Symptom:
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:
Measure the transmit power output and the "DB" terminal voltage at a
dial frequency of 145.020 MHz.
Add the 1 dB circuit if the transmitter power is 60 W or greater and
the "DB" terminal voltage is 10 vdc or less.
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:
Description
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
Caution:
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.