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Bassman 70 repair story

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  • Bassman 70 repair story

    It was a dark and stormy night.....I got a broken Fender Bassman 70 amplifier into my shop. The first thing I did was a quick visual inspection. Then I plugged it into my dim bulb tester and turned the amp on. The red amp indicator light did not light and the dim bulb did not light. I turned the amp off and pulled the fuse. The fuse was blown and I replaced it with same value (2.5 ). I turned the amp on and the dim bulb went to full brightness. I turned the amp off, removed the back cover and pulled all the tubes.

    I turned the amp on and the bulb went to full brightness. I turned the amp off and the dim buld turned off but then the dim bulb turned on.....but the amp switch was off! Then it flickered and I smelled a burning smell. So I pulled the plug from the tester.

    As I was removing the chassis from the cabinet I was thinking to myself that the power or output transformed must have just burned up ($$$). I looked around and did not see anything that was obviously burned but boy could I smell it! After it aired out I sniffed around the transformers but did not smell it that strong. I could not find the offending component.

    As it turns out the lead to the power switch was burned and the switch was burned. I removed the switch and tested it. Sure enought there was a resistance from one of the leads to the threaded shaft (which is grounded to the chasis).

    I have never run across this issue. I have had switches that did not work or where the bats were loose or the case was broken but never shorted especially to ground!

  • #2
    Here are pics.

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    • #3
      Perhaps the switch was made by 'Arco'.
      Must have been arcing for a long time and the owner just ignored the noise of the contacts sparking.
      Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
      If you can't fix it, I probably can.

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      • #4
        Luckily the ground tab was still on the plug......otherwise..........

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        • #5
          Originally posted by Axtman View Post

          I have never run across this issue. I have had switches that did not work or where the bats were loose or the case was broken but never shorted especially to ground!
          It does happen occasionally. One of the reasons in favour of having the fuse before the power switch, not after.

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          • #6
            Good point about the fuse before the switch....otherwise you become the fuse.

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            • #7
              Originally posted by Axtman View Post
              I have had switches that did not work or where the bats were loose or the case was broken but never shorted especially to ground!
              Some amps had DPDT switches wired to reverse the AC line polarity - I've encountered some with apparent internal shorts across the line, their sides blown out and a carbon patch so it looks like a mini grenade went off.

              This isn't the future I signed up for.

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