Help fixing Fender Rumble 150 Bass Combo Amp

[mendonesia]

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!

 

[mendonesia]

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.

 

[Harald Kapp]

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.

A buddy suggested I begin replacing caps - sound logical?

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.

 

[shrtrnd]

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?

 

[mendonesia]

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?

Hi - Sorry for the delayed response. Yes - used a couple different cables - cables that I use regularly - known good.

 

[mendonesia]

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.

Hi - Thank you for the tips. There aren't any obvious signs of failure on any of the caps - none are bulging and I don't see evidence of any leaks. I haven't pulled the boards out of the chasis, so I can't test voltage across caps (the contacts are under the board).

My signal tracing is shit. I tried to follow the circuit in a few spots near the testing points that gave odd values. I could pick up the test tone everywhere I thought it should be.

What I was able to determine is that the signal is intact going into what I am guessing is the actual power amplifier (this guy: https://www.parts-express.com/icepo...er-with-power-supply-module-1-x-170w--326-214), but there is no signal coming out. So I'm guessing the problem is somewhere on that ICEpower 50ASX2BTL, which freaks me out a little, as that is carrying real power. I'm wondering if I should disconnect the power, take it out and see what kind of testing I can do of the individual components on the board, all without being powered up. Is that futile?

Does anyone have experience with these ICEpower boards? Are they known to go out? Are the worth the bother to repair?

Thanks again - any additional help would be much appreciated!

 

[Harald Kapp]

According to the datasheet this amplifier has an enable input (details page 23). Check the voltage on that pin. It should be ~ 5 V. If it is noticeably lower (~ 0 V), the amplifier is disabled either by the internal thermal protection circuit or an external fault.
Enable is located on the signal header P102, pin 1. That would be the red/white single wire to the right in your photos.

 

[mendonesia]

According to the datasheet this amplifier has an enable input (details page 23). Check the voltage on that pin. It should be ~ 5 V. If it is noticeably lower (~ 0 V), the amplifier is disabled either by the internal thermal protection circuit or an external fault.
Enable is located on the signal header P102, pin 1. That would be the red/white single wire to the right in your photos.

There is definitely not 5VDC on pin 1 of P102 - I measured .2VDC. That's also TP9, on the left side of R97, which I didn't have a reading for previously. Now looking at the schematic, I'm not sure where to go next, but I'm looking 'upstream' (I think) from TP9 and I seem to be running into a lot of the testing points that were also out of spec. I'll keep studying this thing, but any thoughts as to the next place to look? Thanks again for your help to this point!

 

[Harald Kapp]

Remove the wire leading to the enable input and measure again. If P102 pin 1 is the still low, it looks like the internal protection circuit has triggered pulling this pin low. If the voltage without wire is high, then the external signal is the culprit and you'll have to trace where it comes from and why it is low.

 

[mendonesia]

wow - it's almost been 2 mos! time flies...

thank you again for your help. pin 1 is reading low when removed - still the .2VDC. i have no idea where to look from here and am thinking about closing this thing up & returning it to my buddy. i simply don't have the expertise to figure this out. i sincerely appreciate your time & effort in helping me. be well!

 

[jedwinsh]

solutions the fault by replacing the mosfet Q9, it is in charge of regulating the voltage, since the input voltage is correct approximately 16 volts in R38