Модернизация платы РА - 200 вт
Вот так DF3UM, предлагает модернезировать выходной каскад трансивера, для получения выходной мощности 200 ватт. Подсмотренно на mods.dk.
The FT757GX and the FT757GXII from Yeasu finds thanks to his size and the many features ,more and more users that like this transceiver.
Thanks to his size this transceiver is very good for use in cars. Because the device has low power ,the people like to have more power from the transceiver.
After study of the Pa circuit it became clear, that the Transistors(2SC2879) could give more power than the default 100W.
Data 2SC2879:
Pd 250W at 25 degrees
IC 25A
BVcbo 45 V
BVebo 4 V
BVceo 18 V
I modified my FT757GX get more power out of it.
These rebuilding instructions are directed to all OMs which also want to raise the output power from the FT757GX and want to use it for mobile or portable use.
The author of this modification is in the clear about the fact, that this rebuilding Manual again will call for several criticism, that we have seen before in the CQ DL. however, this would be permitted to the critics. These rebuilding instructions where published before in the FT757GX INFO-BЦRSE and many phone calls showed them this article was Accepted for publication.
2.) RISE OF THE POWER ACHIEVEMENT OF THE FT757GX
To raise the power of the transceiver you have to have the service manual, wenn you don’t have one try to get one because this is a must to get the modification completed. Further you will need the next things:
2 Resistor 1.5 ohm 2 watts (COAL LAYER!)
1 Resistor 5.1 KOhm
1 Resistor 68 ohms
1 Watt meter
1 Dummy load 50 ohms 200 watts
1 Dummy load 16.6 ohms 100 watts (that simulates SWR 1:3)
1 piece of thin coaxial cable or shielded NF cable
The first thing we have to do is Resistor R205 on the RF UNIT shout be shorted. Now both Resistor R18 and R16 on the PA-UNIT shout be exchanged with the two COAL LAYER resistors 1.5 ohm.
To take care of the cooling from the transceiver we also have to chance the resistor R26 (PA-UNIT) with a 5.1 Kohm resistor.
With this modification we manage to leave the cooling Fan to run longer when ready with transmitting.
This was already everything what we have to do modify.
Now we have to adjust everything.
We do that on the 14 Mhz band around 14.150 mhz Now we connect the Wattmeter with the 50ohm Dummy load to the transceiver.
Switch to mode CW and set the output power to maximum. (DRIVE REGULATOR right full turn) Now both potentiometers VR06 and VR07 are able to turn to the maximum output, till the point the Transceiver starts in bad SWR to regulate the power back.
Put the output in such a way, that when we are adjusting the Potentiometers the output won’t rise anymore!
Now connect the 16.6 ohms Dummy load to the watt meter. (16.6 ohms = 3 x 50 ohms in parallel or 60 x 1KOhm 2 watts parallel)
Now the output is to be regulated so that there will be about 40-50 watts of output! At (SWR 1:3)
80m 40m 20m 15m 10m
210w 210w 205w 200w 150w
3.) ADJUSTABLE POWER WITH THE FT757GX IN ALL MODES
Take off the Bottom plate from the transceiver.
Take a look for Transistor Q50 on the board "RF-UNIT" (in the area around - 8 volts Chinch socket).
Search there the Resistor R 235. Take away the resistor ore make it loose on one site (mass-sided). the connection wires of R 235 will stay on the board and solder a new Resistor with 68 ohms to it.
Take off the front panel off the transceiver. On both sides there are 2 screws that should be removed.
Search now the double potentiometer for MIC-GAIN and DRIVE , desolder to both wires (violet and blue). Connect now both desoldered wires with each other and isolate them.
Take now the thin piece coaxial cable to the hand. On the drive - potentiometer you solder on the left the inner wire of the coaxial cable and on the right the mass (shield) of the coaxial cable.
Now the front panel can be screwed on again. The coaxial cable leads down to the Resistor R 235 , pay attention to a short cable as possible. The middle of the cable should be connected with the mass- side of the Resistor R 235 soldered and isolated, the shield of the cable should be connected with mass.
After these modifications your FT757GX or FT757GXII is adjustable in all modes. Further the transceiver delivers on all bands about 200W. Nevertheless, careful use in all power areas . (no full output in the mode CW and FM).
Although the increase of power with 3 db only it has sometimes appeared, that just this 3 db will make the difference between a connection and no connection.
In the meanwhile i bought also a FT757GXII and also modified this Transceiver. This device is meanwhile in use for over one year in my company , and the modification has proved itself. furthermore because four Transceiver of other OM's also are modified and also these modified Transceiver FT757GX are reliable!
One is able to make local connections with a few milliwatts.
With the use in the car with a mobile aerial it will bring with his increased power a huge difference , and when you use the transceiver at home a extra amplifier isn't needed anymore.
The spectral cleanness of the source signal itself is slightly become a little bit worse.
If there are any questions about the modification you can come in contact with DF3UM
