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)
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.
