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):
- PB-1705A AF Unit. Audio Quality.
This is in my opinion awfull. Very bad bass-response. 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.
- 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.
- 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
I discovered that sintran letter for ohm is converterted to W, so
5kW should read 5k ohm
-----------------
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.
- 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.
- 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.
- 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.
The values for some resistors were later optimized and more than 10dB
variation was possible for 2N5462, but for J177 almost 30dB. By the
way, I believe that FT-101ZE uses similar speech processor circuit, so
it may need the diodes. Later reports on the air 'are possibly the
best RF processor ever heard on the bands, it is tested on 80m/40m,
6m, 2m, 70cm, 23cm, and some rather critical persons who too often
critisize have said this is good and RF processor may be left on
during local QSO's!
- CARRIER UNIT PB-1706A. FSK-mode.
The dc input level is limited to +1.5V, which is an impractical level,
so I have inserted a 4k7 resistor in series with base (Q701). It is
now possible to use normal TTL-level input, noise treshold is
improved.
- PB-1707B FM-unit.(PB-1706A *not cw-ms): Carrier level adjustments.
I believe that it is important to achieve as equal level as possible
for the different operations/modes, ie.(1) Normal SSB, (2) SSB with
RF-processor, (3) AM, (4) CW, and (5) FM. Difficulties arise because
the different units loads the common point, which is NB-unit pin 13.
Diode-switching is therefore used. A diode is inserted at the FM-
input to this point, see 'Overall Circuit for IF signals between the
different boards'. Another diode must be inserted in the other cable
from CARRIER Unit, possibly on this unit, and a similar modification
like the one for FM-unit must be made there. Fig. --- shows the
modification for FM board. In the physical location for TC901 is
mounted a trimpotmeter, a capacitor goes to the output, and resistors
are installed to bias the diode connected at the input to the NB-Unit.
It is now easily possible to adjust the levels from FM-unit, and CW/AM
signals from CARRIER unit) independently without interactions to
occur. When the diode is inserted between CARRIER unit and NB-unit the
capacitor in series with cable must be removed (this has a fixed
value, although it is drawn like a trimmcap). Some interesting details
with the FM-unit is that it is easy to find suitable connection points
for G3RUH 9600 baud modem, but it may be difficult to find some proper
free pins on the connector to wire these to.
FM-DEVIATON.
It should be pointed out the difference between FM operation on VHF
and HF. On VHF/UHF 25kHz channel spacing is used, while on 10M 10kHz
channel spacing is employd, this transceiver is built for FM operation
with transverter. For 10M deviation must be reduced, and it is an
advantage to use a narrower FM RX filter. On VHF/UHF deviation is set
to +5kHz, while for 10m it should be adjusted to maximum +2.5kHz (at
voice peaks). This is important otherwise you will cause severe
interference with the adjacent channels on 10M. Using simple tools,
you can just reduce to audio to the varactor diode to about the half
of the setting as adjusted for VHF. You may whistle 'hard' into the
mike and measure at the input to the varactor diode with a sensitive
audio level meter, and adjust the reading to about half of the first
reading.
- FILTER Unit PB-1995: NOISE-BLANKER-improvement.
I made an ignition noise simulator, this was earlier used to improve
the FT-7 noise-blanker, so it was interesting to compare FT-7 with
FT-902. The noise-blanker seems not to be too bad, but it seems to be
room for improvement. Without NB, S-meter reading was about S9, with
it was S-3, and after modification, the noise dissapeared almost
totally, the background noise could be heared without antenna. Change:
Add 0.1uF to the capacitor C328 (PB-1995 FILTER Unit), bead-tantalum
may be used.
- PB-1718/1720B: Push-buttons.
After some years in use, the push-buttons will not function properly,
the problem is that some small springs, easily seen from above, will
not move too well in the tracks, some thin oil improves this.
- PB-1715 PA-unit (2x 6146B): PA-valves protection.
- To protect the power-supply against anode-to-screengrid voltage
flash- over, a small 1N4148 type diode is connected in series with
screen- grid supply to the 6146B's. This is not shown on the circuit
diagram. This may be good for power supply, but it may be fatal for
the valves in case of some secondary emission due to hard operation.
Some com- promise is therefore needed. The diode must be paralelled
with a 10KW 0.5W resistor. To protect the screen-grid circuit against
flash- over I have connected a pair of SIEMENS varistors, SIOV 14K150,
they draw 1mA current @ 250V DC.
- PB-1715 PA-unit, PB-1708 RECT-A-unit: BIAS-circuit improvements.
To stabilize grid supply against drift-off due to gassy valves, a
diode is connected in paralell with R05, a 1N4007 is preferred because
it is a poor RF rectifier. To avoid that the grid voltage should
dissappear because of bad contact in BIAS POTMETER VR01 a 100KW
resistor is connected from negative side to center. Often this occur
if you have left the potmeter in same position for years, and when one
needs to adjust the idle-current, the trouble sometimes happen.
- PB-1708 RECT-A, PB-1717 RECT-C: IMPROVING THE CW-KEYING (Normal CW).
Orignally the CW-keying is bad, lots of key-clicks. For HF CW ca.
4-6ms delay is optimum. A single capacitor cannot solve this problem,
because of different charge and discharge current. A suitable
compromise was made with simple capacitor/diode/resistor combinations
see figure ----
- PB-1705 AF-Unit, PB-1846 VOX/MARK: AUDIO LINE OUTPUT.
