1967/68 Superbass Presence control not working at all.

[Jos]

Hello Gents, I have a problem on my superbass build where the presence pot makes no audible change at all. I have re-traced the circuit about 50 times now paying particular attention to the feedback circuit and phase inverter section. The kicker is that this is about my 20th build and this ones kicking my _ _ _! I have checked the caps and resistors as well as have the feedback wire hooked to the 8ohm tap off the speaker selector switch. I even tried disconecting the presence lead wire off the pot and no difference at all. To make it better.. The amp sounds amazing and absolutley everything else works correctly! Any advice on what could be causing this issue. I have checked my grounds and all is good there. Member bobtec suggested I post here and add pics which I will this evening. Does anyone have any ideas on signal checking or voltages I can check in the Feedback circuit/phase inverter area? Any help is much appreciated! Thanks in advance. Jos

[axeman]

Check the grounding to the NF pot, Check the jumper wire that goes from the 10k resistor to the NF resistor, if that doesn't work post pic's, if that doesn't work bring it over 90706.

[Jos]

Thanks Axeman,

I have checked the jumper wire from the 10K resistor to the 47K feedback resistor. Is there a test I can perform between that 10K resistor and the 47K NF resistor? voltage/Signal of any kind? Does it suggest any possibilities to the fact I have no audible differenece when completely disconnecting the NF wire from the 5KL pot? I went right to the caps and 10K resistor/jumper wire to the NF resistor as a potential problem but all looked well? My ground wire leading from the 5KL pot to ground connects at the chassis below the pot per a Larry grounding scheme. i have tried a traditional ground buss along the back of the pots as well without change. Looks like pics will be in order as well as a personal visit. I work in Paramount once a week Thanks, Jos

[axeman]

Let me know.

[Jos]

Finally found the problem. Even though the jumper between the 10k resistor coming from the PI was correctly routed to the NFB resistor, I found that it was pinched between the board stud /nut thus ground it to the chassis. I feel stupid but relieved! It was just so hard to see but after a continuity test with the meter I confirmed I had found the problem. Thanks Axeman for your willingness to help out another fellow amp addict. Maybe we can share some amp builds soon. Take care, Jos