[Yaesu] FT-1000MP - main rx AF hiss - followup

[Steve Franke]

Hi All,

I started this thread, so I thought that I should follow up with what I
learned about my FT1000MP.  After some fairly extensive tests, I decided
that my transceiver is within specifications, and performs as the designers
intended. I also discovered that my perception of significantly more high
frequency hiss on the main receiver output relative to the sub-receiver
output (CW mode, EDSP demod OFF, 500 Hz filters) was correct, but I now
have a better understanding of the constraints that led to this performance
characteristic.  I should note that the audio sounded fine to me when EDSP
demodulation was turned ON.

It turns out that the CW-mode AF low-pass filter is different on the main
and sub receivers in my unit.  The sub-receiver employs a filter with a
slightly peaked low-pass response with the peak at about 700 Hz, whereas
the main receiver has a low-pass filter with flat response out to about 1
kHz or so.  I discovered this only after inspecting the AF circuit board,
as the schematic that came with my radio (and the service manual schematic)
showed identical analog AF CW filters in main/sub receivers.

The net result, on my radio, when employing analog demodulation and with
main and sub receiver AF gains set for equal total AF output powers, was 20
dB higher noise spectral density level at 2 kHz on the main receiver.  This
test was performed in CW mode, pitch frequency=550 Hz, 500 Hz IF filters,
no antenna connected, on 21.040 MHz with FLAT preamp engaged.

At least one reason for the use of a flatter and wider filter in the main
receiver is to ensure flat performance over the entire range of audio
"pitch" settings, which can place the center of the AF passband anywhere
between 300 Hz and 1050 Hz.  Since I listen to CW at a pitch of 550 Hz, I
chose to modify my radio so that the main rx CW audio filter has the same
type of peaked AF response as the sub-receiver.  Compared to the unmodified
AF filter, the modified filter has larger response within the passband
(peak at about 550 Hz), and lower response at the higher "hiss"
frequencies.  The net result, after the modification, is an improvement
(i.e. decrease) of about 15 dB in the main-receiver "hiss" components at 2
kHz when main/sub receivers are adjusted for equal AF outputs.  In short, I
am now a happy camper.

I cannot recommend the modification for everyone, as I chose to optimize
the CW mode AF response of my radio for a specific CW pitch setting, and
for use with the 500 Hz and 250 Hz IF filters.  Also, the fact that the
component values in my radio differed from the schematic suggest that not
all 1000MP's are the same in this regard.  That may account for the fact
that responses to my original post ranged from "I agree" to "no problem
here".  In any case, for those who are interested, the relevant component
values are as follows:

AF CW low-pass filter is located on the AF board.  Components that
determine frequency response of the CW-mode AF low-pass filter are R3018,
R3019, C3015, C3026.

Schematic indicates:
R3018=R3019=15K,  C3015=.033uF  C3026=.0047uF

My radio came with the following values:
R3018=R3019=12K,  C3015=.015uF  C3026=.0068uF

I changed C3015 to .062uF by adding a .047uF capacitor in parallel with the
original .015uF cap.

I should add that the modified filter is not in-line when EDSP demodulation
is selected.  Thus, the modification did not alter the performance of the
radio in that mode.

Thanks to all who posted suggestions and who replied to me via direct email.

73, Steve  K9AN
sfranke@uiwpls.ece.uiuc.edu



----------------------------------------------------------------------------
Steven J. Franke                    | phone : 217.333.8128
Space Science & Remote Sensing Lab. | fax   : 217.244.5624
319 C.S.R.L.                        | email : sfranke@uiwpls.ece.uiuc.edu
University of Illinois              |
1308 W. Main Street                 |
Urbana, Il  61801 USA               |
----------------------------------------------------------------------------



--
FAQ on WWW:               http://www.qsl.net/k7on/yaesu.html
Submissions:              yaesu@contesting.com
Administrative requests:  yaesu-REQUEST@contesting.com
Problems:                 owner-yaesu@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm