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Finally having located the two Fender Pro Reverb amps in our inventory, neither ever having been in for service, I got one up on the bench and during initial check-out, the 'Normal' Channel's Volume pot was scary with the noise it made turning it. This is one of the late generation Pro Reverb amps...much deeper cabinet, deep chassis, huge power XFMR. NOT built like the older Pro Reverbs or Twin Reverbs.
When I pulled the chassis out of the cabinet, I was at least pleased not having it lock into place from the aluminum foil on the inside top of the cabinet not peel off and roll up, locking it into place in the process, as too many others do. Upon setting it down onto the lab jacks to support it, I eyeballed the front panel PCB assembly. Ya gotta be kidding me!! You're NOT getting that PCB out of the chassis, the way they designed and built THIS!!
There are 24 short soldered-in jumpers across the 0.1" gap separating the main PCB and the front panel PCB, plus vertical components that would prevent you from extracting the front panel board even if you decided to unsolder or just cut all those jumpers. I looked at this a bit, sizing it up, and came to the conclusion there's no way to extract that board without removing the main board. And most of the wiring is all directly soldered into place all over....some wires from the power transformer ARE at least on quick-disconnects, maybe as an afterhought. But...major surgery to even hope to get a pot pulled.
Trying to seep in Caig DeOxit with a brush at the seam of the pot cover and the resistance wafer was futile, and little success happens spraying thru the tiny open pressed into the cover as the end stops. Not having any small diameter tubing that I could bend to then attach a bent nozzle to the end of the spray can's straight nozzle, I thought about slipping in 22AWG buss wire into the end of the hard plastic nozzle, and apply heat while forcing a right-angle bend with suitable radius. Then remove the wire...it being in place to prevent the small ID from closing during the bending operation. That worked.
It was still clumsy getting the nozzle into place with the can mostly on its' side but I was able to spray up inside the pot from the surface of the PCB...enough to spread the solution onto the pot's resistance track while racking the control back and forth. End result was a quiet pot again. It was odd, finding that the pot was very noisy when the amp was set for 1/4 power (rear panel switch), and not noisy in Full Power mode. After cleaning the pot, I checked the current balance and bias level of the two power tubes. Way out of whack....50mA and 23mA, couldn't bring them into balance, so replaced them. Even after that, I still had to tweak the bottom pair of resistors below the two bias pots in order to have enough range for the new tubes.
So, on this first amp, I did luck out not having to pull any of the PCB's to service it. Hope I'm so lucky on the next one.
When I pulled the chassis out of the cabinet, I was at least pleased not having it lock into place from the aluminum foil on the inside top of the cabinet not peel off and roll up, locking it into place in the process, as too many others do. Upon setting it down onto the lab jacks to support it, I eyeballed the front panel PCB assembly. Ya gotta be kidding me!! You're NOT getting that PCB out of the chassis, the way they designed and built THIS!!
There are 24 short soldered-in jumpers across the 0.1" gap separating the main PCB and the front panel PCB, plus vertical components that would prevent you from extracting the front panel board even if you decided to unsolder or just cut all those jumpers. I looked at this a bit, sizing it up, and came to the conclusion there's no way to extract that board without removing the main board. And most of the wiring is all directly soldered into place all over....some wires from the power transformer ARE at least on quick-disconnects, maybe as an afterhought. But...major surgery to even hope to get a pot pulled.
Trying to seep in Caig DeOxit with a brush at the seam of the pot cover and the resistance wafer was futile, and little success happens spraying thru the tiny open pressed into the cover as the end stops. Not having any small diameter tubing that I could bend to then attach a bent nozzle to the end of the spray can's straight nozzle, I thought about slipping in 22AWG buss wire into the end of the hard plastic nozzle, and apply heat while forcing a right-angle bend with suitable radius. Then remove the wire...it being in place to prevent the small ID from closing during the bending operation. That worked.
It was still clumsy getting the nozzle into place with the can mostly on its' side but I was able to spray up inside the pot from the surface of the PCB...enough to spread the solution onto the pot's resistance track while racking the control back and forth. End result was a quiet pot again. It was odd, finding that the pot was very noisy when the amp was set for 1/4 power (rear panel switch), and not noisy in Full Power mode. After cleaning the pot, I checked the current balance and bias level of the two power tubes. Way out of whack....50mA and 23mA, couldn't bring them into balance, so replaced them. Even after that, I still had to tweak the bottom pair of resistors below the two bias pots in order to have enough range for the new tubes.
So, on this first amp, I did luck out not having to pull any of the PCB's to service it. Hope I'm so lucky on the next one.
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