Hello,
A friend of mine has a 1976 Fender Quad Reverb with some issues that I've been commissioned to repair. The screen grid resistors on two of the four output tubes released their magic smoke! Both of them were on the "brown" lead of the output transformer. I traced back the circuit to the "Balance" pot. It appears that someone has modified this section of the amp in the past. The electrolytics have been replaced, and it seems that whoever did this failed to re-attach the surrounding components to the "Balance" pot properly. The pot has four leads on it, and on the schematic all four seem to be used. On the amp, only three of the four have any components soldered to them, although the fourth has solder and flux residue, like something was soldered to it at one point. I have never worked with these four terminal pots before, so I'm not sure how they physically correspond to the schematic. Here's how it is attached at the moment:
Photos of "Balance" pot
The second photo has labels explaining all of the connections. Here is the schematic for the amp:
Fender Quad Reverb Schematic
Could this cause one side of the push-pull to burn up its screen resistors, or am I just chasing a red herring? Even if it was connected properly as is, could this possibly happen if the "balance" was adjusted all the way to one "half", causing those two tubes to run much hotter, eventually causing enough screen current to burn those resistors up? My friend is notorious for accidentally leaving the amp on overnight on several occasions.
A friend of mine has a 1976 Fender Quad Reverb with some issues that I've been commissioned to repair. The screen grid resistors on two of the four output tubes released their magic smoke! Both of them were on the "brown" lead of the output transformer. I traced back the circuit to the "Balance" pot. It appears that someone has modified this section of the amp in the past. The electrolytics have been replaced, and it seems that whoever did this failed to re-attach the surrounding components to the "Balance" pot properly. The pot has four leads on it, and on the schematic all four seem to be used. On the amp, only three of the four have any components soldered to them, although the fourth has solder and flux residue, like something was soldered to it at one point. I have never worked with these four terminal pots before, so I'm not sure how they physically correspond to the schematic. Here's how it is attached at the moment:
Photos of "Balance" pot
The second photo has labels explaining all of the connections. Here is the schematic for the amp:
Fender Quad Reverb Schematic
Could this cause one side of the push-pull to burn up its screen resistors, or am I just chasing a red herring? Even if it was connected properly as is, could this possibly happen if the "balance" was adjusted all the way to one "half", causing those two tubes to run much hotter, eventually causing enough screen current to burn those resistors up? My friend is notorious for accidentally leaving the amp on overnight on several occasions.
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