Úprava audiovýstupu

Bob Nagy, AB5N - Throttling back the audio output level

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 TDA2000
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..separate 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.

Good Luck and 73's!

Bob Nagy  /  Chief Eng.  KVRX FM  /  KVR-TV9
Amateur Radio - AB5N    442.15/R    1292.3/R
email:  www.botany.utexas.edu/facstaff/stfpages/bnagy