Yaesu FL-50B Transmitter

1.   Introduction
2.   Modifications to the keying circuit
3.   Modifications to the receiver mute circuit
4.   Additional 'RX Mute' Circuit

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 and functions.

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.

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 station equipment.

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.