Concert Bass "Distort" control failing

[MarkG]

I have a Concert Bass that the Distort control does not seem to work .It used to but had not tried it in a very ling time. When I was testing it out getting it ready for sale,it wouldnt work.It is not a footswitch problem,as I can hear the volume increase when I switch it on as it does with the boost switch,and as I increase the distort control the volume increases also,but never fuzzes up,as it should. Any idea what component may be failing?

[JoeArthur]

Take a look at the Concert Bass Schematic available on this site...

http://www.sunnamplifiers.com/schematics/concertbass.gif

First locate Q102.  This FET is primarily responsible for the distortion.  Half of the DISTORT control is used to variably bypass the source of the FET to ground.  This FET is in a split rail configuration, and at maximum settings of the DISTORT control (which provides a minimum resistance), the gain of this stage is primarily determined by the value of R107 and any "resistance" provided by C105.

Based on this, there are a couple of possibilities.  First, I would suspect footswitch, which could not be completely grounding the connection.  You can measure the resistance at point 52 to ground (with the amp unplugged) and step on the footswitch.  The reading should go from 68K or there abouts (off) to zero (on).  A higher reading than zero could either be the footswitch switch, cord, or dirty jack.

Second would be point 52 itself.  These amps have connections between components like jacks and controls, to the printed circuit board made with lugs that fit over pins on the circuit board.  Pull off the connector and clean it up.

Third would be the DISTORT control.  You can check for this by measuring resistance between point 52 and the lug of the pot that has a connection (to R110 and C105).  With the control fully clockwise (max distort), you should get a reading of zero.

Fourth, would be C105.  Electrolytics can develop a high ESR value as they age.  You can temporary bridge another cap across the existing one to check for this.  If the distortion comes back, replace this cap.  Fourth number two would be R107... just measure the resistance across it and you should get 220 ohms or less.  If it is much more, then replace it.

Finally, if all that checks out, then the remaining possibility is that the first stage Q101 is not providing enough for Q102 to be driven into distortion.  C125 would be the likely culprit here and can be bridged temporary with another cap to check it out.

Hope this helps!!