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Thread: Want a bit of help fixing a guitar amp

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    Default Want a bit of help fixing a guitar amp

    Hey, I got given an old guitar amp that powers a quad box. The quadbox works fine, but I'd like to get the amp working. It wont turn on at all, I have taken it apart and I can see a few disconnected wires and a couple of broken diodes that form what I assume are bridge rectifiers. I have tried to include as many pics as possible of the inside of it, and would like to get a bit of info on how to proceed. The amp looks home made:














    Cheers, Gab



Look Here ->
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    LSemmens
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    Looks like a child of the 60s! Where abouts are you, Gab. I'd be checking the voltage at the tranny first, and then following the circuit through. Probably a simple analogue PSU which would involve a bridge rectifier and a few caps.
    I'm out of my mind, but feel free to leave a message...

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    Jeez, you really need a better camera with more resolution and one total shot in landscape in a decent resolution so I can see a bit better where all the wires go and use a flash to avoid all the shadowed areas. You may have to tilt the camera if the flash reflects in the image.
    Then make popper closeups of the PCBs, full frame, so we can see the components and read their values. Please hold the camera in landscape, not portrait in that thin 16:9 format. We shouldn't have to download and convert them so they fit properly on our computer screens, in order to view the details.

    First check if there is a short across the main filter capacitor while it is still wired to the PCB. If there is a short, find what is causing it and then replace the rectifier.
    It looks like the garden variety 1N5408 would fit based on the size of the diode bits you are holding. However it could also be assumed that they are not original and somebody else mucked that up to replace a proper block rectifier higher than 3A and that could be a reason why one of them has crumbled apart.

    I would go to Jaycar and get this one:
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    Quote Originally Posted by lsemmens View Post
    Looks like a child of the 60s! Where abouts are you, Gab. I'd be checking the voltage at the tranny first, and then following the circuit through. Probably a simple analogue PSU which would involve a bridge rectifier and a few caps.
    60's, no. More middle to late 70's with those green epoxy PCB's.
    Back in the 60's valves were the way to go for musicians
    Pretty sure that Strauss emblem does not belong on that amp. Melbourne company that are known for their quality valve amps.

    Even today I will not plug my guitar into anything that does not have at least one valve in it's path.
    Last edited by Uncle Fester; 22-01-18 at 11:42 AM.
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    Actually nomeat, I think you are on the money, my date was more a reference to the fact that with those picces it looked like it was wired up by some hippy whilst he was high. I have a 70s tranny amp that looks just like it (wiring wise). I agree with you about the valves, but the tranny amps do the job for me and are nowhere as near temperamental when it comes to bump in and bump out especially if they've not had a chance to cool down.
    I'm out of my mind, but feel free to leave a message...

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    If you are in the Sydney or Newcastle region, I will fix for you for cost of parts.

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    I reckon it's a Savage 85 by Soundlabs, from the mid 70's. have a look at these two sites, and see what you think.






    and scroll down to the pics on this one
    Last edited by bob_m_54; 24-01-18 at 03:10 AM.

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    I would say you nailed it with this link:

    At least we got now decent images of the PCBs but more interesting is what is noted under:"Discussion and Modifications"

    Looks like that hippy Lsemmens mentioned, also designed it.
    Could be a lot of work and a propper heat sink is required.
    Maybe if he can rewire the speaker cabinet to 16Ω that should keep the heat down and can leave this minimalist heat dissipation as it is. It looks like the driver transistors have already been replaced, hopefully with BD139/140.
    Last edited by Uncle Fester; 24-01-18 at 06:52 PM.
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