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Those that service these aging but usually reliable bass amps no doubt have encountered the hard failure they produce when over-stressed. Two of these just came in from a steady client, both being rental amps. In both cases, the output xstrs of the 100W Amp (normally used for the HF output cabinet along with the 300W Amp driving the Bass Bin) had shorted power xstrs. One was a melt-down. PCB scorched where resistors sat and burned a hole thru the board, with many other resistors burning open. Soot all over the insides, along with little fragments that got airborne inside during the meltdown.
I've no details in how the failure occurred on the two amps. Only one burned, to the extent of having to first remove the burnt parts, then had to scrape away the carbonized fiberglass enough to allow the remains of the PCB to be coated with Corona Super Dope to seal it and prevent the damaged board from being a high-resistance path between circuit branches.
I cleaned up the soot and debris scattered throughout the chassis and the boards. All the damage appeared to be on this 100W amp circuit. But, in checking out the rest of the amp, I found the readings of the power xstrs on the 300W section reading bad, though none were a hard short as was the case on the one pair in the 100W circuit. I de-soldered all of the power xstrs in the 300W amp and verified I had full isolation of the base and emitter terminals passing thru the large solder pad. That done, I checked all eight (drivers and the three output pairs), those now measured nominal using the DMM in Diode Test mode. Checked the short circuit protection devices, bias, emitter resistors, and all was ok over there. So, set about the repair of the PCB first.
After removing all of the failed parts. I had to scrape away all of the carbonized PCB from both sides...mostly on the component side, then coated it with Super Corona Dope and left to dry overnight. I thought I had taken photos of that prior to restoring the circuit components, tack-soldering leads where pads had burned off, and such. No photos of that step.
Having also pulled the second amp apart, which didn't have this severe damage to the PCB, I just found after putting this first one back together yesterday evening, so it would be ready to connect probes thruout the two amp sections, composing this thread, I saw the same original emitter resistors on this burnt unit in place, and see it was the other board which had both of these emitter resistors open and were the ones I had to chiesl off of the heat sink. I just checked and see one of the two resistors is open on this amp, so I have to take it apart again and replace the pair of 0.33 ohm/5W resistors that should have been done when it was all apart. Sigh.
At least I found it before applying power.
Onward
I've no details in how the failure occurred on the two amps. Only one burned, to the extent of having to first remove the burnt parts, then had to scrape away the carbonized fiberglass enough to allow the remains of the PCB to be coated with Corona Super Dope to seal it and prevent the damaged board from being a high-resistance path between circuit branches.
I cleaned up the soot and debris scattered throughout the chassis and the boards. All the damage appeared to be on this 100W amp circuit. But, in checking out the rest of the amp, I found the readings of the power xstrs on the 300W section reading bad, though none were a hard short as was the case on the one pair in the 100W circuit. I de-soldered all of the power xstrs in the 300W amp and verified I had full isolation of the base and emitter terminals passing thru the large solder pad. That done, I checked all eight (drivers and the three output pairs), those now measured nominal using the DMM in Diode Test mode. Checked the short circuit protection devices, bias, emitter resistors, and all was ok over there. So, set about the repair of the PCB first.
After removing all of the failed parts. I had to scrape away all of the carbonized PCB from both sides...mostly on the component side, then coated it with Super Corona Dope and left to dry overnight. I thought I had taken photos of that prior to restoring the circuit components, tack-soldering leads where pads had burned off, and such. No photos of that step.
Having also pulled the second amp apart, which didn't have this severe damage to the PCB, I just found after putting this first one back together yesterday evening, so it would be ready to connect probes thruout the two amp sections, composing this thread, I saw the same original emitter resistors on this burnt unit in place, and see it was the other board which had both of these emitter resistors open and were the ones I had to chiesl off of the heat sink. I just checked and see one of the two resistors is open on this amp, so I have to take it apart again and replace the pair of 0.33 ohm/5W resistors that should have been done when it was all apart. Sigh.
At least I found it before applying power.
Onward
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