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Introduction
The FT102 is a Yaesu hybrid HF transceiver using 3x6146Bs in
the PA section. It is a high performance radio with a low distortion
high power output on transmit and a high performance receiver. The
examples of these radios I have handled lately have all suffered
from VFO drift, so I did a search around the net for an aftermarket
“Huff and Puff” stabiliser. A Google search for “PA0KSB” and
“Hans Summers” will reveal some background information on the
principles of “Huff and Puff” stabilisers and their origins.
I decided to use the X-Lock kit from Cumbria Designs. The
X-Lock uses a PIC micro-controller and includes a tri-colour LED
that indicates loop status. The kit was quick to assemble, the
documentation clearly described assembly instructions as well
circuit operation.
Preparation
My
aim was to not dismantle or modify the FT102’s VFO in anyway so I
applied the X-Lock correction signal via the clarifier connection to
the VFO.
On examining the circuit diagram for the “Local Unit” of
the FT102, I found that the clarifier or RIT relay, selects either
an internal potentiometer or the external one depending on whether
TX (XIT) or RX (RIT) is selected and depending on whether the radio
is on transmit or receive. The external potentiometer is the same
value as the internal one (5k) but the resistors that set their
ranges are different, the internal potentiometer has more range than
the external one. This actually makes little sense as the internal
pot is merely used to adjust the centre position of the external
control.
Circuit Changes
I decided to replace the resistors R163, R164,
R165 and R166. These are now all 10k. The original plus or minus
3kHz of RIT range is still retained. The resistors are circled in
figure. 1 (right). Whilst the local board was out I also connected
an extra wire to a pad on the moving contact of the relay RL01. This
track also connects to R167 and to the clarifier pin of J03.
(Note.
This is also a good time to change the relay if your FT102 has been
jumping about in frequency!) The resistors that are changed in this
modification can be seen in the circuit diagram in figure 2. I felt
that with the original values the “control range” of the X-Lock
would vary significantly with adjustment of VR07 and the external
clarifier control.
The underside of the Local Unit is shown in
figure 3. The yellow wire vanishing from the bottom of the picture
is routed up to the X-Lock. (Note. The flux marks indicate where I
changed the resistors and the relay!). Route this wire through the
loom to the region of the display board but do not connect it at
this stage. Refit and reconnect the local unit and check for loose
wire cuttings. Connect a speaker and switch the radio on. With the
marker on (rear panel switch), centre the external clarifier control
and tune to the marker to produce a clear beat note. Adjust VR07 so that the marker produces the same
beat note whether RX RIT is depressed or not. Once this stage is
complete the bottom cover of the transceiver can be replaced.
Installing the X-Lock
Unit
I will leave it to the reader to decide how to
mount their X-Lock, mine is still a breadboard so is temporarily
attached with double sided pads to the top of the frequency counter
chip on the counter board. I think it is best to remain in roughly
that area of the radio as it is well away from the receiver and IF
stages. Also it means the signal paths associated with the X-Lock
are kept very short. Another bonus is that the LED can be observed
whilst adjusting the radio with the lid removed.
The layout of the Counter Unit is shown in Fig. 4. The various
signals and power needed by the X-Lock are available on this board.
The brown wire to J5005 carries +12v, Gnd is available in a number
of places such as the screens of the wires on J5002. An input signal
which tunes from 0.5Mhz to 1MHz appears on J5002 however, I found
the level here was inadequate to drive the X-Lock so I extracted the
signal from Pin 9 of Q5005. Please note that this signal is a mixed
product of the VFO and crystal band oscillator, so the correction
signal will be applied to the whole Local Oscillator system
correcting any drift produced by the mixing oscillators as well as
the VFO. As these are all crystal sources drift is minimal but they
are included in the correction process.
A close up of the installed X-Lock is shown above
in figure 5. The kit includes all of the mating connectors used to
make the connections to the board.
Observations
The time constant
can be relatively “fussy”. The default 390k
is fine as a starting value but if the correction introduces a
“warbling” sound quality to demodulated audio, it may have to be
increased somewhat. Mine currently uses a 1 M Ohm resistor. I have
also included a resistor in series with the “VAR” correction
output of the X-Lock. In the three FT102s that have been modified so
far, we have used a value of 47k for this. This value produces a
maximum correction roughly plus or minus 2 kHz. The FT102 drifts
less than this but I believe there is scope to having a little
overkill. The input adjustment of the X-Lock needs to be turned up
until the LED only just goes green. In mine, if I turned it up too
high it seems to see one of the clock signals on the display board.
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