testing JFETS: J201 higher gain than 2N5457?

[Jay Doyle]

The voltage jumps from 3.3V to 6.8V by a microscopic turn of the trimpot. I had this in the Flipster as well, then some board members suggested using a smaller value trimpot, some adding a resistor in parallel to bring down the resistance.

OK, are you sure that you used a 100k Trim? If so then you are right to try to use a smaller trim or place a resistor in parallel. This does happen sometimes. You may want to take an educated guess as to the resistance in that area of the trim and then see if a lower value trim can be used for a little more control. A multi turn trim would be the best solution, but you etched so...

With the third Pro Tweed trimpot, that was something different, I couldn't get the 9V down with any credible value (100k-800k), but did go down to 6.5V with a 3M4, so I'm sure it's not a solderbridge at hat location, and am sure connetions is ok. I etched the layout from ROG.

OK, don't take this the wrong way, but I wouldn't be too sure about your not having a solder bridge, a short or an open connection between the trim and Q3s drain. I took a look at the layout on ROG for the Prof Tweed and the distance between the trace from Q3s trim connection and where it passes between C8 is two pixels wide on the transfer and that trace looks to be maybe 10 mils thick. I've etched a lot of boards and I wouldn't be confident in my PNP ability to get that one done right. Plus, leave it in etchant for too long and a thin trace like that can have the copper underneath it eaten away and you would never know it.

My guess: is that you have an open connection on the drain of Q3, try soldering a jumper from the wiper of Q3s trim to Q3s drain and see if that lets you dial it in. When you get up to 3M4 and you are able to drop the voltage to 6.5V, it sounds like the 3M4 and the input impedance of your DMM are forming a voltage divider, dropping the voltage. Which leads me to think open connection.