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Help fixing Fender Rumble 150 Bass Combo Amp

Discussion in 'Troubleshooting and Repair' started by mendonesia, Sep 13, 2020.

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  1. mendonesia

    mendonesia

    2
    0
    Sep 13, 2020
    Hi All - new member here, hoping to get some help repairing this amp.

    Back story - a band member wanted me to play his amp that doesn't get used, to see if I liked it - maybe I would buy - unsure. He brought it to rehearsal, and although the XLR line out sent signal to the board, there was no sound coming from the local speaker. I took it home, opened it up, nothing looked blown, or otherwise faulty. I plugged in headphones and was able to play through the amp (hearing through the headphones). I sent signal in through the aux inputs and was able to hear it through the headphones.

    I found the schematic online and tested all testing points, recording values and noting which were outside of tolerance. TPs 30-34, 38-39 & 41 all seemed out of wack (+20% over what the schematic says they should read). For reference, I tested these with a 100mV 100Hz sine wave being fed into the input, with all dials set to 12:00 and with all buttons out, as directed on the schematic.

    One thing to note: when I was testing with the 100Hz sine wave input, I could juuuuust barely hear a slight bit of that coming through the speaker. Nothing I did with onboard controls seemed to make it louder, though I could turn it down all the way.

    I don't know what to do now. I've attached the schematic, and pics, some of which had to be heavily compressed to get under 1MB for upload. Help? Please and thank you!
     

    Attached Files:

    Last edited: Sep 13, 2020
  2. mendonesia

    mendonesia

    2
    0
    Sep 13, 2020
    I would settle for pointers to other online forums/resources that might help me. A buddy suggested I begin replacing caps - sound logical? Any help/pointers is appreciated. Thank you.
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    10,223
    2,204
    Nov 17, 2011
    Do you have an amplifier with a high impedance input? Preferably battery operated. You could use that to trace the input signal from the XLR input to the speaker along the signal chain of (within) the amplifier:
    1. Connect headphones to the separate amplifier's output.
    2. Connect the GND input to GND of the Bass amp.
    3. Connect a fine tip to the signal input of the separate amplifier.
    4. Follow the signal (use the 100 mV, 100 Hz sine from your own test setup) through the bass amplifier to see hear where it disappears. Between the point where it is still present and the point where it disappears you'll have to search for the problem.
    5. Alternatively, if you have one, use an oscilloscope to trace the signal.
    6. Once you have located the black hole for your signal, let's discuss what may be the issue and how to proceed.
    Google "signal tracing" for more online info.

    Yes and no. Your friend probably means the electrolytic capacitors which typically lose capacitance over the years. But that is not necessarily the issue. What you can do:
    1. Inspect all electrolytic capacitors for signs of bulging or leakage on the PCB. Visibly defect capacitors need to be replaced by ones with the same capacitance (voltage rating being the same or higher, case size not important as long as the replacement fits in the respective position).
    2. Check for correct voltages across the DC supply bypass electrolytic capacitors. These are those connected from positive supply to GND or negative supply to GND. These voltages should match the values from the schematic. Check these voltages also using the AC range of your multimeter. Measuring DC in the AC range may sound illogical, but you'll be able to detect excessive AC on these voltages by this method - that is unless you have an oscilloscope as the better alternative. If you see excessive AC, this means the respective capacitor i probably defect, too, even if there are no visible signs. Replace it.
    But: defect electrolytics usually result in bad audio quality and audible hum, not the signs you describe.
     
  4. shrtrnd

    shrtrnd

    3,752
    482
    Jan 15, 2010
    My input is that it sure sounds like a short (but I guess is could be an 'open') in one of the cables.
    Did you try using a different set of cables and see what happens?
     
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