For complaints of either induced power supply hum, or intermittent frequency stability while using the DFC, perform the following changes to the VCO unit X50-1550-00:
- Install a 330 Kohm resistor between Q11 G1 & the 8V line.
Connect from L7 at C12 to Q11 G1, at R35.
- Replace L12 68uH with new part L40-6831-21.
- Replace L13 150 µH with new part L40-1541-27.
- Install magnetic shield as illustrated. Use the two existing screws, plus double faced tape to prevent vibration.
No adjustments are necessary.
Note: On page 37 of the Service Manual, the VCO Board view shows reversed callouts for Q11 & Q15. D & S, and G1 & G2 are transposed.