FT-736R
- F2887103A audio board fault location and repair.
The FT-736R VHF/UHF transceiver
developed a start-up fault that appeared when powered on as a
momentary flicker of the frequency display and other instrumentation
back lights or failure to start. When pressed on several times the
radio would start up and operate normally, this fault appeared to be
identical to a common power supply fault that has been identified
for this radio. Totally convinced that this was in fact the problem
I panned to carry out the recommended power supply repair by
replacing the power supply electrolytic capacitors and re-solder any
heat stressed connections on the power supply PC board.
When powering the radio up on one
occasion the radio started up normally first time with no flickering
of the instrumentation lights, however there was no audio not even a
click when powered up or when the mute was opened.
The fault that has taken out the
receiver/audio had also caused the power supply to trip off until
the fault had finally destructively cleared itself from overloading
the power supply. The audio board was felt to be the likely faulted
section and with the aid of a sniffer audio amplifier the audio
signal into the AF board was sampled and good clean audio was
detected when opening the mute, the fault has now been isolated to
the AF board.The AF
board was replaced with a new board and the radio is now fully
operational.
Photo 1 AF UNIT F2887103
installed to the right of the picture circled in red.
The recovered faulty AF board has
been repaired and kept as a spare. Investigation revealed that the
R30 (1 ohm 0.25 watt) resistor had open circuited as result of a
failure of either the C20 (2200uF) electrolytic capacitor or the Q06
amplifier chip by causing excessive current flow through that
resistor.
While the C20 electrolytic capacitor
seemed to be okay based on a capacitance measurement and leakage
test it was replaced due to the age of the component and that it
still may have been the problem. The elimination of the C20
electrolytic capacitor as the cause of the fault left the unlikely
possibility of the Q06 amplifier chip as the culprit which has also
been replaced.
R30 (1 ohm 0.25 watt) resistor is
apparently a known fault for this radio and can fail on its own
probable due to 0.25 watt being under rated for its location in
the circuit. The R30 resistor was replaced with a 0.5watt resistor
and should really be a 1.0 watt.
The power supply tripping indicates that there was more going on
here probably relating to the C20
(2200uF) electrolytic capacitor or the Q06 amplifier chip.
Bottom line is that the radio is now
operating normally with a good spare AF board available. The power
supply will have the preventative maintenance of the replacement of
the electrolytic capacitors and any heat stressed components in the
near future.
Fig 1 AF UNIT F2887103 Schematic
Fig 2 AF UNIT F2887103 PCB component layout viewed from top,
Fig
3
AF UNIT F2887103 PCB component layout viewed from bottom,
Video
of the F2887103A audio board fault location and replacement.
