Quote:
Originally Posted by old_coot88
I would try a couple of things first. Run a jumper from the 12BY7 cathode (pin 1) to pins 3 & 9. See if contrast improves.
If it does*, check resistance from the wiper to the top lug of the contrast control when it's rotated to max contrast. It should be zero or near zero ohms. If it is, try shorting out the 39 ohm resistor R109. See if that gives more grunt.
*Be sure jumper is removed.
|
When I jumper pin 1 to 3 & 9 contrast improves. Not a whole lot but an improvement.
When I short out R109 (with the jumper removed from pin 1 to 3 & 9), the contrast improves just barely.
Because the contrast pot is 5K, it doesn't take a lot of rotation to really change the contrast. BTW, with the contrast all the way up, I read 94 ohms from wiper to the pot end.