SB-018

Service Notes on TR-7400A                             10-6-77

PLL and LED Display

1.   No transmit or PLL unlock light on:  R-27 shorted to shield at D-1 on
VCO Unit (X50-1370-10).

2.   LED display changes when pushing on LED boards:  Remove LED display
boards on display mother board.  Trim all wires as close to the circuit
board as possible.  Insulate the foil side of the board with thin plastic
and reinstall.

3.   PLL unlock light on:  Check for 9 volts at 9V terminal on VCO Unit
(X50-1370-10).  Check Q10 (2SC496) and zener D5 (WZ-061) on this board.

4.   PLL unlock light on and no signal at PI terminal on PD Unit (X50-1380-
10):  Check for open ferri-inductor L1 in emitter lead of Q1 on VCO Unit
(X50-1370-10).

5.   PLL unlock light on and no signal at pin 3 of IC4 on PD Unit (X50-
1380-10):  Check for proper divide down signals on IC10, 5, 6, 7, 8.

6.   Digits in display are intermittent or some segments are not properly
lit:  Reseat the connectors on the Display Unit (X54-1210-10).  (Push them
with a small screw driver.)

7.   PLL unlock light stays on when unit is first turned on or cold:
Change R4 on the PD Unit (X50-1380-10) from 470 ohms to 47 ohms.

8.   PLL unlock light is on for about 2 minutes when the unit is first
turned on and the inputs to IC4 on the PD Unit are ok, but there is no
output:  Defective IC-4.

9.   LED display changes when slightly turning or pressing down on channel
select knobs:  Check for loose solder connections on the circuit board
under the BCD switches.  (ON bottom of unit.)

10.  One segement of one LED is dim:  LED may be defective or the resistor
network may not be properly soldered.

11.  Speaker howl at high volume levels:  Reseal the VCO coil (inside small
shield box) on the VCO unit (X50-1370-10).  use Q dope or a small amount of
Epoxy glue.  The VCO must be retuned after the glue dries.

12.  -600 KHz offset does not work:  Check for open ferri-inductor L1 on PD
unit (X50-1380-10)

13.  LR frequency cannot be adjusted to meet specifications:  Check for
proper frequency at TP1 on the PD unit.  It should be 2.56000MHz.  (Must
use a 33 pf capacitor when counting this frequency)