Although Yaesu did a fantastic job of gain distribution in the FT-847's receiver,
they bombed when it came to the last stage...the audio output. The volume control's
lowest setting results in room filling audio. The TDA200 audio amp chip is not actually
being over-driven, it just is getting too much audio signal at the lowest input level
from the previous stages. A great improvement can be gotten with the following mod.
Take off the covers and turn the rig over on its top....exposing the bottom. Near the
rear, opposite side from the power input connector, you will easily be able to locate
the audio amp IC. It has 6 legs and is next to the purple 470mf capacitor which it's
output goes through. The input to that amp comes in on it's #1 pin - which is the long
leg -at the end nearest the rear of the rig. You can see the small 1uf cap feeding
it -quite close to it. The mod is to add a 47K resistor in line with that leg. The
easiest way is to snip it in the middle of the leg..seperate the snipped conductor
and add the 47K in series with the two leads. Prepair the 1/4 watt 47K resistor with
just enough lead length to contact the seperated amp leg. "Tin" resistor and leg
contact points and tac solder the resistor in. (be sure that the leg is really
seperated). Remember-You can always remove the resistor and bridge the leg together
again at a later date.
The 47K gives just enough level reduction that you will now set your volume control
at 11am instead of 10am, and the innitial lowest level point will be about half what
it was. You may find that 68K or 100K may still provide acceptable volume levels.
There is still enough audio to "blast out" the internal speaker...or drive any external
unit. I even think that the audio sounds smoother and cleaner at this input level.
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