Barry – in the Super Drive 7-‘s build doc you mention the following regarding “TR1” . . . .
“TR1 / R18– either the trim pot or fixed resistor is used but not both. When using TR1 the trim pot should be set so that the voltage reading at the collector of Q3 is approximately 4.5V to 6V.”
To your attention I draw the point that the ‘Gain’ pot is also part of that optional trimmer adjustment to set the voltage of Q3’s collector to “approximately 4.5V to 6V”. Being part of that section of the circuit, my question is: “At what setting should the ‘Gain’ pot be set when adjusting Q3’s collector voltage?”
I’ll take a stab at this. It’s been years since I built my Super 70’s, but I believe the Gain pot must to turned full CW (to set max resistance). That way it is combined with TR1 to set the max resistance seen by the Emmiter of Q3. That should permit the correct bias range for the collector to be 4.5V to 6V.
Barry may know better. I see that the build doc does not specify this.
Wilkie – thanks! That’s as I anticipated, but wanted to ask all the same. Once I get ‘er built, I’ll play with the ‘Gain” pot setting whilst measuring Q3’s collector voltage. Maybe I’ll learn something.
Also, this Super Drive 70’s look very ripe for a number of mods. 🙂