Help! Gibson Medalist 212

Discussion in 'Musical Instruments' started by BobCraver, Apr 5, 2018.

  1. BobCraver

    BobCraver New Member

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    Hey Folks, I recently picked up a SS Gibson Medalist 212 guitar amplifier that needs some work. It's got two channels that operate off of one transformer. Each channel has its own PCB and they are identical. One of the channels is completely functional, but one is DOA. I'm pretty sure that I have a bad filter cap on that board (after visual inspection). I have contacted Mouser about a suitable cross reference, but I struck out there. I have tried to get schematic from Gibson, but since that amp was only built for one year, they didn't keep any records of it. There are a few things that I will really need to resurrect this Lazarus: Schematic and/or spec sheet and a lead on replacement caps for the PCB's. The info on the cap is as follows:
    Sprague 945-003861-1 1000-40DC 070891 69398 (the 8 might be a B) . It’s about 2” long and 3/4” in diameter. I'm trying some forums hoping that an old school electronics buff or professional is familiar with these caps and can point me to a suitable replacement. If anyone has a copy of the schematics in a repair manual or on the amp itself, PLEASE make a copy and post it here. Any help will be greatly appreciated!
     

     

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  2. BobCraver

    BobCraver New Member

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    Here are a couple of pics of the PCB's and close up of suspected cap. IMG_0581.JPG IMG_0584.JPG
     
  3. philo426

    philo426 Super Member

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    2,454
    Go to YouTube and ask Uncle Doug.He is a master at tube amp technology .
     
  4. BobCraver

    BobCraver New Member

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    Thanks philo426, I just posted on his page. None of the activity was very recent there, so hopefully he's still YouTubing!
     
  5. Celt

    Celt Peanut Head Staff Member Super Mod

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    Paragould, Arkansas
  6. BobCraver

    BobCraver New Member

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    Celt, thanks for your input. This particular cap is not marked with a uF symbol. What it says is: Sprague 945-003861-1 1000-40DC 070891 69398 (the 8 might be a B). I have been getting some advice from an electrical engineer that I know.
    This was his interpretation of info on the cap: " I guessed the value to be 1000uF, 40VDC Electrolytic but I can't cross the number. I am sure you can find a substitute but without knowing the specs of that part I can't say which is an acceptable one. The problem is what is known as Equivalent series resistance (ESR). When AC signals pass through a capacitor, there is a loss due to the ESR and just like a resistor; there are wattage tolerances. The lower the ESR the less heat is generated. The higher the ESR the more heat. It can effect the sound quality too and if the ESR is too high and the wattage passing though it too much.....it could result in premature failure or an exploded part. If it is lower than what was designed into the circuit; it could pass more power to downstream components and put strain on them which could also lead to premature failure."
    Based on that info, I can't find anything on the Tube Depot site. I found a couple on Mouser, but not knowing the ESR requirements, I'm still stuck.
     

     

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