Yaesu FT-902-DM Mods

IF frequency schematic showing carrier frequencies at the IF on the different modes, this is very important
to understand which frequencies are on RF compared to audio for RX and TX
 

Just change the C511 in PB-1705A to 47nF (.047). Fortunately it is conventional
component here, another Yaesu transceiver has 0.1µF instead of 0.01µF capacitor fitted - as a fault, but
it was SMD type, so it is two hours procedure to remove and replace.
 

FT-902/901 isn't among the worst transceivers in this matter, but I'd
made up my mind to check the key circuit long before I put it on the air for the first
time. Some serious construction (it is not called design!) faults were traced, with different
time constant for key-down and key-u

I always wish to have a crisp and clean cw signal, the received reports indicate that the result was good. It was some problems encountered, because the time constants for KEY-UP and DOWN would be quite different if only a capacitor was added, so the time constants needed to be divided into two parts with diodes to increase timeconstant when needed for a symmetrical keying shape. Suppose I have marked all changes with read colour, but it might be some more component values, they should be correct as they appear on the circuit diagram.
2) The key circuit isn't easy to understand, so I put a copy of it to the right, and at least, you may see the pin 16 connection at upper right (pin 14 goes to mode switch!).

It is also another problem with the screengrid circuit for 6146B's. The power supply is protected against flash-over from anode to screen grid, but secondary emission in the valve could cause damage to the valve. This may be allright to do in times when valves are easy to obtain, just go to any shop and pay some amount of money because you choose to protect the electrolytic capacitors instead of 6146B's. But now is 6146B's worse to find than electrolytic capacitors, so why not protect both?
The problem is a diode (D01) in series with L04 on the PA board - PB-1715 (No.17). A better idea is to connect a 10k resistor across the diode and a varistor from screengrid to ground og in the power supply, suggested type is SIOV14K150 which will limit the voltage to 240VDC.
I've never experienced any problems, so my circuit might work, or it wasn't needed, but I am not interested to prove it



The power amplifier board PB-1715 (No.17), see more details on next circuit diagram


The handbook is not correct, it is a power reduction on 28MHz with a 2k2 resistor og the bandswitch. Here is shown my modification of the screen grid circuit.

Improving the FT902 speech processor. The original circuit had little effect, so I added 1N4148 diodes in anti-parallel.
The PROC. LEVEL also had little effect, this is greatly improved using a P-type JFET. 2N5462 will work, but J177 is much better.
On-the-air reports have been very convincing, it is no distinct clipping, and I am told that it is usually no problem to use the
RF-clipper (processor) in local SSB QSO's on 80, 40, 6, 2m, 70 and 23cm bands. It is approved by otherwise very critical friends,
like Morgan SM6ESG, although he don't like my old Microwave modules 70cm SSB transverter, hi.
The modification is described in more details in Technical Topics (G3VA), Radcom
 

The first and simple solution to reduce the range, particularly valuable for CW operation

Usually it is practical to have different RIT range for CW and SSB, HF and VHF operation,
here is a circuit which suits my needs with 1.5kHz for CW and 5kHz on SSB, and most part
of my operation with this receiver has been CW and SSB on 144, 432, 1296MHz bands.
 

RF output level is quite high with FT-200, FT-250, FT-101B, FT-901, FT-902. Suppose it is 100mW, while
most transverters hardly needs milliwatt level drive. Some transverters are constructed for high drive level, but
I don't see the point, so I've always modified it using a 1000W resistor in series with 2p2 capacitor,
instead of the original 10pF from PA-grid circuit. The larger capacitor may load the grid circuit and
tuning on higher bands may be upset. Also connected a 10K resistor to the transverter terminal, such
that +12V follows the signal during transmit, it simplifies the arrangement, no need for PTT lines,
and PTT lines to external equipment is very bad practice, if they are grounded the external
equipment may start the transmitter and cause problems.




FT-901/902 Transmitter modification to drive a transverter
 

Made external RX antenna connection for the FT-902 (FT-901), similar system to those I made for FT-250 and
FT-101B earlier. A cable from RX side of antenna relay goes to a BNC connector board, and another
cable goes back to the receiver input. So I may choose between which receiver I'll listen to, possibly
Drake 2-B or Drake R-4C, or use FT-902 with a transverter.



