REH Receiver
The REH is an amenities receiver used by the United States Navy. It was
made by the Belmont Radio Corporation, Chicago 39, Illinois, USA,
in 1945 and has the designation of CDL-46271.
An "amenities" receiver is intended to provide radio news and music to
off duty serviceman. In the USA it was known as an "Entertainment" receiver,
or called a "troop morale" receiver. In the UK it was known as a "NAFFI"
receiver because it was placed in the canteen, and also known as an "Invasion"
receiver because that is where you would hear of the invasion landings.
This type of receiver is normally a ruggedised version of a domestic receiver,
with the additional ability to operate from several voltages, and in hostile
climatic environments.
INTRODUCTION
The REH is a 7 valve superheterodyne receiver with a 455 khz IF, and has
5 frequency bands covering the ranges:
Broadcast 540 - 1600 kcs
49 Meters 5.96 - 6.19 mHz
31 Meters 9.1 - 10.0 mHz
25 Meters 11.45 - 12.16 mHz
19 Meters 14.14 - 15.46 mHz
This is the normal broadcast band and the short wave broadcast bands. It
also has a phonograph input. It can operate on 110 or 240 volts AC and 50
or 60 cycles. The receiver is in a strong steel case, and slides out the
front when 5 captive screws are released. It has built in feet, a carry handle,
a telescopic antenna, and is painted with grey wrinkle paint. The nameplates
on the front panel, carry a warning that it is not to be used on ships, as
it does not have radiation protection, and so may betray the ships position
to the enemy.
CONTROLS
The controls are very simple. There are only 3 knobs. The left hand
knob is the volume control, the centre knob is the band change switch, and
the right hand knob is the station tuning knob. There is a row of push buttons
to select pre-set stations. By pushing these buttons, the tuning knob
spins and the station is selected. The RIGHT most button is the ON/OFF control.
The LEFT most button is the TONE control. Next to the TONE push button is
the PHONO button, (but this has to be pulled, not pushed). The manual has
a sheet of American station call signs, that can be cut out and affixed
to the push buttons. Each button has a clear removable cap, that can be popped
out, and then replaced with the station call sign visible behind it.
To set a station, you first push a button to set the old station, then pull
the button out, then you tune in the new station, and press the button in.
The new station is now set. Above the buttons is the square dial, showing
the frequency. Above the dial is a magic eye tube to show the signal strength.
On the RIGHT hand side is the speaker. The power cord comes out the back.
Also on the back are the PHONO terminals, the ANTENNA terminals, and a telescopic
antenna. The manual recommends operation on the telescopic antenna for nearby
stations, and a 50 foot wire and earth for other stations. There is an internal
chassis mounted toggle switch, to select 110 or 240 volt operation.
DESIGN
The radio has a conventional steel chassis, with the octal valves mounted
in the normal fashion. The RF tuning section is a separate module
that can be removed. It contains the mixer, tuning coils, RF amplifier, dial,
and push buttons. On top of the tuning module, is a rack containing a spare
set of valves.
Most of the valves are the metal type. It has a mains transformer, with
a switch to select 110 or 240 volts AC. It has no provision for DC or battery
operation.
There is a 5Y3 rectifier, with a capacitor choke HT filter. The audio output
valve is a 6V6 pentode which drives a large oval speaker. The audio pre-amplifier,
detector, and AGC rectifier is a 6SQ7 triode double diode. There is a TONE
control circuit on the triode plate, that switches in a fixed capacitor,
or a fixed resistor with the capacitor. This is activated by the left hand
push button, that cycles through 3 positions, one for each push button press.
The triode grid is switched to the rear PHONO terminals for external input,
or the IF transformer for normal radio reception. This is activated by the
second most left hand push button, but instead of pushing, you pull it for
PHONO and push for radio. The single IF amplifier is a 6SK7 pentode, and
the 455 kHz IF transformers are slug tuned with fixed capacitors. The mixer/oscillator
is a 6SA7 heptode. The RF amplifier is a 6SK7. The receiver has AGC and
this is applied to the mixer and RF amplifier only, not the IF amplifier.
The tuning method for station selection is not the normal variable tuning
capacitor, but variable slug tuning. There are 3 slug tuned coils for the
broadcast band (RF, mixer and oscillator). The short wave bands are combined,
and all are tuned by just 3 slug tuned coils (RF, mixer and oscillator).
To change the short wave coverage, there are additional fixed coils switched
in circuit, and the oscillator coil is tapped. There are 6 slugs in total.
A note on the circuit diagram, explains that there was a loop antenna, but
the telescopic antenna has replaced it.
RESTORATION
When I acquired the radio, it was described to me as not performing very
well. Upon examination, it showed that several people has worked on it, as
there was evidence of many different repairs. I had no circuit, so I put
the radio aside until I managed to locate a manual. A friend scanned
the manual and emailed to me, complete with the station list for the push
buttons, and the original purchase receipt. Thank you very much, it has been
a great help.
