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Yaesu FR-50B Receiver

I bought this receiver, shown on the left, (and the matching FL-50B transmitter) 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 receiver uses mainly valves, with transistors in the switched-range VFO, and in the IF conversion oscillator.  The general design produces a receiver that is effective on most bands, but is a little quiet on 10 m.   The main problem is poor VFO stability which is mostly due to the lack of an enclosed VFO compartment within the chassis, such that most of the frequency changes occur due to temperature variation within the open-backed case.  Receiver selectivity for all modes is accomplished through the use of tuned circuits in the 455 kHz IF stages.   As designed, there is no provision for a narrow IF (CW) filter. 

Further details of the FR-50B can be found on this ElectronicsNotes web page.   Samuel, EI9FZB has compiled a lot of technical information relating to the FR-50B which can be found here.

Being a CW-only operator, this receiver 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 this receiver would simply gather dust if I didn't improve its features and functions.

To improve VFO stability, I have replaced the 9 v zener diode supply with a 3-pin regulator, and made other mechanical changes around the VFO coils. I have done my best to clean the range-selecting wafer switch contacts, but they still affect VFO frequency.

Because the AGC 'slow' and 'fast' options didn't actually affect the delay, I have re-designed the AGC line and am delighted with the results. 'Slow' is now plenty slow, and 'fast' is fast enough.

The muting circuit in the receiver has been modified to accept 'ground on transmit' operation using a IRF830 switching transistor.

I have added a switchable 1000 c/s CW filter in the 455 kc/s IF stage. I did think about using a narrower filter, but the VFO simply isn't stable enough yet!