Although Yaesu did a fantastic job of gain distribution in the FT-847 receiver, they bombed when it came to the last stage, the audio output. Even at lower settings of the AF Gain, the resultant audio is room filling and control is very critical. The TDA2003H audio amplifier integrated circuit (IC) is not being over-driven, it is just getting too much audio signal from the previous stage at the lowest input level. Operational and distortion improvements can be gotten with this modification. Read the entire procedure before starting the actual modification. Later model FT-847s may not have these audio problems. Also remember that if your rig is under warranty, and you modify it, the warranty is void.
Take off the FT-847 covers and turn the rig over on its top, exposing the bottom. Near the rear of the rig, on the opposite side from the input power connector, locate audio amplifier IC Q1086; it has five (5) pins and is close to the purple 470 F audio output capacitor, C1242. The input to the audio amplifier is on Pin 1, the pin with the long leg at the end nearest the rear of the rig. There is a small 1 F audio input capacitor, C1225, close to the pin. The modification consists of opening input Pin 1 of the IC and inserting a 47 Kę resistor in the opened input lead. The easiest way is to snip (cut) the middle of the IC pin, separate it, and add the resistor in series with the input lead.
To insert the series resistor:
- Cut input Pin 1 of audio IC Q1086, observing ESD precautions.
- Separate the two halves of the cut lead.
- Prepare the 1/4 W, 47 Kę resistor with just enough lead length to contact the separated IC input lead.
- Tin the resistor leads and the IC leg contact points.
- Tac solder the resistor to the separated leg of the IC.
The 47 Kę resistor gives just enough level reduction, allowing setting of the AF Gain control at 10 or 11 o'clock instead of 8 or 9 o'clock, the initial lowest level settings of the control . There is still enough audio to "blast out" the internal or external speaker.
With this modification, the audio seems even smoother and cleaner, and the problem of erratic audio for the first quarter turn of the AF Gain control, as reported in older units, is also gone. With the above mod and the replacement of the original ceramic SSB IF filter with International Radio #702 SSB crystal filter, for better receiver selectivity, there is a feeling of having acquired a new receiver.
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