Yaesu FR-50B User manual

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INSTRUCTION
MANUAL
FR-50B

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Notes: Changed kc/s and mc/s to kHz and MHz respectively
Changed sized and weights to metric
Any errors I found have been corrected.
I have resisted making wholesale changes to the language
used in the original instruction, where I have added
comments these are in {italics}.
References to AVC have been changed to AGC.
Version: 17 December 2018

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******************************
FR-50B COMMUNICATION R ECEIVER
*******************************
The model FR-50B Communication Rec eiver, designed for the amateur
bands, provides a high degree of sensitivity, selectivity and
stability.
Basically, it is a double conversion super-heterodyne receiver
employing a variable oscillator for the first mixer stage, and a
crystal controlled oscillator for the second mixer stage.
Adequate selectivity is provided for SSB, AM and CW with the
utilization of two 4 Kc/s mechanical filters .
When used in conjunction with the FL-5OB Transmitter, transceive
operation - receiving and transmitting on the same frequency -
is possible. This is a useful feature for SSB communication.
A built-in monitor circuit enables monitoring of the station
transmitted signal at any time .
SPECIFICATIONS
Frequency range 80m 3.5 -- 3.8 MHz
40m 7.0 -- 7.5 MHz
20m 14.0 -- 14.5 MHz
15m 21.0 -- 21.5 MHz
10m 28.0 -- 29.2 MHz
JJY/WWV 10.0 -- 10.5 MHz (can be installed)
{The AUX position on my receiver covered 1.2 – 1.7 MHz and
appeared to be factory fitted.}
Sensitivity CW/SSB Less than 0.5 µV for l0dB S/N ratio
AM Less than l µVfor lOdB S/N ratio
Selectivity At ± 5 kHz, -50 dB. At ± 1.8 kHz, -6 dB
Image ratio More than 50 dB.
Calibrator 100 kHz (crystal option)
Audio output 1.5 W into 4 ohm/600 ohm, speaker built-in
Power source 220 / 240 V, 50 Hz, 50VA
Dimensions 33cm wide, 15.24cm high, 26cm deep
Weight 8 kg approx.

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Figure 1. Receiver Block diagram {V8 changed from 6AW6 to 6BM8}
CIRCUIT DESCRIPTION
1. RF Amplifier
The high Gm tube V1 (6BZ6) provides a minimum of cross modulation.
It is a semi-remote cutoff pentode and has an excellent AGC
characteristic. A 5 MHz trap is inserted to reject a spurious
signal which could enter directly into the first I.F. stage. To
avoid complexity, two coils in each of the grid and plate tuning
circuits, together with switched capacitors, cover the full range.
A FM type variable condenser is used with electronic band spread
for pre-selector tuning. The centre of each amateur band is
approximately at the centre of the range of the pre- selector
control. On the 3.5 MHz band, the pre-selector covers 3.5 - 3.8
MHz.
2. 1st Mixer
The amplified signal and local oscillator output are mixed by
introducing to the grid and cathode of triode mixer V2, 12AT7. The
triode mixer is very effective because of its low conversion
noise. The first I.F. is 5 173.9 kHz.
3. VFO
Silicon transistor Trl, type 2SC373, is employed on a colpitts
oscillator circuit. Oscillator frequency range is selected by
switching a coil for each band. C61, C63, C65, C67, C69, C7l and
6BM8

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C72 are temperature compensating capacitors which insure stability
even in the 28 MHz band. Using the same dial mechanism as the FR-
lOOB, with the 50 kHz variation per revolution of the knob, it is
very effective for SSB and CW tuning.
Buffer amplifier Tr2, silicon transistor type 2SC372, is employed
in an emitter follower circuit. Its output is injected to the
first grid of mixer tube V2 (12AT7), and is also used as a VFO
source for the transmitter when connected for transceiving. The
output terminal is located on the rear panel of the receiver.
{This output terminal is where I have connected a frequency
counter that displays the received frequency.}
4. 1st I.F.
The signal converted into the 1st I.F., 5 173.9 kHz, by the 1st
mixer, is applied to the 2nd mixer via transformer L6.
5. 2nd Mixer
1st I.F. and 2nd local oscillator signals are added at the control
grid of V3, a 6CB6, and results in a second I.F. at 455 kHz at the
plate of the tube.
6. 2nd Local Oscillator
Tr3, silicon transistor type 2SC372, together with a quartz
crystal (X1) and other parts are installed in an I.F.T. case.
Output of the 2nd local oscillator is applied to the control grid
of V3 (6CB6) through transformer Ll9. Oscillator frequency is 5
628.9 kHz.
7. 2nd I.F. Amplifier
This is a two stage amplifier using two 6BA6 coupled by two 4 kHz
mechanical filters. "S" meter is inserted in the cathode circuit
of the second 6BA6 (V5) and must be adjusted to indicate zero with
no input signal. The "S" meter indicates cathode voltage
variation in V5, which is proportional to the AGC voltage. A
diode in series with the "S" meter protects it from inverse
deflection. Cathode return of V4 together with that of the RF
tube (V1) is connected to the RF GAIN potentiometer (VR1). {My
receiver as well as the schematics I have seen do not have this
diode in series with the “S” meter}
8. AM Detector & Noise Limiter Circuit
Gold bonded diode, 1Sl007 (D4), is used for AM detection. Series
type automatic noise limiter (ANL), with silicon diode 1Sl941 (D2)
effectively suppresses pulse noise. The ANL may be switched off at
the front panel.
9. SSB & CW Detector

