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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.
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