Modifications for the Yaesu
FT-227
19-07-1998 FT-227R & 9600 baud
Packet
Modifying the Yaesu FT227R for 9600 baud operation
by James Miller
G3RUH
1993 Sep 29
The FT227R "Memoriser" dates from 1977, and was
one of the first amateur base/mobile FM radios to use a PLL synthesiser. Power
output is 10 watts.
Its classic design lends itself well to 9600 baud
operation.
The synthesised local oscillator tunes from 133.3 - 135.3 MHz
in 10 kHz steps and for any particular channel uses the same frequency for both
RX and TX. Being unswitched, the synthesiser doesn't chirp when changing from RX
to TX. 5 kHz intermediate steps and 600 kHz repeater shift are effected by using
other internal crystals selected with the FUNCTION switch.
The RX IF
chain is at 10.7 MHz (144.0 - 133.3), while the TX modulation is applied to a
separate 10.7 MHz crystal oscillator for final output at 133.3 + 10.7 = 144.0
MHz.
Reception Notes
On the RX side, the IF filtering
consists of a 15 kHz 2-pole roofing filter FMT-15A at 10.7 MHz, two ceramic 4
element LFB-15 (muRata CFU455E2) ceramic filters at 455 kHz, and the
discriminator based on a muRata SFD455S4. The frequency response is some -12 db
down at 4800 Hz, and is much too heavy for 9600 baud where -6 db is the target,
but is perfect for 4800 baud operation.
Most of the damage is done by the
ceramic discriminator; changing two resistors partly cures that. Changing the
455 kHz filters to CFU455D2 does the rest, and allows for several kHz of
mistuning. You can also change the crystal filter to a 10M30AZ for a slightly
better eye, but this is only a marginal improvement and admits some of the
adjacent channel. TXEPROM selection 10 or 12 gives the best results.
The
discriminator output is very low, 20 mv/kHz, so a typical +/-3 kHz deviation
signal gives only +/- 60 mv output. (G3RUH's own PCB implementation of the 9600
baud modem will accept from 8 volts down to 10mv before the circuit noisefloor
is encountered. Other implementations are noisier.)
The existing FT227R
audio circuit loads the discriminator with a big kerchunk when a signal is
initially received; changing (or removing) capacitor C158 cures
that.
Transmitter Notes
On the TX side varactor D401
directly frequency modulates the oscillator X401 via the screened cable named
MOD. This can be disconnected at the mic amplifier output, and 9600 baud audio
injected instead. The load impedance is 18k seen through series 1uf capacitor
C401. The charging of C401 when the TX is keyed results (even in standard form)
in a 10 kHz swoop of TX frequency with a 20 ms time constant which takes about
40 ms to die away.
This can (must) be reduced by 50% in amplitude and
duration by decreasing the capacitance.
Because of this, and only this,
the recommended minimum TXDELAY is 50 ms. Extra may be needed to accommodate the
distant station.
TXAudio drive of 1.2 volts peak-peak gives +/- 3 kHz
deviation.
Other Notes Although the antenna changeover switching
is by PIN diodes, 8 volt power to the TX and RX sections is switched by a
miniature G2E style PCB relay. This operates in about 5 ms, but its clicking
might annoy. A semiconductor alternative would appeal to some operators, maybe a
couple of power JFETs.
The radio has a convenient 5-pin 180 DIN socket on
the rear panel. This can be re-wired with data audio input and output as per a
conventional TNC for a very neat interface.
Modifications to
Radio
You will need:
2 x 2k7 resistors. 1n ceramic capacitor
small, 100n ceramic capacitor small, 470n capacitor polyester or ceramic, 2 x
muRata CFU455D2 filters, 50 cm fine screened cable, e.g. RG174 coax (or tightly
twisted pair).
Tools: small Pozidriv (crosshead) screwdriver, pliers,
cutters, solder sucker and soldering iron, solder, sharp knife.
- Access to the rear of the MAIN UNIT is gained by hinging up the PLL UNIT.
This has the big black D857C IC on it. Change the following components:
R132 to 2k7
R133 to 2k7
C158 to 100n
CF101 to CFU455D2
CF102 to CFU455D2
Now put the radio correct way up, exposing the top of the MAIN UNIT.
- About 1cm to the right of the red discriminator block CD101 there is a
post, marked CENTRE METER on the circuit. It is joined to R134. Connect a 1nf
capacitor from the post to ground. The exposed leg of R135 should be used for
this. Now connect a fine screened lead to the post and ground (across the
capacitor), route it to the left of the radio, then to the rear and down the
corner to below. This service is RXAudio.
- On the centre left edge of the MAIN UNIT are a pair of posts marked MIC
OUT. This is MOD on the circuit diagram. Disconnect the inner of the screened
cable from the rear post of the two. Connect a 470 nf capacitor to the inner.
Make another screened lead and route it below as previously described. Connect
the inner of the new lead to the other leg of the 470nf capacitor, and the
outer to the ground post of MIC OUT. Thus the capacitor is in series with the
cable run. This service is TXAudio.
- Turn the radio over, hinge up the MAIN UNIT again and expose the DIN-5
socket. Remove all the wiring from pins 1, 4 and 2. Remove the short blue wire
from the adjacent loudspeaker socket, discard, and connect the long blue wire
in its place. Connect the TXAudio inner to pin 1, the RXaudio to pin 4, the
screens to the ground blade, and connect pin 2 to the ground blade. PTT is
already on pin 3, and pin 5 already has +12 volts on it.
Other
Modifications
The default frequency shown at switch-on is loaded into
the BCD up/down counter ICs Q707, Q708 and Q709 according to the state of their
preset pins 3,13,12 and 4 as per the table below.
Some neat work with a
sharp knife will be necessary, but all the tracks are easily accessible by
removing the cardboard cover.
Q709 does the MHz. N = 0 corresponds to 143
MHz, N=1 to 144 MHz and so on.
Q708 does the 100 kHz digit, and Q707 the
10 kHz.
PIN PIN
N | 3 13 12 4 N 3 13 12 4
----+---------------------------------------------
0 | L L L L 5 L H L H
1 | L L L H 6 L H H L
2 | L L H L 7 L H H H
3 | L L H H 8 H L L L
4 | L H L L 9 H L L H
----+---------------------------------------------
L = LOW (pin 8), H = HIGH (pin 16)
The ICs are type MC14510. Q709 is at the front centre close to the screw,
Q708 is next left and Q707 on the far left.
Thanks
To G4AJJ for
the loan of two FT227Rs and carte blanche to hack them.
Notes prepared by
James Miller G3RUH @ GB7DDX.#22.GBR.EU 1993 Sep 29