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Product detector V6, 6BE6, has a very efficient performance for
SSB and CW detection. BFO voltage is derived from V7,6BA6, in a
self-excited oscillator circuit. BFO pitch is controlled by
trimmer condenser VC4.
10. Automatic Gain Control (AGC) Circuit
AGC voltage from the AM detector is applied to control grids of
Vl, V4 and V5. The AGC circuit keeps the output level of the
receiver almost constant regardless of input signal strength. In
the case of reception of DX (CW & SSB signals), the AGC circuit
may be switched off and RF GAIN control adjusted for good
reception quality.
{My receiver does not have a switch to turn the AGC off – I
suspect that this is legacy text from the first model of the
receiver (FR-50) and instead a slow/fast AGC switch took the place
of the AGC on/off switch on the FR-50B model}
11. Mute & Monitor Circuit
Adoption of a muting circuit enables smooth switching of transmit
and receive. RF GAIN control VRl, and monitor control VR2 are in
series to cathode return circuit of Vl and V4. For muting with a
transmitter, relay contacts must open on "transmit" and close on
"receive". Thus, on "receive", the mute terminal is grounded. On
"transmit", the monitor potentiometer increases the bias.
{The relay mentioned here is part of the FL-50B transmitter}
12. A.F. Amplifier
Output from the detector, through the mode switch (S3), is applied
to the triode section of V8 (6BM8), from which output is coupled
to the power amplifier, the pentode section of V8.
Since the same circuit is used for the speaker connection and
headphone jack, low impedance speaker or headphones, 4 - 8 ohms,
should be used. The output transformer (T2) also has a 600 ohm
winding to supply anti-trip signal for a VOX circuit.
13. Power Supply
HT supply consists of four silicon diodes (D5, D6, D7 & D8) in a
bridge circuit and with a filter choke performs with good
regulation. Supply for the transistor oscillator is stabilized by
a zener diode 1S225. Heater supply is 6.3 volts, same as that of
the FL-5OB.
{The original diode in my receiver is a 1N4740 which appears to be
an equivalent zener diode}

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OPERATION OF RECEIVER
The following operating conditions should be observed.
1. Antenna
The antenna input impedance of the FR-5OB is 50 - 70 ohms. Use a
matching antenna with coaxial lead-in. The receiving sensitivity
is directly dependent on the quality of the antenna. However, a
length of wire will suffice for casual listening.
2. Speaker
As the output impedance is 4 ohms, a speaker impedance close to
this is recommended.
For headphone use, connect the leads to the plug supplied. Low
impedance, 4 - 16 ohms, headphones should be used.
AM RECEPTION
Position of Controls
Mode Switch: AM
Preselector: Set to noise peak
Band: Desired Band
RF GAIN: Maximum
AF GAIN: For suitable volume
ANL: Off
CALIB: Off
Monitor: Minimum position
AGC: Fast
CH-select: VFO
Main dial may be accurately set for each band at the 100 kHz points
with the aid of a crystal calibrator and adjusting the zero set
control to zero beat.
Turn the main tuning knob to receive the desired signal and adjust
for maximum "S" meter reading, then peak with pre-selector. When
impulse noise such as generated by auto ignition systems is strong
switch on ANL. The tuning knob has 50 graduations, and one
revolution of the knob covers about 50 kHz on each band, except 28
MHz where one revolution covers about 100 kHz.
3. SSB & CW Reception
Set knobs to the same position as above, except the mode switch is
set to the SSB/CW position.
Tune for maximum "S" meter indication, then adjust BFO PITCH to
resolve the SSB signal, or, for CW, to obtain a comfortable
listening tone. Most SSB stations use LSB on 80 and 40 meters and

