15m CW transceiver MFJ-9315K

Japanese & English : Apr. 23, 2016
Last update : Apr. 23, 2016
Japanese page is here.

Flower Overview of MFJ-9315K

I built a 15m CW transceiver 'MFJ-9315K' in September 2015. Its manufacturer's webpage is here. We can download the product and assembly manuals from this site. RF output of my MFJ-9315K is 1 watt.
In addition, I built a 80m CW transceiver 'MFJ-9380K' at the same time.

My MFJ-9315K (upper) and MFJ-9380K (lower)
（ For modification, a push switch and an LED are added on the front panel. ）
( There are holes on the top plate for speaker addition. )

MFJ-9315K PC board before building

MFJ-9315K PC board

Rear panel of my MFJ-9315K
( A BNC connector is installed at the upper hole, instead of a RCA jack at the lower hole. )
( 'FREQ-mite' is fixed by the screw on the upper-left. )

Flower My MFJ-9315K overview (modification points)

Inside of my MFJ-9315K
（ There is a 'FREQ-mite' in the right front. )
( There is a speaker on the top plate. )

Main modification points are as follows:

Speaker addition.
Expansion of the frequency range.
Frequency indication using 'FREQ-mite'.
Upgrading of low-pass filter after the final transistor.

Front panel of my MFJ-9315K
（ A push switch, an LED and some letters are added on the front panel. ）

Flower Expansion of the frequency range

In order to make the frequency range wide, I changed C9 to be shown in the table.

C9	Frequency range	Remarks
Original : 10pF	21.000～21.059MHz
15pF	21.000～21.092MHz	Current set value

C9 which influences the frequency range - 15pF at the center of the image

As frequency range is more than 90kHz, tuning is slightly difficult for me. Therefore, I think that 70-75kHz is better.

Flower Frequency indication

In order to indicate the frequency, Dave Benson K1SWL's 'FREQ-mite' supplied by the Four State QRP Group, a Morse-readout frequency-annunciating device, is installed, and a push switch is added on the front panel.

　 AF signal injection point

'FREQ-mite' ( left photo )
AF signal ( Morse code sound ) injection point is the additional 100k ohm resistor in the center. ( right photo )

Flower Upgrading of the low-pass filter after the final transistor

One more stage is added to the low-pass filter after the final transistor.

Added stage of the low-pass filter after the final transistor

Flower Alternative final transistor

According to the assembly manual, the RF output could improve if the final transistor 2N5109 is changed for 2N3553. Therefore I purchased some 2N3553s from an American shop and tried to change it.
However, I gave up the changing, because the RF output became zero.

2N5109 　 2N3553

Original final transistor 2N5109 (left photo)
Alternative final transistor 2N3553 (right photo)