A simplified circuit shows what was done with FT250, FT101B, FT902 to provide RX-ANT
output and input so I can use which receiver I want with FT902 transmitter, and use FT902 RX
as a separate receiver for another purpose without any risk of transmitting into the RX circuit.
This is important feature for use with transverters, and often one receive on another band, than tx,
only 14 or 28MHz is common.
 

Originally the transmit drive levels varies very much. FM has the highest drive level. Decided to equalize the
drive levels for the different modes, some minor modifications to different boards are necessary

The AMGC switch comes to practical for selecting between SSB and CW filter, but few extra components are
needed to activate it. When the switch is operated the voltage in base circuit (see schematic above) is pulled
down from 12V towards +8V, and a 12V relay is energized. One might have used lot of time to look for
an 8v relay, but I had a small 12V relay, and it draws much less current than an 8V type. It is usually a good
practice to measure which voltage the relays need to operate, and which voltage it falls back on, so it is often
possible to use a 24V relay on 12V, but it must usually not have higher operate voltage than 9V for safe operation
on 12V system, and 6v for 9V operation.

 

Logarithmic S-meter for FT-901/902.

S-meter deflection is far from ideal, and some improvements were made with the original circuit, but
it was never somewhere near the desired result. I would accept that an S-unit varies between
3-7dB, which is not much accurate, so I looked for other ideas



It was a need to make more accurate measurements with the receiver. An S-meter where an S-unit could be
anything between 0.5-7dB is regarded as useless, but believed to be a feature for the average operator who
wish to give 59 reports to everybody. This circuit has 70dB range - provided it is set up correct, and this is
more than you need, I've never experienced that the meter peaked, but I've got some complaints
that I don't give a 59-report to an unreadable station.
It should be noted that the gain and current of the input stage must not be set too low, it is better
to attenuate the signal with resistors in the drain circuit! I suppose the reason is that the dynamic range
for the amplifier is reduced when it doesn't draw much current, so it will not drive sufficient when
signal to pass is "strong". The minimum voltage from MC3356 output is 0.6V. It is a bad idea to
use a simple diode here, the voltage drop with varying current may vary between 0.4-0.6V, you may
loose indicating range. It is much better to rely on a transistor B-E junction diode, or just use an
emitter follower. It also solves the problem with time constant in the detector, but this is an unsolved
problem, it could be better. It is also a problem with FT-902 that it is some false signals in the
IF and the logaritmic detector cannot be set to indicate signals lower than 2µV on the receiver
input connector. MC3356P is a very sensitive device. Another device used for similar purpose
is Philips TDA1576, but I suppose it won't work properly here because it is not sensitive enough,
or perhaps is it just what one need to operate the input stage with some decent current and gain?
 

                   

PB1726A (FT902 VFO switch) obscure circuit diagram
Had a fault and needed to follow the VFO signal, but the handbook is obscure on this point.
Perhaps the worst is that details shown in the handbook are incorrect, so it was only one
thing to do; loosen the board and inspect the pcb to draw my own circuit diagram
 

FT-902 overall installation

                   
 

                                                

PTT facility on the paddle for CW operation.

Have mounted a PTT-switch on the paddles for my shack. It simplifies operation and you have 100% control, a
nd easy to switch between RX and TX, and back again if you wish to check that you are not transmitting when you shouldn't.
Since it will always be a time delay when you throw the button to you can start transmitting it is no need for time delay, but
it may be another idea if you use no-control cw meteorscatter operation

The paddle is good quality, but haven't the faintest idea who made it, bought it from a very drunk Finnish OPR at
Aannaboda VHF meeting, who may be the producer, and definitely wanted to buy it back the next day, but I refused.
 

                                      

The Vibro-keyer paddle.
Since I worked at Rogaland radio in 69-70 we had some ideas what was the best paddles and it was two different makes to choose
between, Autronica and Vibrokeyer paddles. Bought my own vibro-paddle, and used it for a year, until I got a better German construction
which also could operate SQUEEZE, and it was used until 1987, when I got the Finnish paddle mentioned above. Vibrokeyer is a part of the
older vibroplexer. It seems possible to drill some holes in the iron block, and mount a PTT switch on the paddle, but I keep it only as a reserve.
 

All this information are from LA8AK. For any other explanation send email to him.

ATTENTION
The KB2LJJ takes no responsibility for any damage during the modification or for any wrong information made on this modification.