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USB on 20, 15 and 10 meters, so the BFO knob will point somewhere
near the 453.5 kHz or 456.5 kHz position.
To receive a weak signal, set RF GAIN to maximum [and turn off
AGC]{See previous note – there is no off switch for the AGC}. For
strong signals, decrease RF GAIN and increase AF GAIN.
4. Operation in Conjunction with a Transmitter
It is possible to operate in the transceive condition if the
transmitter is FL-5OB. Refer to the Figure 2.
Use as short a length of coaxial cable as possible to connect to
the VFO. When using a transmitter other than the FL-5OB, the mute
terminal must be so connected as to be open circuited to ground
during transmitting, and short circuited during receiving.
If receiver is used while disconnected from the transmitter, M & G
terminals must be shorted together.
5. Transceive Operation
Set the Xtal switch of the FL-50B in EXT OSC position. Switch to
SPOT on the transmitter and set the carrier control to maximum,
then adjust BFO PITCH for zero beat. Return carrier control to the
SSB position. Then the transmitting and receiving frequencies are
the same.
Figure 2. Connection of FL-50B transmitter and FR-50B receiver for transceive operation.

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In the case of adjusting while communicating, turn the BFO PITCH
to give the same tone from the received signal and monitored
signal. Note that the BFO PITCH control can be used to provide a
degree of "off-set" tuning in the transceive mode.
ALIGNMENT PROCEDURE
1. Test Equipment Required
A. Circuit tester
B . VTVM & RF probe
C. Standard signal generator (SSG)
{To protect your precious signal generator please make sure you AC
couple to the receiver. VTVM refers to a Vacuum Tube Volt Meter –
these instruments have an input resistance of around 11 M Ω , most
modern Digital Mustimeters have a input resistance of 10 M Ω - this
will slightly affect the voltage measurements made as per Table
1.}
2. Voltage Measurements {to assist with fault finding}
Use the VTVM to measure voltages. Refer to Table 1 for approximate
values. A circuit tester used for measurement will indicate some
lower voltages than those shown in the table.
{2bis. Resistance Measurements
To further assist with fault finding Table 2 contains expected
resistance measurements between ground and the indicated point in
the circuit. These measurements are made with the receiver
switched off (and preferably with the plug pulled out of the mains
socket)}
3. 455 kHz I.F. Alignment
Apply a 455 kHz signal to grid 1 of V3 (pin 1). Adjust MFl, MF2
and L7 for maximum "S" meter reading. Use care if adjusting MFl,
MF2. The adjusting screws have a fine thread and can be easily
damaged, if too much pressure is applied. However, re-adjustment
of these should not be necessary as there is little to get out of
order. {The filters in my receiver where not adjustable.}
4. 1st I.F. Alignment
Apply 5 173.9 kHz signal to grid 1 of V2 (pin 7). Adjust L6 (both
cores) for maximum "S” meter reading.
{L6 on my receiver had only one core and peaked satisfactorily}
5. Adjustment of 2nd Oscillator Output
Connect RF probe of VTVM to grid 1 of V3 (pin 1). Adjust the upper
core of Ll9 for 70% of maximum reading, and the lower core for maximum
reading.

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6. Alignment of VFO Scale
For correct alignment a 100 kHz marker is necessary, and if this is
built-in it should be used. Adjust coils at the low frequency end,
and trimmer condensers at the high end of each band for zero beat.
Repeat this adjustment for greatest accuracy. Note that turning of
cores or trimmers too far may lead to incorrect frequency by an
amount of ±100 kHz.
7. RF Alignment {alignment of the pre-selector}
Set mode switch (S3) to AM, band switch (S1) to 10m, and the
preselector (VC1) to mid-scale. With the main dial (VC2), tune in
a 28.5 MHz signal from the SSG. Adjust cores of L2 and L4 for
maximum "S" meter reading.
Set the band switch to 15m and tune in a 21.25 MHz signal from the
SSG, and adjust TC3 and TC8 for maximum "S" meter reading.
On 20 meters, with SSG output 14.25 MHz, adjust TC2 and TC7 for
maximum "S" meter reading.
On 40 meters, with SSG output 7.25 MHz, adjust L3 and L5 for
maximum "S" meter reading.
On 80 meters, with SSG output 3.5 MHz and preselector (VC1) at
“1”, adjust TCl and TC6 for maximum “S” meter reading. On this
band, the upper limit of the preselector range is about 3.8 MHz so
a slight sensitivity decrease results between 3.8 and 4 MHz.
During the alignment, keep the SSG output level down to the
minimum of that required. {at least below -60 dBm}
{TC4 and TC9 are used to adjust the AUX frequency, I have no idea
what TC? does}
8. Adjustment of Trap
Ll is a trap to prevent a spurious signal from entering directly
into 1st I.F. stage. Set the SSG output on 5 173.9 kHz and adjust
Ll for minimum “S” meter reading. Adjust Ll9 so that no beat is
heard from the speaker. This adjustment is unnecessary unless set
up for transceive operation.
9. BFO Alignment
Set mode switch (S3) to SSB/CW. Tune receiver to a non-modulated
signal from the SSG, adjusting for maximum "S" meter reading. Set BFO
PITCH to "0" and adjust Ll9for zero beat.
10. "S" Meter Zero Set
After removing antenna, adjust VR3 so that “S” meter indicates
zero. Note that the “S” meter will not read inversely due to the
installation of a diode. {My receiver as well as the schematics I
have seen do not have this diode installed.}

