FT-736R - F2887103A audio board fault location and repair.

The FT-736R VHF/UHF transceiver developed a start-up fault that manifested as a momentary flicker of the frequency display and other instrumentation backlighting, or a failure to start altogether. Upon pressing the power button multiple times, the radio would eventually start up and operate normally. This fault appeared to be identical to a common power supply issue that had been identified for this particular radio. Convinced that this was indeed the problem, I planned to carry out the recommended power supply repair, which involved replacing the power supply electrolytic capacitors and re-soldering any heat-stressed connections on the power supply PCB.

During one instance of powering the radio up, it started normally without any flickering of the instrumentation lights. However, there was no audio output, not even a click, when powered up or when the mute function was opened. The fault that had affected the receiver/audio had also caused the power supply to trip off until the fault finally cleared itself through destructive means, overloading the power supply. The audio board was suspected to be the faulty section, and using a sniffer audio amplifier, I sampled the audio signal into the AF board and detected good, clean audio when opening the mute. The fault has now been isolated to the AF board. Subsequently, the AF board was replaced with a new one, and the radio is now fully operational.  

Photo 1 AF UNIT F2887103 installed to the right of the picture circled in red.

The faulty AF board has been repaired and kept as a spare. Further investigation revealed that the R30 resistor (1 ohm 0.25 watt) had open circuited, likely due to a failure of either the C20 electrolytic capacitor (2200uF) or the Q06 amplifier chip, which caused excessive current flow through the resistor.

Although the C20 electrolytic capacitor seemed fine based on capacitance measurement and leakage test, it was still replaced due to its age and the possibility that it could have been the source of the problem. With the C20 electrolytic capacitor eliminated as the cause of the fault, attention turned to the unlikely possibility of the Q06 amplifier chip being the culprit, which was also replaced.

The R30 resistor (1 ohm 0.25 watt) is apparently a known issue with this radio and can fail on its own, likely due to being under-rated for its location in the circuit. The R30 resistor was replaced with a 0.5-watt resistor, but ideally, it should be a 1.0-watt resistor. The power supply tripping indicated that there were other factors at play, possibly related to the C20 electrolytic capacitor or the Q06 amplifier chip.

In the end, the radio is now operating normally, and a good spare AF board is available. The power supply will undergo preventative maintenance, including the replacement of 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.

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Page last revised 07 September, 2024