Subject: FT902 mods. 1/5
If you are interested I can send you the circuit diagrams by post, here is the
text. I have done some more modifications but there are so far no English text
for this, see OZ (danish magazine) nr 7/93.
73 de jan-martin
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Improving the YAESU FT-901/902 type transceiver (90.12.20)
By LA8AK, Jan-Martin Noeding,
Voielien 39/B, N-4623 Kristiansand S.
Several printed circuit boards in either model are very much equal.
There has been some modification kits available to obtain the new
bands for FT-901, and the RF-unit has also been replaced to achieve
the better intermodulation performance as with FT-902.
The FT-902 has been in use here for 3 years, and some changes has been
found important. Too often it seems not possible to buy a new trans-
ceiver at the required specifications, but many features now found on
receivers add only marginally to the on-air performance and just too
often there are many new bugs, some of these are caused by incorrect
ways of operating a semiconductor device, some rigs have some boring
background noise which does not disappear, even for strong signals
(IC-730/740). Some of the bugs with FT-902 has been experienced with
the later YAESU models. 5-10 years old rigs are much easier to modify
and find room for the additions you prefer, compared to last models.
But there is of course a limit for when a piece of equipment is
worth modifying, exceptions are of course rigs with historical
attraction. The first owner has paid most part of the price to the
dealer. Therefore it seems quite interesting to buy a secondhand rig.
The following changes/improvements has been made for FT-902 (FT-901):
1) PB-1705A AF Unit. Audio Quality.
This is in my opinion awfull. Very bad bass-respons
e. Looking at the
FT-7 audio circuit, will show the solution. C511 to the AF-power-
amplifier is decreased to 47nF (.047). Improvement is just inbelie-
vable! Measuring the audio RF to audio response now indicate that
there are still more room for improvement, but it sounds good .......
even with the built-in load-speaker, at least.
2) PB-1994 NB unit. Carrier Level Control.
This is a 5kW potmeter, with effective 'range' of about 1000 W, ie.
80% of the rotation is outside practical control range. This potmeter
is shunted with 1200 W, it is easiest to connect the resistor on
PB-1994 (NB-unit) pin 5, and ground, or under the PCB-socket.
3) PB-1720B RIT. (Receiver Incremental Tuning = Clarifier)
One must decide self which RIT-range should be. In my experience the
requirements depends on the operation.
7000000000000080000000000888000000000080000000008880000000000800000009
.HF SSB . + 5kHz ... HF CW . + 1kHz ... 6M aurora. + 5kHz.
.6M CW . + 5kHz ... 6M SSB . + 5kHz ... 6M CW MS . + 5kHz.
.2M CW . + 2kHz ... 2M CW MS. + 5kHz ... 2M SSB . + 5kHz.
.2M Aurora . + 5kHz ... 70cm CW . + 5kHz ... 70cm SSB . + 5kHz.
.70cm Aurora . +10kHz ... 23cm CW . + 5kHz ... 23cm SSB . + 5kHz.
1000000000000020000000000222000000000020000000002220000000000200000003
From :LA8AK
To :MODS
Date :931024/1744
Msgid :BF 38941
Subject :FT-902(901) mods 2/5
I discovered that sintran letter for ohm is converterted to W, so 5kW should
read 5k ohm
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First I modifyed my rig for HF CW because it is very important to use
correct tuning in HF CW contests. The modification shown in fig.2
makes +1.6/-1.5kHz (original is +5.5/-4.5kHz). VR06 is shunted with
1500W resistor. Switch RIT on, set VR06 to center, tune RX to
7000.0kHz, switch off RIT and adjust VR01 to frequency reading 7000.0.
4) Mode-dependent RIT.
The modification as shown above was used for about one year, but it
was a desire to have some further changes. The circuit shown in Fig.3
switches automaticly between two RIT ranges, dependent on modes
1:AM/SSB/FM "wide" (+4.8kHz), 2:CW/FSK "narrow" (+ 1.4kHz)
This is solved by using a dual-OP-amp, LM358=MC3358P1 (TL082 will not
function). The circuit is connected on a printed circuit board. Some
time was needed to find a suitable location, but at last it was
mounted next to RIT-control, at the XTAL OSC. A little angle- bracket
was soldered to one end of PCB and fixed using the two available
screws which fixes the oscillator assembly. VR01 is adjusted such that
RX with and without RIT operate on the same freq. The transistor is a
P-FET, which is not too commonly available, I used the only available,
a J177 Switch-FET. For "narrow RIT" the FET reduces the gain, but
center-frequency remains equal.
5) PB-1994 NB-unit:RF-clipper (so-called RF Processor)
It was not possible to notice any operation of this circuit, reports
on the air did not indicate that this circuit did operate at all! I
made some measurement using RF mV-meter connected to PB-1994 pin 17.
Whistling into the mike produced 70mV from NB-unit, while "blowing"
into the mike produced 200mV rms, this indicate that RF clipper does
not work on most sort of speech, while perhaps very good for single
tone transmission....... The LIMITER (Q206 TA7060P) was suspected, a
pair of 1N4148 diodes were mounted in antiparalell across the output
from this IC, and the RF CLIPPER functioned !! Just as easy it was to
improve this. The same suspect circuit has been seen in some other
FT-line transceivers.
6) PB-1994 NB-unit:Processor Level Control. (NB unit Q206).
This strange circuit did not have any practical function. I used a P-
channel FET, the only one available was 2N5462, while it may be
possible to use the J177. Some resistors were changed too. 6-10dB
variation was achieved. The rotation is now opposite way, so the
center connection at the potmeter must be connected to the other side.
VR203 must be at maximum, but this was later removed and a strap was
inserted on the PCB to get maximum output.
