Yaesu FT 2000 low audio output level

After I connected FT 2000  with my backup radio FT 950 together on SP-2000 where two audio input jack can be connected there, and one of  two transceivers audio output  can be selected with switch on the front panel, I have found, that FT2K has  a rather low audio output level compared to FT 950

This is surprising because the both transceivers have very similar audio circuit.  There is also   audible hiss, which is present also when the radio´s volume is turned down. I noticed that the FT2K audio   output level on the headphone jack as well on the external speaker jack  seems really low as compared to other radios.

When I have had inspected the audio circuit i found that my FT2K version Lot 2-5 has some minor changes in audio chain compared to Lot 1. The result is, that audio preamplifier Q111D has to low gain. To raise the audio  gain for about 9dB I have changed R 1338 ( 4,7kOhm) to 12 k Ohm.  

I also  decided that the most effective way of addressing the rig audio  noise was to modify the AF passband. This audio noise which sound like a white noise or hiss is something disturbing  listening weak signals if using sensitive HI-FI headphones. You can simply get rid of it to roll off the highs with    2700pf  cap added parallel to new resistor 12kOhm. The receiver is now super quiet and now I have enough audio gain. ( See picture 2 and 3 )

Picture 2: Audio circuit Lot 2-5,6

Preparing for installation:

1.      Locate the working area on the Main unit PC board

2.      Unsolder SMD resistor R1338 – 4,7kOhm

3.      Solder SMD resistor 12kOhm

4.      Solder SMD cap 2700pF parallel to resistor 12kOhm

The cap and resistor size are in the SMD 0603 category.

Picture 3:  Main Unit Lot 2-5 ,6 ; preamplifier  Q111D and R1338  location

Before changing some elements simulations were done in EWB program which is a fast utility for these kind small changes on this circuit. I mostly work CW but I   decided on a  2,3 kHz/3dB  corner frequency target because I sometimes do SSB but never FM.  



My observation after this mod is that the hiss level is far less with the cap changes. I also made some measurements (see picture 4)

Picture 4 : New  6dB  audio spectrum taken at headphone AF Output  - Mode FM

Warning !!! Changing any hardware inside FT2000 will void your warranty. I am not responsible for any hardware failure. You made this mods on your own risk.! This modification requires a high level of soldering skill, possibly beyond that normally possessed by the average HAM. Professional assistance is advised if you are not confident that you have  this ability.