The underneath showed that the 110/240 volt switch was disconnected and
a new mains transformer had been fitted. There was indication of pitch leakage,
so the original transformer may have failed and been replaced. I began by
connecting it up to a VARIAC and bringing the voltage up slowly to 240 volts
AC, while monitoring the HT voltage. It came up to full voltage without and
smoke or any other problems. I let it stay that way for a some time, while
checking the filter capacitors for heating. I found that the audio coupling
capacitor was passing positive voltage to the power output valve grid. This
is a common fault. I removed the capacitor, pulled out the insides, and concealed
a modern capacitor within the cardboard tube, then filled it with wax. The
scratchy volume control proved that the audio stages were now working properly.
The control was lubricated to remove the noise. The manual said the IF was
455 khz so I connected a signal generator into the mixer grid and attempted
to tune the two IF transformers. The slugs came all the way out to their
stop, without peaking, very strange. I aligned one of the RF bands,
but could only get a 160 mv sensitivity, which indicated something was wrong.
The RF amplifier seemed to be very low on gain, and when I checked the grid
it had a positive voltage on it, indicating a gassy valve. I replaced the
valve, the voltage was now zero, and the gain improved.
Nothing else seemed to increase the gain, so I removed the IF transformers.
I took the can off, and there was the normal pile wound windings with a
capacitor across each one. I unsoldered one end of each capacitor, and checked
it with the capacity meter. They should be 75 pf, but the reading was 1200
pf and changing as I watched it! It was not steady, so there was something
wrong with the capacitors. I checked the resonance and it was 350 khz. I
fitted new capacitors, and used a signal generator and oscilloscope to set
each winding individually to 455 khz. The level was much higher, and the
peak very sharp. The method I used, was to remove one capacitor, and use
this as the primary untuned winding, driven from the low impedance signal
generator, set at 455 khz. The IF secondary was connected to the high impedance
oscilloscope. I tuned the secondary slug, for maximum level which was
resonance. In this way there was only one tuned circuit, so there were no
interactions. I then reversed the procedure, and set the other winding to
resonance using the other slug. I reassembled the IF transformers, put them
back in the radio, and realigned the IF. There was much more gain this time,
the slugs peaked properly, and the radio came alive, with a sensitivity of
50 micro volts at the mixer grid.
I aligned the broadcast band mixer and RF coils, and then the shortwave
bands. One band was dead, which proved to be an open circuit oscillator coil.
I repaired the broken wire and then this coil aligned properly. There was
a disconnected capacitor, with a new one tacked on to the band change switch.
This was C8 which coupled the signal from the RF amplifier to the mixer.
I checked and this was passing DC from the RF plate circuit. Here was another
capacitor of the same manufacture which had failed. I snipped out the faulty
one, and the unsightly replacement, and concealed a new one within the switch
wafers where it should be mounted. The magic eye did not open or close when
a station was tuned in. I removed the rear cover from the magic eye base,
where it is mounted above the dial, and examined the 1 meg ohm resistor there.
It was open circuit. I replaced it with an identical style good resistor,
and the eye now worked properly. The green fluoresent screen, was a light
green, and on the section where it closed, it was a darker and brighter green.
This is probably due to the radio being operated for a long time in the faulty
condition, and the screen wearing out. The part that was never used
is as good as new.
The radio was distorted on some of the stronger broadcast stations. I checked
the AGC and it was only reaching negative 6 volts, whereas I would expect
about negative 30 volts. I checked at the AGC diode, and its voltage was
in this order, but it was low by the time it reached the grids. I found that
the AGC bypass capacitor was measuring 2 meg ohms, so this was loading the
AGC line. I replaced this capacitor, the AGC voltage was now much larger,
and the distortion went away. The oscillator grid resistor had been replaced
with a modern style resistor, and it was sitting up on long leads. I found
a suitable resistor in the junk box, that matched the style of the other
resistors and fitted it to the radio.
The radio now has sensitivities of:
Broadcast band 3 uv
49 Meters band 2 uv
31 Meters band 2 uv
25 Meters band 2 uv
19 Meters band 3 uv
The case and front had been painted with light grey paint. I checked under
the removable panel, and discovered that it should be dark grey wrinkle paint,
so I repainted the front and the case, with grey wrinkle paint, and baked
it in a n oven. It came up nicely.
CONCLUSION
The radio reforms well for a domestic receiver, considering it only has one
IF stage and one RF stage. It is easy to use, with the push button controls
and the external whip aerial. The large speaker gives a good sound. Having
spare valves inside is very convenient for remote repair. The radio could
be improved, by using all the push buttons for station TUNING, and adding
a separate ON/OFF TONE control switch. There could also be a RADIO/PHONO
(and even MICROPHONE) switch. Having one switch as a "pull out" for PHONO
is strange. The manual has a comment in the "Fault Finding" section, which
says that "if the radio does not work, check the setting of the internal
110/240 switch, and if it is set wrongly, replace all the valves".
REFERENCE
Manual
Copyright
Ray Robinson VK2NO
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