SB-827
Service BulletinAmateur Radio Division
Subject: TS-120S - VFO StabilityDate: June 6, 1980
VFO drift can also be reduced by replacing the dial lamps with high brightness LEDs. Perform this mod ONLY if the VFO drift, after warmup, is more than 200Hz.
(Supercedes 7/9/79 bulletin)

VFO drift characteristics may be improved by changing component values and location in the VFO. This change applies to serial numbers before 0010001.

NOTE: DO NOT disturb the lead dress or component placement while working inside the VFO.
  1. Remove the VFO unit by four 3mm hex bolts.
  2. Withdraw out the front panel and unplug the leads.
  3. Remove the VFO cover by 5 phillips head screws.
  4. Remove C17 and C18 from the PCB. These capacitors may be cut off flush to the board.
  5. Reposition the GROUND terminal for the Variable Capacitor by heating the post and pushing it through to the opposite side of the PCB.
  6. Install paralleled 10pf green (CC45SG1H100J) and 18pf yellow (CC45RG1H180J) caps from the bottom of the tank coil terminal (LH) to the modified GROUND post. Position the caps near the coil, not the PCB. Make certain the GROUND lead does not touch the coil or coil mounting.
  7. To reassemble, perform steps 3, 2, and 1. Snug all VFO case screws. A later complaint of frequency jump would indicate a loose VFO case.

Analog Dial Recalibration

  1. Turn the main dial fully CCW. The red cursor should line up with the VFO mark on the sub dial. If it does not, remove the main knob, loosen set screws and align the scale start with the red cursor.
  2. Turn the main dial to 50kHz analog. Adjust the aluminum slip subdial to line up with any one of the larger black dial marks.
  3. Note the digital error. If it is MORE than 2kHz adjust the VFO trimmer cap TC1 (center under the seal tape) to exactly 50.0 on the digital readout.
  4. Turn the main dial to 450kHz analog. If the error is LESS than 2kHz it is in spec. If the digital error is greater, proceed:
    1. For instance if the digital display shows 14.454.0 (+4kHz error) then multiply the error by four (+16kHz) and adjust the VFO trimmer cap to the desired frequency (14.450.0) MINUS the error value (14.450.0 - 0.016.0 = 14.434.0). Next adjust the VFO inductor (front under the seal tape) back up to the desired frequency (14.450.0).
    2. If the error was in the minus direction, reverse the direction of correction adjustment above.
  5. Check VFO linearity. The digital readout and the analog dial should agree within +/- 1kHz at every 100kHz dial point.
©1980 Kenwood