Yaesu FL-50B Transmitter
2. Modifications to the keying circuit
3. Modifications to the receiver mute circuit
4. Additional 'RX Mute' Circuit
I bought this transmitter, shown in the centre, (and the matching FR-50B
receiver) from an eBay seller who was selling a significant amount of
equipment belonging to her late father, Mike G0JVB (previously G6MUI).
This mainly valved equipment was probably made in the late 1960s and had
been designed to meet the requirements of novice licensees. The
transmitter uses valves (and semiconductor diodes) throughout. The
result is a
transmitter that is effective all five bands: 80, 40, 20,
15 and 10 m. The operator has the option of using the internal
crystal oscillator, or an external VFO. It was intended that the VFO
in the FR-50B would be used as the VFO. However, VFO stability
problems inherent in the design of the FR-50B switched-range VFO would tend
to rule out such operation in these days of high stability 'black boxes'.
Further details of the FL-50B can be found on this
RigPix web page.
Being a CW-only operator, this transmitter needed modifying so that it stood
any chance of being used on a regular basis. While my modifications
may detract from the previous 'original' status of this equipment, I knew
that this transmitter would simply gather dust if I didn't improve its features
I have modified the keying line so that the external keying device (such as
a morse key) now pulls an internal + 5v rail to ground,
and interfaces to the internal grid block keying using a high voltage
transistor. I have also modified the muting arrangements so that the FL-50B
now mutes an external receiver by extending a ground on 'transmit' (rather than
the original design that extended a ground on 'receive').
Because the VFO in the companion receiver is very unstable, I have designed and built a
DDS VFO for use with the FL-50B. My design is based on the work of Paul
VK3HN. You can see my DDS VFO in the above photo, to the
right of the transmitter.
I also added an additional RCA ('phono') socket to extend the keying line for
use with an external sidetone oscillator.
Further details of the above
changes are provided below.
Finally, I altered the PA pi-tank coil taps on all five bands to improve PA
efficiency into 50 ohms. The PA neutralising capacitor needed tweaking also.
If you would like more specific information included, please let me know and I
will update this page accordingly.
Modifications to the keying circuit
Many rigs designed in the 1960s used the 'grid block' keying method to
generate CW signals. This involved applying a large negative voltage to the
control grid of the PA valve(s) which resulted in the PA being turned off at
'key up'. To key (turn on) the PA at 'key down', the negative
control voltage was shorted to ground by operation of the morse key.
Apart from extending a high voltage to the terminals of the key, this worked
well enough. Operators soon learned not to touch the terminals!
However, modern semiconductor morse keyers are often designed to key a low
positive voltage (within the transmitter) to ground. With this in
mind, and to make the
FL-50B compatible with my electronic keyers, I decided to provide an
internal interface circuit so that the keying device 'saw' a low positive
voltage rather than a high negative voltage.
I used a small piece of tinned, copper clad board to secure the
necessary components using ugly construction. The board was then held
in place using double-sided tape.
The circuit uses a readily available high voltage transistor (2N6520) which
is powered from the 6.3 v AC heater supply via a half wave rectifier.
The next change I made was to add a little bracket to the rear runner of the
transmitter upon which two RCA (phono) sockets were added: an 'RX MUTE'
socket, and a 'KEYING OUT' socket.
I'll say more about the RX MUTE socket in a separate section (below).
The KEYING OUT socket simply extends the 'KEYING IN' line from the morse key
so that external devices (such as a sidetone oscillator) can be keyed at the
same time as the FL-50B. This was required to ensure compatibility
with other elements of my station equipment.
Modifications to the receiver mute circuit
Both the FL-50B and the companion FR-50B receiver were designed such that
receiver was muted by providing a ground on receive. I never liked
this protocol on any of my station equipment, so I needed to change this
part of the design. But there was another issue! The FR-50B
needed an internal +150v line to be grounded in order to un-mute the
receiver. As with CW keying of the FL-50B transmitter (see above), the
presence of high voltage control signals was incompatible with my other
For this reason, I provided an interface circuit within the FR-50B receiver
so that the receiver is now muted by pulling a low positive voltage to
ground on 'transmit' (rather than pulling a
high negative voltage to ground on 'receive'). The new RX MUTE
socket on the transmitter provides the required ground on 'transmit'. To retain compatibility with other Yaesu
equipment, the ground on transmit is still available at the Accessory socket
of the transmitter.
Additional 'RX Mute' Circuit
This circuit generates a 'RX Mute' signal (pulls a
positive voltage to ground) when the
transmitter is in the 'Operate' ('transmit') mode.