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FT-736R F2887103A AUDIO BOARD REPAIR 

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,

 

 

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Page last revised 23 April, 2018
 

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