Service BulletinAmateur Radio Division

Subject: TM-732A - APC Circuit ImprovementDate: March 28, 1994

Failures of APC circuit transistor Q232 can occasionally be attributed to the gain dispersion characteristics of the VHF power module. Some of these devices allow normal transmit power output even with reduced "DB" terminal voltages. If the unit is then operated for extended transmit key down periods Q232 can overheat and fail.

Corrective Action:

  1. Measure the transmit power output and the "DB" terminal voltage at a dial frequency of 145.020 MHz.
  2. Add the 1 dB circuit if the transmitter power is 60 W or greater and the "DB" terminal voltage is 10 vdc or less.
  3. After adding the attenuator, re-adjust the transmitter output power for 50 W and check the "DB" voltage. The "DB" voltage should now fall between 10 and 11 vdc. If it does not adjust the values for R98, R100, and R99 until this voltage range is obtained.

Parts Required:
Part No.
Circuit Description
820 ohm chip resistor RK73FB2A821J R98, R100
470 ohm chip resistor RK73FB2A471J R98, R100
270 ohm chip resistor RK73FB2A271J R98, R100
5.6 ohm chip resistor RK73FB2A5R6J R99
10 ohm chip resistor RK73FB2A100J R99
18 ohm chip resistor RK73FB2A180J R99

Note: Quantities vary depending upon level of attenuation required to meet requirements of step 3.

Pad Attenuation R98, R100 R99
1 dB 820 ohms 5.6 ohms
2 dB 470 ohms 10 ohms
3 dB 270 ohms 18 ohms
This modification requires soldering equipment rated for CMOS type circuits. It also requires familiarity with surface mount soldering techniques. If you do not have the proper equipment or knowledge do not attempt this modification yourself. Seek qualified assistance.

Time required for this modification is 60 minutes or less. Service code A:84 B:X57-4000 C:R99 D:91

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