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Thread: Ampeg b-100R Output Short?

  1. #1
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    Ampeg b-100R Output Short?

    Hey All-


    Got a B-100R Ampeg Bass Rocket on the bench. Did my homework, seems like these have problems with their output trannys. Read a short on one of the two TIP142s, replaced it and started it back up on my current limiter. Still shorting.... Here are some measurements. Can you solve the mystery?

    Diode Checks-

    Q5 TIP142 (new)
    Base - Emitter: 0
    Base - Collector: .543
    Emitter - Collector: .543
    Emitter - Base:0
    Collector - Emitter: .658
    Collector - Base: .658

    Q6 TIP142
    Base - Emitter: 0
    Base - Collector: .542
    Emitter - Collector: .543
    Emitter - Base:0
    Collector - Emitter: .636
    Collector - Base: .636

    Q7 TIP147
    Base - Emitter: 0
    Base - Collector: .806
    Emitter - Collector: .807
    Emitter - Base: .001 / .000 (fluxuates)
    Collector - Emitter: .561
    Collector - Base: .561

    Q8 TIP147
    Base - Emitter: 0
    Base - Collector: .805
    Emitter - Collector: .806
    Emitter - Base: 0
    Collector - Emitter: .561
    Collector - Base: .561

    Voltages:
    Emitters -.08mv across the board

    Base Q5 20.7mv
    Base Q6 1.8mv
    Base Q7 -3mv
    Base Q8 -1.8

    Collector Q5/Q6: 4.1mv
    Collector Q7/Q8: 6.4mv


    On the Schematic, where it says +40v and +20v, I don't even get a single volt.

    AND I just found out all emitters have about 1.4ohms resistance to ground Should I be looking for the short to ground and ignore the trannies?

  2. #2
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    Quote Originally Posted by Mr_bibbles View Post
    AND I just found out all emitters have about 1.4ohms resistance to ground Should I be looking for the short to ground and ignore the trannies?
    Disconnect the speaker and see what happens.

  3. #3
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    was already running it without a load, same result with speaker disconnected or connected.

  4. #4
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    Quote Originally Posted by Mr_bibbles View Post
    was already running it without a load, same result with speaker disconnected or connected.
    Start at the top with the power transformer check the voltages with it disconnected. Cause if it's not right nothing else will be.
    Then move to the power supply once power supply is supplying correct voltages then move down the chain. They have a thing around here about doing you homework. Each amp is like an individual different issues. Kinda of like not all muslims are terrorists even if if says so on the internets.

    nosaj
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  5. #5
    Supporting Member The Dude's Avatar
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    I'll add, it's a good idea, when you post a new thread, to include a schematic so the rest of us don't have to go hunting for it. Posting diode check readings is great, but without a schematic, we have no idea what you're talking about.
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    Supporting Member Jazz P Bass's Avatar
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  7. #7
    Supporting Member The Dude's Avatar
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    What nosaj said. We need to figure out why no positive rail first. Also, leave the speaker disconnected until you get it fixed and verify that there is no DC on the speaker output.
    Last edited by The Dude; 03-15-2017 at 11:42 PM.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  8. #8
    Senior Member Enzo's Avatar
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    If the transformer were bad or unpowered, we'd be missing the negatives as well. Are you? +20 is derived from +40 in the power amp. DO you have the +16 in your preamp for the ICs? If you do, that means the +40 supply is there, but not reaching the power amp. If the preamp is not seeing +16 either, then we suspect the +40. Point being the +40 mioght exist but due to a cracked solder joint or something is not reaching the power amp circuits.

    Now when you say you are missing +40, are you also missing -40?
    Education is what you're left with after you have forgotten what you have learned.

  9. #9
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    Sorry, should have mentioned I did the transformer continuity/short tests and also can power up the amp without drawing excess current while the secondaries are disconnected. (dim lightbulb limiter)


    Coming off the secondaries leading to the rectifier:

    j5: 23mv and j21: 20mv

    Exiting the rectifer is -23 mv and 73mv instead of +40 and -40


    At D10 and D14, 40+ reads as about 70mv and 40- reads as about -1 mv

  10. #10
    Supporting Member The Dude's Avatar
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    Measure and post the AC voltage from J5 to J21, please.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  11. #11
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    Thanks Dude-

    To Clarify, do you mean measure the flow of AC voltage BETWEEN the secondaries while connected to circuit? Or just the between the secondaries? Or from each Secondary to ground?

  12. #12
    Supporting Member The Dude's Avatar
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    One probe on J5 and one on J21- meter set to read AC volts. It's not really "flow" but rather a voltage potential we are looking for. We need to find out if the rectifier is getting AC. No need to disconnect anything. If we had a short, the fuse would be blown.
    nosaj likes this.
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  13. #13
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    Thanks for clarifying and explaining!

    I have been using a lightbulb limiter this whole time and it's glowing bright as the sun... assuming if I took it off I would blow a fuse immediately.

    Voltage between the secondaries measured on the Rectifier is 2v AC, and it starts to decrease. Watched it get as low as 1.8, but it was decreasing very slowly at this point.

  14. #14
    Supporting Member The Dude's Avatar
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    Ah! I wasn't aware we had the bright bulb. With the low voltage readings there and the bright bulb, I suspect we have one or both shorted rails. With power off, measure the resistance of each of the 40V rails to ground. When you replaced the outputs, did you make sure any sil-pads were put back, isolation washers installed, etc. Maybe one of your new transistor's tabs is shorted to the heat sink?
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  15. #15
    Supporting Member Jazz P Bass's Avatar
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    "Maybe one of your new transistor's tabs is shorted to the heat sink?"

    Or all of them.

    All of the Collectors could be sitting on the heatsink.

    B-100R Output Section.pdf

  16. #16
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    Transistors installed with proper paste / silicone pads / properly bent legs. Customer wanted it back today so this mystery will go unsolved, thanks though everyone.

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