For packetradio/RTTY/CW-meteorscatter it is desired to use an in-
dependent AUDIO LINE OUTPUT. Surprisingly, there was an unused 1/4-
LM324 on the AF board. There was marked but unused component-designa-
tions around this, one point was used only to connect two components,
they were removed from this common point and connected in the air.
Then the components as shown were mounted, the un-numbered components
are mounted below PCB or which is practical. It was an unused cable
from AF 'MJ6' to VOX 'MJ7', and further to J17 SIDETONE out on rear
side. I could not see any practical use for this TONE output, so I
decided to use this for AUDIO LINE OUTPUT. The 4k7 resistor is changed
to 1KW or removed, IN VOX UNIT R52 is loosened in one end.
- Other changes (PB-1728A and other components):
MIKE contact is changed to the KENWOOD circuit for 4pin connectors,
such that it is possible to use same mikes for different rigs. CW-jack
is rewired. One contact is KEY, while the other is PTT in/out. The
ELBUG was removed, I like my own HB9ABO Keyer/Memory keyer better. A
separate CW-high-speed board was installed (keyfilter and keyed IF
amplifier). AM position is wired for CW-Wide (2.4kHz BW) for VHF/UHF
CW (normal CW/- Aurora/Meteorscatter) where larger bandwidth is
preferred. Perhaps someone else would prefer to have a switch to
select between CW-Narrow/CW-Wide, the AMGC switch may be used for this
purpose.
- PB-1715 PA: TRANSVERTER RF OUTPUT.
RF output level for transverters is far too high, this is reduced by
connecting a capacitor in series with C02=10pf. I used 3P3 with 1000
ohms in series (the resistor is used to avoid parasitics). About 20mW
is available on 10M.
- PB-1717A RECT-C: +6V Regulator for RIT.
OZ1HDA (with FT-901) and I (with FT-902-DM) have been troubled by VFO
instantaneous instability, the +6V IC-regulator (Q01-TA7089) is sus-
pected. I changed the +6v regulator with a 78L06 regulator and the
problems seem to have disappeared.
- PB-2154 RF-Unit:
Forward conductance for Q03=J310 must be adjusted to 20mmho (20mS) for
50W mixer termination.The drain-current must be ca 20 mA. Correct
value for R13 ca. 18 W, according to G4DGU, Chris (MUTEK).
Pse note that this manuscript is written in sintran and converted to
DOS, therefor some letters will be wrong, here W may mean ohm!
! Table 1 !
! COMPARING CIRCUIT BOARDS USED IN THE TWO MODELS !
!-----------------------------------------------------------------!
! Board . FT-901 . FT-902 Note !
!-------------------!-------------------!-------------------------!
! RF unit ! PB-1702 ! PB-2154 Different !
! Filter ! PB-1716 * ! PB-1995 * Equal ? !
! IF ! PB-1704C ! PB-1704C !
! FM ! PB-1707B ! PB-1707B !
! CARRIER ! PB-1706A ! PB-1706A !
! AF(RX) ! PB-1705A ! PB-1705A !
! NOISE BLANKER ! PB-1703C * ! PB-1994 * Equivalent !
! Keyer ! PB-1728A ! PB-1728A !
! VOX/MARK ! PB-1846 ! PB-1846 !
! VFO ! PB-1440B ! PB-1440B !
! XO ! PB-1711A ! PB-2165 New bands !
! VCO ! PB-1710A ! PB-2166 New bands !
! PLL ! PB-1709A ! PB-1709A !
! Counter ! PB-1729A/PB-1730 ! PB-2086 New !
! Memory ! PB-1787 ! PB-1787D !
! Capacitor ! PB-1713 ! PB-1713 !
! RECT-B ! PB-1712 ! PB-1712 !
! RECT-A ! PB-1708 ! PB-1708
!
! RECT-C ! PB-1717A ! PB-1717A !
! Driver(12BY7) ! PB-1714 ! PB-1714 !
! PA (2x 6146B) ! PB-1715 ! PB-1715 !
! Miscellaneous ! PB-1718A/PB-1720 ! PB-1718A/PB-1720B !
!-------------------!-------------------!-------------------------!
Almost equal boards but new numbers are marked with an asterisk *)
Note the availability for modification kit to have the 'new' bands with
FT-901.
Table 2.
Some IF measurements to be used for fault-tracing. All readings are in
mV-RMS(eff.Werte) +/- 3dB * Adjusted level, see text for modifications
!------------!-----------------------------!-----------------------!
! P.C.B. ! PB-1994 (PB-1703C) NB-UNIT ! PB-1706A CARRIER-UNIT !
!------------!---------!--------!----------!-----------!-----------!
! MODE (TX) ! Pin 8 ! Pin 13 ! Pin 17 ! Pin 4 ! Pin 2 !
!------------!---------!--------!----------!-----------!-----------!
!FSK CW AM ! - ! 55 ! 70* ! 65 ! 75 !
!FM ! - ! 65 ! 70* ! - ! - !
!SSB(Normal) ! 60 ! - ! 70* ! 160 ! - !
!SSB(Proc.) ! 60 ! - ! 20-80 ! 160 ! - !
!------------!---------!--------!----------!-----------!-----------!
! RX ! - . - . - ! 30 ! - !
!------------!-----------------------------!-----------!-----------!
|
