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Having serviced this Fender Bassman TV Duo Ten Combo back in October last year, doing preventative maintenance at that time, it had low output, plus only the Treble control worked....Bass and Midrange controls didn't work, nor did the Deep Bass Switch. It's a hybrid....SMPS with a Class D pair of amps in Bridge, with a Tube front end, passive tone stack and preamp output before handing that off to some TL072 op amps to drive the Class D amps.
Back then, I had found a tiny lead cutting that got trapped underneath one of the tiny DPDT pushbutton switches, managed to extract it, and all was working again. It just came back yesterday with the complaint of Low Output. Powered it up, and, found it was doing the same thing as before....no Mid or Bass EQ, no Deep Bass Switch, and low output. So, pulled it apart, extracted the tone stack PCB to see if it was again measuring low resistance from the Treble pot's CCW terminal. It was, same reading as I got before (143 ohms). This time, no debris found trapped under the switches, though removed them just to be sure there wasn't something hiding. Traced the short down, kept returning to the Treble pot's CCW terminal, with nothing on the board in the range of 143 ohms. Removed the treble pot (having support brackets, like the pots on the Hot Rod series amps). Once removed, there was no more low resistance from that CCW solder pad to ground, but still existed between the pot's terminal and the support bracket.
Nothing visible, so opened up the pot, extracting the cover. Nothing inside to be causing that near-short, and now, with the cover removed, no continuity at all between the resistance track/wiper and support bracket. Looked inside the cover, nothing caught up in the little bit of grease inside. Odd. Put the cover back into place, checked for shorts....closed two of the four tabs, still ok.....closed the other two tabs, still ok. Put the pot back onto the PCB and soldered into place. Still ok. Re-connected my pink noise generator to the top of the tone stack, output to my analyzer/headphone amp, and the tone stack now working proper.
I hate that! Pull something completely apart, with it yielding a short, open it up, nothing there to be causing it, plenty of gap between the grounded cover and the inner working of the pot, put it back together and it's all good again? Ah, well, it racked up 3.5 hrs of shop labor to solve it. Coulda been worse.....like needing to make a Peavey 5150 quiet again.
OM_leg_bassamp_Bassman_TV_Amps_20092012.pdf
Fender Bassman TV Series Schematics.pdf
Back then, I had found a tiny lead cutting that got trapped underneath one of the tiny DPDT pushbutton switches, managed to extract it, and all was working again. It just came back yesterday with the complaint of Low Output. Powered it up, and, found it was doing the same thing as before....no Mid or Bass EQ, no Deep Bass Switch, and low output. So, pulled it apart, extracted the tone stack PCB to see if it was again measuring low resistance from the Treble pot's CCW terminal. It was, same reading as I got before (143 ohms). This time, no debris found trapped under the switches, though removed them just to be sure there wasn't something hiding. Traced the short down, kept returning to the Treble pot's CCW terminal, with nothing on the board in the range of 143 ohms. Removed the treble pot (having support brackets, like the pots on the Hot Rod series amps). Once removed, there was no more low resistance from that CCW solder pad to ground, but still existed between the pot's terminal and the support bracket.
Nothing visible, so opened up the pot, extracting the cover. Nothing inside to be causing that near-short, and now, with the cover removed, no continuity at all between the resistance track/wiper and support bracket. Looked inside the cover, nothing caught up in the little bit of grease inside. Odd. Put the cover back into place, checked for shorts....closed two of the four tabs, still ok.....closed the other two tabs, still ok. Put the pot back onto the PCB and soldered into place. Still ok. Re-connected my pink noise generator to the top of the tone stack, output to my analyzer/headphone amp, and the tone stack now working proper.
I hate that! Pull something completely apart, with it yielding a short, open it up, nothing there to be causing it, plenty of gap between the grounded cover and the inner working of the pot, put it back together and it's all good again? Ah, well, it racked up 3.5 hrs of shop labor to solve it. Coulda been worse.....like needing to make a Peavey 5150 quiet again.
OM_leg_bassamp_Bassman_TV_Amps_20092012.pdf
Fender Bassman TV Series Schematics.pdf
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