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RM Italy KL-503 Lost Bias?

Discussion in 'Amplifiers' started by buickid, May 21, 2013.

  1. buickid

    buickid Active Member

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    I found a schematic somewhere, I'll have to go find it again. It was strange, it didn't include part values...


     

  2. 9C1Driver

    9C1Driver Sr. Member

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    Now that we got that straight, have your friend run the S45 barefoot into that sweet sixteen. 60-80 watts should make her sound real good.
     
  3. buickid

    buickid Active Member

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    That's already being done.
    Doesn't help the KL amp any.
     
  4. paws264

    paws264 Active Member

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    I looked but, could not find a 1 X 4 RM-Italy Amp, here is a roadmap for a straight 4 transistor amp. In the colored in area, you will find the bias components, at the base of the input x-formers, there should be .6 vdc. There are 2 transistors, TR5 and TR6 as well as diodes which form a voltage diver and a regulator system.

    Work back from from L9 and L6, D17, D18, D19. R13 has source voltage from Vcc.

    .
     

    Attached Files:

  5. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    KL-503 is a different critter from the 500.

    The 503 is all fet finals. While I have not seen a schematic for one yet, most other manufacturer's use a regulated five volt then resistor divider network to get the bias to where the FET's operate in the proper area of the transistor's specification for the amp they have designed. Look for a regulator circuit like a 7805 or the European equivalent. Also check your traces and feed through, and look for wires with insulation of the transformers inserted into the solder joint. I have rebuilt so many of the KL-500's and it usually came down to a quality of construction problem.
    Most of the time it was a customer abuse problem with OVERDRIVE as the number one cause of failure. I really would like to see the schematic of the KL-503, They always put a schematic of the unit in the box, do you still have yours?
    Sorry about the book!
     
  6. 9C1Driver

    9C1Driver Sr. Member

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    So which is it, you first said," I have rebuilt so many of the KL-500's and it usually came down to a quality of construction problem."

    Then you turned around and said this," Most of the time it was a customer abuse problem with OVERDRIVE as the number one cause of failure."

    The RM amplifiers are low end low drive light duty amplifiers. I guess maybe it could be a combination of the two issues you listed. Maybe that was what you intended on saying.

    In this example the S45-HP radio just plain overdrove the piss out of that amp. I would sell it on E-Bay as a fix or repair and be done with it. Chaulk it up to a lesson learned.
     
  7. 9C1Driver

    9C1Driver Sr. Member

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    chrisman686 likes this.

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