.
          Last edited by Jazz P Bass; 06-12-2019, 03:29 PM.

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          • #50
            Its blowing fuses so I'm not gonna turn it on to troubleshoot until I replace all the electrolytic caps, power transistors, bridge rectifier and those TIP30C/29C transistors. Its my best shot. I figure all 50 year old electrolytics can probably benefit from being replaced.
            I dont have a bulb limiter, oscilloscope or any of that stuff. I got a DVM and thats all.
            How do you troubleshoot when the amp blows its fuse on power up?
            Last edited by czech-one-2; 06-11-2019, 04:58 PM.
            https://soundcloud.com/damalistik/ro...ival-dubplatessigpic

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            • #51
              Originally posted by czech-one-2 View Post
              How do you troubleshoot when the amp blows its fuse on power up?
              You build a light bulb limiter so it will, hopefully, stop blowing fuses. No point powering it up without the light bulb limiter as that will only blow more fuses and possibly keep destroying more fragile semiconductor based parts from the current draw. I would only replace transistors, diodes and burnt resistors at this stage of the game. Then start the amp with a 60 watt light bulb in the limiter. If the bulb lights up bright there is still something pulling too much current. If you have no schematic I would make sure that I have great pictures of every nook and cranny of the amp, that way if some resistor burns up you can figure out the value.
              When the going gets weird... The weird turn pro!

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              • #52
                If you've got a DVM, why not just check the solid state parts and see if they need replacing?
                "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                • #53
                  Thats kinda what I'm doing, but I ordered a better multi meter as mine sucks.
                  I'm also gonna build a light bulb limiter or else you guys may ban me from this forum. Got lots of music to learn now so project on hold a couple weeks...
                  https://soundcloud.com/damalistik/ro...ival-dubplatessigpic

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                  • #54
                    You will not regret building a limiter , .. don't mix your Bb with an A#
                    If you don't know where your going any road'll take you there : George Harrison

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                    • #55
                      I want to build a simple one without a switch. I'm not sure how to wire it up using EU two prong plugs...
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                      https://soundcloud.com/damalistik/ro...ival-dubplatessigpic

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                      • #56
                        Originally posted by czech-one-2 View Post
                        I want to build a simple one without a switch. I'm not sure how to wire it up using EU two prong plugs...
                        [ATTACH=CONFIG]53965[/ATTACH]

                        [ATTACH=CONFIG]53966[/ATTACH]
                        Regarding safety it is mandatory to use a 3 wire arrangement that provides safety grounding of the load/amplifier connected. I.e. the yellow/green ground/earth wire must connect the ground contact of your wall outlet and the amplifier chassis. Your lamp fixture may not need to be grounded (if it's safety Class II), but the amp does. Also any accessible metal parts of your bulb limiter must be securely connected to the ground wire.
                        Last edited by Helmholtz; 06-14-2019, 01:53 PM.
                        - Own Opinions Only -

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                        • #57
                          Ok, got it. It was pretty easy, didnt blow up my wifes hairdryer anyway
                          https://soundcloud.com/damalistik/ro...ival-dubplatessigpic

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                          • #58
                            My bulb limiter design:
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                            Used the housing of a household transformer which already provided the outlet. Just had to drill a small hole for the lamp holder, connect the wires and ground the housing.
                            Last edited by Helmholtz; 06-14-2019, 09:00 PM.
                            - Own Opinions Only -

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                            • #59
                              The amps bridge rectifier shows continuity between all four lugs, that cant be right...right?
                              Also, it has a plastic base, my new BR is metal. Does it need to be isolated from the chassis in SS amps?
                              https://soundcloud.com/damalistik/ro...ival-dubplatessigpic

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                              • #60
                                ''I built a fuse tester once. If the fuse flashed, that meant it had been good''. ENZO

                                :-DDDD
                                https://soundcloud.com/damalistik/ro...ival-dubplatessigpic

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