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After pulling apart this Fender Blues Deluxe Reissue amp, removing and salvaging the component leads of the two 470 ohm 5W bathtube resistors used in the +/- 16V Zener Regulator circuit (one had broken away from the PCB, req'd lead extenders wrapped/soldered to it's short leads), and replacing the bias pot, I got the main board back into place, and powered it up. RED LED working again, nominal residual noise coming out as the controls were exercised & volumes turned up, I then stopped to check the power tubes. Inserted my bias probes (Compu-Bias), and found while in standby, one of the 6L6GB power tubes was drawing plate current. Plate voltage reading was about 40VDC, which was odd. Moving that one tube to the other tube socket yielded the same plate current reading, now on the other socket, so associated with the tube. The original pair of 100uF/350V supply caps and their 220k ballast resistors had been removed, replaced with a single 100uF/500V cap. Residual charge from first powering up on the 1st stage cap?
Plus, the amp had a odd hum sound to it, so I found a replacement pair of tubes that were matched, and installed them, biased them up. The previous tech was a hack, leaving shabby fixes around, beginning with the one 470 ohm 5W resistor that had broken free of the board. He had never bothered to remove the main board to do proper surgery, or if he did, he managed to break two of the six ribbon cable conductors off the second cable that feeds V2. He had tacked in replacement wires to restore the connections. I removed and cleaned up those with freshly stripped/tinned ends so they were solid, and cut away the left-behind broken ribbon conductors.
So, powered it back up, all sounding fine again. I let it burn in while I got busy at the computer, it running with burst pink noise at substantial level (90dB SPL at my desk). After about 30 minutes, I began hearing odd transients in the otherwise smooth character and got up to investigate. I patched into the power amp input, cranked up the source 20dB to get to similar level, and that didn't have the odd distortion that began occurring. Patched back into the input, listened to the Drive Channel, and it too had that distortion. Patched into the 0dB input jack, Normal Ch, and turned it up a bit more, and it suddenly burst into this loud howling sound, startling me! Turned down the volume, no change, and switched to Standby to stop it. WHAT THE F**K?
Powered back on, same jack, Normal Ch, and turned up the volume again. Again, it burst into that loud howling, and I switched back to S/B, and stepped back to ponder this. Whatever it is, it has to be excited into that mode. Doesn't do it at idle.
Connected my DMM and checked the voltages on the preamp stages first. V1 was measuring fine, 250VDC on the plates, 1.9V on the cathodes. Plate supply for them at 376VDC. Checked the plate resistor for V2A (V2B is strapped off, all connections grounded), but no current flowing thru V2. Hmmm.........looked at the ribbon cable, to find the conductor next to the single replacement wires had broken free. So, I cut the rest of that ribbon cable off at the main PCB, extracted the remains (the PCB on this Reissue amp is double-sided, with plate-thru holes). Had to remove the main board again to do this right, stopped to prepare the ribbon cable, which is a PITA, but got that restored, main board back into place. Powered up again, but still no current flowing thru V2. Looked at the tube PCB end of it, only to find that other end that had broken was also broken at this end now. Sigh...... Cut that away, managed to get all four holes clear of the wires and opened up, then prepared it's mating end of the ribbon with freshly stripped/tinned ends, and got it back into place, that ribbon cable now much shorter.
Finally have current flowing thru V2. Gave a listen again, now solid. Let it burn in for the rest of the day without any further mishaps.
Odd one, though........I'd never heard this loud howling out of this series amp before. At least I didn't just pack it up after having gotten it back up and running the first time, believing all was healthy again, and send it on it's way to the customer. I thought back to Stretch's shop a few miles away (Valley Sound Music Technology), who managed to pick up a small acoustical isolation chamber, about 4' x 4' x 8' that he had in his shop to put amps in, letting them run at high SPL levels inside, while only hearing that a good 30dB or more lower outside the booth.
Fender-Blues-Deluxe-Reissue-2004-Schematic-Rev-A.pdf
Plus, the amp had a odd hum sound to it, so I found a replacement pair of tubes that were matched, and installed them, biased them up. The previous tech was a hack, leaving shabby fixes around, beginning with the one 470 ohm 5W resistor that had broken free of the board. He had never bothered to remove the main board to do proper surgery, or if he did, he managed to break two of the six ribbon cable conductors off the second cable that feeds V2. He had tacked in replacement wires to restore the connections. I removed and cleaned up those with freshly stripped/tinned ends so they were solid, and cut away the left-behind broken ribbon conductors.
So, powered it back up, all sounding fine again. I let it burn in while I got busy at the computer, it running with burst pink noise at substantial level (90dB SPL at my desk). After about 30 minutes, I began hearing odd transients in the otherwise smooth character and got up to investigate. I patched into the power amp input, cranked up the source 20dB to get to similar level, and that didn't have the odd distortion that began occurring. Patched back into the input, listened to the Drive Channel, and it too had that distortion. Patched into the 0dB input jack, Normal Ch, and turned it up a bit more, and it suddenly burst into this loud howling sound, startling me! Turned down the volume, no change, and switched to Standby to stop it. WHAT THE F**K?
Powered back on, same jack, Normal Ch, and turned up the volume again. Again, it burst into that loud howling, and I switched back to S/B, and stepped back to ponder this. Whatever it is, it has to be excited into that mode. Doesn't do it at idle.
Connected my DMM and checked the voltages on the preamp stages first. V1 was measuring fine, 250VDC on the plates, 1.9V on the cathodes. Plate supply for them at 376VDC. Checked the plate resistor for V2A (V2B is strapped off, all connections grounded), but no current flowing thru V2. Hmmm.........looked at the ribbon cable, to find the conductor next to the single replacement wires had broken free. So, I cut the rest of that ribbon cable off at the main PCB, extracted the remains (the PCB on this Reissue amp is double-sided, with plate-thru holes). Had to remove the main board again to do this right, stopped to prepare the ribbon cable, which is a PITA, but got that restored, main board back into place. Powered up again, but still no current flowing thru V2. Looked at the tube PCB end of it, only to find that other end that had broken was also broken at this end now. Sigh...... Cut that away, managed to get all four holes clear of the wires and opened up, then prepared it's mating end of the ribbon with freshly stripped/tinned ends, and got it back into place, that ribbon cable now much shorter.
Finally have current flowing thru V2. Gave a listen again, now solid. Let it burn in for the rest of the day without any further mishaps.
Odd one, though........I'd never heard this loud howling out of this series amp before. At least I didn't just pack it up after having gotten it back up and running the first time, believing all was healthy again, and send it on it's way to the customer. I thought back to Stretch's shop a few miles away (Valley Sound Music Technology), who managed to pick up a small acoustical isolation chamber, about 4' x 4' x 8' that he had in his shop to put amps in, letting them run at high SPL levels inside, while only hearing that a good 30dB or more lower outside the booth.
Fender-Blues-Deluxe-Reissue-2004-Schematic-Rev-A.pdf