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VOLTAGE MEASUREMENTS (Volts d.c.)
TUBE PIN NUMBER
1
2
3
4
5
6
7
8
9
V1
0
♯
1.0
0
AC 6.3
145
70
0
V2
75
ѻ ♯
-3.0
ѻ
0
AC 6.3
AC 6.3
40
♯
-0.4
♯
0.3
0
V3
0
♯
0.4
0
AC 6.3
80
20
0
V4
0
0
0
AC 6.3
140
85
♯
1.6
V5
0
0
0
AC 6.3
135
85
♯
1.2
V6
♯
-0.7
∆
1.5
∆
0
AC 6.3
80
∆
55
∆
0
V7
0
∆
0
0
AC 6.3
35
∆
35
∆
0.3
∆
V8
0
7.2
0
AC 6.3
0
140
120
♯
0.6
35
Table 1. Voltage measurements.
ѻ= Calibrator switched on
∆= Mode switch in SSB/CW position
♯ = {no idea and no indication on any of my reference documents}
RESISTANCE MEASUREMENTS (ohms)
TUBE PIN NUMBER
1 2 3 4 5 6 7 8 9
V1
∞
150
6K
18K
0
V2
100K
ѻ
1M
0
100K
100K
270
0
V3
100K
330
52K
500K
0
V4
∞
0
5K8
30K
180
V5
∞
0
5K6
11K5
150
V6
22K
220
34K
∆
20K
∆
4K7
V7
47K
0
70K
∆
70K
∆
220
V8
500K
•
220
470K
3K6
9K5
2K2
300K
Table 2. Resistance Measurements
ѻ= Calibrator switched on
•= Audio gain set to maximum
∆= Mode switch in SSB/CW position

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MAINTENANCE
This receiver has been thoroughly adjusted with the aid of much
measuring equipment by the manufacturer, so re-adjustment should not
normally be necessary for a long time. If, due to component change or
development of some fault, re-adjustment or repair should be found
necessary, then this should be attempted only after full
understanding of this booklet.
1. Removal of Chassis
After removing the four screws from the
underside, push on the
front panel and withdraw from the rear.
2. Location of Faults
Measure
voltages and make resistance checks using Table 1and
Table 2 for reference. Any large departure from the values
shown in the tables could be an indication of the faulty
section.
Use the VTVM for checking.
(1)
Receiver not operating:-
Examine AC power cord and plug.
Check fuse. If the fuse has gone, there may be a short
circuit in the
H . T . or heater circuits, or failure of one
or more of the silicon diode rectifiers (D5, D6, D7 &
D8).
(2) No sound from speaker:-
Examine speaker connections, making sure that the
headphone plug is removed from the jack on the front
panel.
Check audio amplifier section by touching grid of V8
with a screwdriver.
Check detector stages.
AM --- detector diode (D4)
SSB --- BFO and product detector
(3) Audio and detector stages OK but receive fails to operate:-
In checking IF and RF stages make use of the "S" meter.
Examine the lst converter if the receiver fails only on
one band.
If click is apparent when grid of V3 is touched with a
screwdriver, fault is in the stages preceding V3.
Local oscillator injection voltage must be measured with
the VTVM.
NOTE - Dial Calibrations
On the main dial, the bottom scale is for 80 meters. "500"
represents 3 500 kHz, "600" represents 3 600 kHz, and so on.
The centre scale serves for the 40, 20 and 15 meter bands where
"0" represents 7 000, 14 000 or 21 000 kHz, "100" represents 7
100, 14 100 or 21 100 kHz, etc., depending on the band in use.

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The sub-divisions on the main dial are 10 kHz points. The
graduations on the tuning knob skirt closely represent 1 kHz
divisions. The upper scale on the main dial is used for 10 meters
but in this case the knob skirt graduations represent 2 kHz
divisions.
The knob skirt may be adjusted independently of the knob itself by
firmly holding the knob while turning the skirt, and can be set at
0 for 100 kHz points with a crystal calibrator. Since, on all
bands except 10m, the knob rotates two full turns for each 100 kHz
segment, a double numbering system is adopted on the skirt, thus,
read 0 to 49 kHz using the black numbers and 50 to 100 kHz using
the red numbers.
A plug-in 100 kHz crystal is available as an optional extra.

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PART LOCATIONS

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CIRCUIT DIAGRAMS

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