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Spec-1 Preamp - Massive gain using 100Hz (+)bass control on L ch only

E

Electman

New Member
Hello All, hope you can help me.

Before:
Normal sound without using tone controls. With Tone ON, and the 100Hz bass control set to any positve value - there is massive gain on the LEFT Channel. Right channel is fine. Doesn't matter where you set the 50Hz bass control. Treble is fine. Bass works on Right channel when using the (-) side of the bass control.

Steps I have taken:
Rebuild all tone switches - disassembled each, cleaned contacts of oxidation/ contamination, lubricated and tested - then reassembled. Process done to each of the 4 rotating tone control switches and the Tone on/off switch.
Replaced ALL semiconductors on Tone PCB.
Replaced all electrolytics throughout unit with audiophile grade caps.
Tested each and every value of resistor on entire Tone PCB.
Removed and tested for value each and every film and ceramic capacitor on entire Tone PCB. Didn't test polystyrene's, but did ohm them out for shorts.
Reflowed all solder connections on entire PCB.
Checked/ adjusted 48V supplies.

Problem still exists. Scratching my head over here... Any help with this problem would be gladly and cheefully accepted.

Thanks!
 
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The 100Hz bass control, S10, is in the feedback circuit so massive gain means NO feedback. Broken solder joint or copper trace?

Craig
 
not looking at the schematic but a bad pot was going to be my guess. Ohm meter ought to tell you. If the switches are also in that circuit, ohm check those to verify both channels are actually working like expected. I've had plenty of 2 section pots and switches fail on one side only.
 
gadget73 said:
not looking at the schematic but a bad pot was going to be my guess. Ohm meter ought to tell you. If the switches are also in that circuit, ohm check those to verify both channels are actually working like expected. I've had plenty of 2 section pots and switches fail on one side only.
Click to expand...
Yeah, thanks for that. That switch was the first thing that I removed and disassembled. I went through each wafer and made sure of all connections on all three wafers. I even reflowed the connections on the wafer. I think the only things left are either the switch (i missed something) or a broken trace or connection. Thanks again!
 
llwhtt said:
The 100Hz bass control, S10, is in the feedback circuit so massive gain means NO feedback. Broken solder joint or copper trace?

Craig
Click to expand...
Yes sir - that piece of information is extremely helpful. If the switch tested fine (I'll double check all the connections on it again) then there must be a break in the circuit - I now have a direction to head in. Can't than you enough for this insightful advice!
 
gadget73 said:
not looking at the schematic but a bad pot was going to be my guess. Ohm meter ought to tell you. If the switches are also in that circuit, ohm check those to verify both channels are actually working like expected. I've had plenty of 2 section pots and switches fail on one side only.
Click to expand...
Hi, thanks for replying. This unit uses rotary switches, not pots, for the bass and treble controls. I've already rebuilt that switch and tested every connection - but I will do it again. Between that and looking for a broken trace or connection will be the answer. Thanks again.
 
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