Modifications for the Yaesu FT-227
Modifying the Yaesu FT227R for 9600 baud operation
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.
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
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.
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
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.
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
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.
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.
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.
This counter shows the number of hits since the
9th February 2000
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