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TR-8400 mods | Radioaficion Ham Radio

TR-8400 mods

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TR-8400 Transmit Delay Time

Author: Trio-Kenwood Communication, inc.


Service Bulletin no. 855 (25-1-1982)

PLL lockup time when switching from receive to transmit may be shortened for faster transmitter response. This change applies to units before serial number 1060455.

On the PLL unit X50-1670-xx, check the values of R31 (390 ohm) and C47, C48 (6.8uf) near Q7 and Q8. If these component values differ, change as required. This will give approximately a 0.12 second delay, which should be satisfactory for practical use. However, if shorter delay is required, replace R31 with an 820 ohm resistor. This will yield an approximate 0.096 second turn-on delay.

No adjustment is required after this part change.

Note: When R31 is 820 ohms, CTCSS (Subtone) will be slightly distorted.

Installation time for this procedure is 1/2 or less.


TR-8400 Transmitter Freq Response, Voice or DTMF

Author: Trio-Kenwood Communication, inc.


Service Bulletin no. 853 (25-1-1982)

Operator complaints of insufficient lows in transmit, or faulty Autopatch operation traceable to incorrect twist (high to low tone group balance) may be remedied by changing two capacitors at the mic amplifier input.

On the RX/TX unit X44-1340-11, change C62 and C65 from 0.1uf to 1.0uf, 35v tantalum (CS15E1V010M) to increase low frequency transmitter frequency response. Adjust Deviation (VR3) or mic gain (VR2) as necessary.

Installation time for this procedure is 1/2 or less.


TR-8400 Sub-audible Encode Installation

Author: Trio-Kenwood Communication, inc.


Service Bulletin no. 843 (18-2-1981)

When installing a sub-audible encode in a first-lot radio, change the Tone switch power source from the 8C (common) to the 9T (transmit) line. This will prevent the encoder from distorting the receiver when the Tone switch is ON.

Preocedure.

  1. Remove the bottom cover (2 long screws, rear, 3 short screws, front) and unplug the speaker.
  2. Cut the GRAY lead to the Tone switch wiper one inch from the switch. Tape-insulate the long lead to the harness.
  3. Connect the switch wiper to the Mic connector, pin 5.
    (See Fig. 1)
  4. Install the encoder, connect the speaker, and replace the bottom cover.

For complaints of distorted sub-audible tone in a first-lot unit, replacing components and readjusting the PLL will reduce or eliminate distortion.

Procedure.

  1. Remove the top cover (4 screws).

  2. Remove the rubber conductive shield.

  3. remove the aluminum cover from the PLL unit X50-1670-xx (17 screws).

  4. Unplug connectors J2-J6.

  5. Remove 11 screws and 2 threaded studs securing the PLL printed circuit board. Shift the board to the rear, then swing up and tothe right.

  6. Use Wikit or a desoldering tool to remove and replace:
    Component      was      replace width (vicinity of Q7)
    R30            12 Kohm  18Kohm 1/4W
    R31            820 ohm  390 ohm 1/4W
    C47            6.8uF    15uF Tantalum (observe polarity)
    c48            6.8uF    15uF Tantalum (observe polarity)
    
    Note: Don't create solder-bridges or lift the foil.

  7. Reverse steps 5, 4, 3 to partially reassemble.

  8. Perform the PLL adjustment listed in the Service Manual, page 24, item 2.

  9. Reverse steps 2 & 1 for final assembly.

TR-8400 Sub-audible Encode Installation


TR-8400 RX internal sporious signals

Author: Trio-Kenwood Communication, inc.


Service Bulletin no. 845 (22-5-1981)

Receiver internal sporious signals occuring at xx1.7 MHz and xx6.7 MHz may be reduced or eliminated by adding an additional ground and bypass capacitor in the PLL. These are third overtone (21.6 MHz) products of the PLL IF (7.2 MHz). This applied to all units before S/N 111xxxx.

Procedure.

  1. Remove the top cover (4 screws).
  2. Remove the rubber conductive shield.
  3. Remove the aluminum cover from the PLL unit X50-1670-xx (17 screws).
  4. As illustrated, solder a jumper between the DBM and VCO compartments.
  5. Solder a .01uF 50V disc ceramic capacitor (CK45F1A103Z) from the DBM shield to the case of Q4 (3SK76 or 3SK92).
  6. Reverse steps 3-1 to reassemble.

TR-8400 RX internal sporious signals

Note: Installation time for this procedure is ½ hour or less.

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