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Thread: Ampeg BA600 repair

  1. #1
    Senior Member
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    Ampeg BA600 repair

    Hi there!

    So, it is an Ampeg BA600-115 it came with a massive electrolyte leakage from a filter capacitor (C16) that exoplode after some PSU issue.
    PSU was shorted, one diode bridge (D8).
    I replace Mosfets Q1 and Q2 PWM controler U1, both rectifiers D7 & D8 and four large filter capacitors D14/15/16/17

    This is like a dual PSU as far as I can tell, One for the preamp rails with the HT for the preamp tube and another one for the AMP.
    Now everything works but I have an issue with the amp rails (+/-30VDC) the positive rail is about +25V but the negative is -12V, the Amp actually works but when it reaches a volume threshold the tension drops too muchs and it cuts out.

    any clue will greatly apreciated īcouse this is driving me nuts.

    ampeg_ba300_ba600_sch.pdf

  2. #2
    Old Timer J M Fahey's Avatar
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    Just thinking aloud, no experience with SMPS.
    1) you practically replaced all main components of the large power supply.
    2) +30 and -30 are 2 identical supplies in all aspects, just one has negative grounded, other has positive.
    They *should* provide exact same voltage.
    3) that supply is regulated, so you should at least have one +30V rail, yet best is +25, let alone the "officially bad" -12V one
    4) "just to do something" Iīd scope U1 pin 6 to check whether itīs putting out a symmetrical square wave (it should) so as to drive Q1 and Q2 the same.
    5) then would scope secondary T1b , then T1c, and find if there are any differences
    6) then cathode of D7 and of D8
    "just thinking it as if it were a conventional supply and looking for differences or abnormalities"

    Certainly somebody SMPS savvy will be able to offer more approppriate analysis or suggestions, Iīm just mentioning what Iīd be doing for starters if I had it on my bench today.
    Juan Manuel Fahey

  3. #3
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Please note you will need to use an isolating transformer if you are going to 'scope anywhere on live side of the power supply.
    J M Fahey and The Dude like this.
    Experience is something you get, just after you really needed it.

  4. #4
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    The two secondaries T1-B and T1-C are off the same primary T1-A. I would expect the voltages to be symmetrical even if the primary voltage is too low. The main cap failure C16 is on the negative rail, and you still have a problem with that rail (ignoring for the time being that the +25v isn't correct). Re-check the rectifier solder joints and make sure you don't have any AC leakage superimposed on the rectifier outputs (maybe from a faulty 1nf snubber cap) that could pull down the filtered voltage.

  5. #5
    "Thermionic Apocalypse" -JT nickb's Avatar
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    I see there are +24V and -15V rails too which are very close to what what you measured. So, I have to ask and don't be mad with me, are you sure you measured the 30V rails in the correct place?
    J M Fahey and Mick Bailey like this.
    Experience is something you get, just after you really needed it.

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