general HF receive coverage to the Yaesu FT-301
No modification on your FT-301 is required!
Before you attempt any modification on any commercial radio, you should first study it's manual really well. Here
is the manual of the Yaesu FT-301 transceiver. It is a great radio, in
fact one of the best looking radios ever produced to my taste. To me,
when I am in front of the panel of this radio, it feels like being in
front of an old military plane cockpit. This alone is an excuse for
more time on HF transceiving and DX contacts. I have two of them, both
with analogue dials (versions FT-301 and FT-301S), because I do not
particularly like the digital readout versions of this radio, the
FT-301D and FT-301DS.
It is too bad this radio does not have a general HF coverage
capability, so I can enjoy broadcast SW listenning. Well, many amateur
radio transceivers of that era didn't, so it is not to blame the radio
for it. However, I have decided to try to change that. With the
addition I describe in this page, you should be able to receive all HF
frequencies, so you can enjoy listenning to your favourite HF
programs. More importantly, and this was a requirement, no
modifications to the radio are allowed to be done, because I like
keeping these radios in their original state.
Having this in mind, the original idea was to build an external VFO
and use this to tune to other bands/frequencies. But if you study the
circuit closer you can find out why this is not possible. If you use
the external VFO option, the external VFO must be in the
5-5.5MHz range, like the internal one. Like the internal VFO, the
external would be mixed to one of the band crystals to produce a signal
with a 9MHz offset from the one you want to receive/transmit. So by
using the external VFO option you do not gain anything as far as
concern the frequency range. Only frequency stability is increased and
only if you use a DDS or PLL external VFO.
Another idea was to use the internal VFO of the transceiver, but bypass
the bands crystal oscillator and use a DDS in place, to produce any
fixed frequency required for the transceiver to operate on other
bands/frequencies. This would allow tuning the transceiver from it's
elegant big VFO knob in the front panel and use the DDS for switching
to different bands. However, this was not possible. If you closely look
at the schematic of the transceiver, There is a bandpass filter after
the MC1496 in the premix unit, which does not allow for this to happen,
at least not without severe modification to the transceiver.
One has to completely bypass the internal vfo AND the band crystals AND
the premixer AND the following BPF and feed directly a CLEAN signal
after the BPF, 9MHz offset from the one you want to receive/transmit.
The disadvantage of this, is that you cannot longer use this nice big
tuning knob of the transceiver any more. You only tune the transceiver
from the DDS now. This is the price you have to pay for general HF
reception. However, by switching back to the internal VFO and disabling
the DDS, you can again tune the transceiver from it's tuning knob
within the amateur bands, as you would normally do prior to the
addition of the DDS.
I have tried this idea. I used my DDS to
drive pin 17 of the premix unit. The DDS frequency must be set at 9MHz
above the actual frequency that you want to receive/transmit. The front
panel oscillator choice must be set to EXT (external). This disables
the internal VFO but not the the bands crystal unit.
However, in operation I did not notice ANY ghost signals from the
crystal oscillator at any band switch settings. You see the signal out
of the crystal oscillator passes to the MC1496 mixer, which expects 2
signals to mix them. But if there is only one, only this one appears at
the output of the mixer. And because after the mixer there is a BPF,
this signal is effectively removed.
So when I directly fed pin 17 of the premix unit from the signal
output of my DDS, things worked like a charm! The DDS signal on the HF
range is clean, I have tested this on the spectrum analyzer, no need
filtering. So when it is fed to the wideband amplifier on the
transceiver, composed from
Q705, Q704 and Q703, the output level is just right for normal
operation of it's receiver (audio signal levels equal to these produced
with the internal VFO). Success!
In operation, the variable preselector as well as the band switch must be used in
conjunction with the DDS frequency tuning of course. Normally (without the external DDS addition) the
preselector markings on panel are calibrated in conjunction with the
band switch settings. So for example, on 40m band switch setting, the
preselector must be set on the 40m range marking. However, when using
the DDS oscillator to extend the range, the preselector settings may be out of
the range of the band markings. Also, sometimes you could receive the same signal on two
different preselector and band setting combinations.
For example, say you want to receive 8MHz, which is normally out of the
receiver range. The procedure coes like this:
In practice, tuning is much easier than it sounds, you just set the
band switch at a setting close to the frequency you want to receive and
tune the preselector for max noise. When the received frequency is
between two ham bands, to find the best preselector setting, switch to
the higher band and tune the preselector for max noise. Then do this
for the lower band. The setting that gives you the most audio noise out
of the receiver is the best. Once you have determined the band setting,
you do not need to find it out again the next time.
- Set the oscillator setting to EXT (external), so as to
use the DDS instead of the internal vfo.
- You have to set the DDS at
8+9=17MHz to receive 8MHz (9MHz IF).
- Then set the band switch to 20m. Now the
preselector must be tuned way below the 20m panel marking so as to
cover this lower-than-20m frequency (8MHz). However, you could set this
the other way. You could set the band switch to 40m instead. In this
case, the preselector must be tuned way above the 40m panel marking, so
as to cover this higher-than-40m frequency (8MHz). In fact you will
find that one of the two settings works better than the other (greater
noise volume), so choose this one.
This procedure allows the conversion of the FT-301 to a general
coverage HF receiver. But it cannot be used for converting it to a
general coverage transmitter, because there are fixed selectable bands
LPF units at the output of the transmitter. Well, HAMs are not allowed to
transmit out of bands, do they?
A major advantage of this solution is that you do not need to convert your
precious vintage transceiver. I hate when people are doing conversions
that ruin these transceivers. You just need to inject the DDS signal on
pin 17 of the premix unit. This means soldering a cable to this pin and
another one to the ground.
To perform the addition you have to do the next steps. Open the bottom
cover of the FT-301 to expose it's components. Inside, there is a big
lid that shields the sensitive RF components. Unscrew that as well to
expose the components inside it.
In one of
the "pockets" of the circuits below this lid, you will find the bottom
of the green connector to which the premix unit is connected. We are
interested in the two top pins on this connector, as shown in the
picture below. The top pin is the ground and the pin below it, is the
pin 17 of the premix unit.
piece of thin coaxial cable and connect it to these pins on the
connector. How thin? Well, it was to be able to pass through the fin
openings of the bottom cover of the enclosure of the transceiver. If
you haven't got a source for such thin xoaxial cables, note that thin
coaxial cables can be found inside Wi-Fi equipment or old laptops. A
piece about 50cm long or even less, should be enough.
There is a
small opening at the walls of the metal shield, so pass the coaxial
through this opening out of the shield. Then don't forget to screw the
shield back again in it's place.
clippers that hold the rest of the cabling in place and clip the
coaxial cable along them, just to look pretty. Then bring the bottom
cover of the transceiver close to the circuit and pass the coaxial
cable through it's fins out of the enclosure.
I choose to output the coaxial
from the bottom of the transceiver, because I wanted to place the
external DDS at the bottom of it or at it's sides. But if you choose to
place the DDS at the top of the transceiver, then it would be better to
pass the coaxial cable through the top cover fins instead. In that
case, pass the cable from the bottom of the transceiver circuit,
through the circuit, to it's top. There is plenty of room to do so.
Then guide the cable through the top cover fins out of the transceiver.
I am not going to cover the DDS oscillator details here. Whichever DDS
you choose, make sure that it's controlling firmware has an IF shift
option. You want the DDS to operate at 9MHz above the actual displayed
frequency on it's LCD!
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