The following description is concerned with my last construction
of a Fremodyne. I decided on
the simpler two RFC circuit as per the "Meck" as experience showed
that there was no difference in performance between it and the three RFC
circuit. An audio amplifier was included to form a mantel style FM receiver.
A simple plywood cabinet was constructed for the Fremodyne and its
8" speaker.
The circuit of my Fremodyne; click here for larger view.
The chassis was made from aluminium; its dimensions being 200 x 110
x 50mm.
This
diagram shows the component layout of the Fremodyne portion of
the receiver.
As can be seen there is not much to it. Construction is
basically a matter of connecting the
components correctly, keeping in mind the necessity of keeping leads
straight and short in the
VHF portions of the circuit. It is also adviseable to use thick (eg.
18 or 20SWG) tinned copper
wire for connecting between points due to skin effect. That is, at
VHF and higher, current flows
on the outside of conductors. Likewise it is necessary to have a good
groundplane. The 12AT7
pin connections assist with layout as the oscillator can be confined
to one side of the valve
socket and the super regenerator to the other, with the heater and
earth connections in between.
The centre sheild of the socket should be earthed, and as for the socket
itself, a teflon or ceramic
one should be used if available, but a bakelite or wafer type seems
to work just as so long as it is clean and not leaky. If the socket used
is second hand, it would be worthwhile cleaning away the old flux with
metho.
With a circuit of this type there will always be the question
of parts. Fortunately there are
always alternatives and it would be possible to build the receiver
with all new parts. The audio
amplifer and power supply are the easiest sections to deal with first
as they offer the greatest
flexibility.
My audio stage is unconventional in that a 6AW8 is used
with the output pentode triode
connected. This valve was used as not enough heater current was available
for alternatives. The
pentode is meant for resistance coupled video amplifier service and
does not perform well with
a speaker transformer as a load. This is not surprising as I have never
seen data on the 6AW8
for audio output use. Triode connected, however, it works quite well
with sufficient gain and
output power for the Fremodyne. Incidentally, the European version
of the 6AW8 , the 6DX8
does have ratings for audio output use and performs like it, but it
requires more heater current
than was available.
I imagine the reader would use something more conventional
such as 6AV6, 6AQ5 or 6J7, 6V6
type of circuit as per a typical mantel set or radiogram. I have included
a circuit using a 6BM8 for those who want to use a more easily obtainable
valve. It is a well tried circuit in manyof my
projects.
Of course, one can save time and parts by using an existing set with
P.U terminals, or
as stated erlier, high impedance headphones.
The power supply will be largely dictated by the audio
stage as the Fremodyne itself only
requires 100V at about 4mA. Again, any standard type radio power supply
can be used as a
basis. Some radiograms even have a socket to deliver B+ to an FM tuner
which could be used
via a suitable resistor to provide power with the P.U terminals providing
the audio stages. To
cater for those who don't have any valve power transformers, back to
back transformers can be
used as shown here.
It will be necessary to check the supply voltages and adjust the filter
resistors accordingly with the particular power supply used. It is
easiest to adjust the main filter
resistor first to get the audio stages working correctly, then adjust
the second filter resistor so the
Fremodyne is supplied with 100V.
Now to the actual receiving part. The IF coil is not critical.
Something with a former around 1/4" diameter with a ferrite core suitable
for 30Mc/s operation is required. I have used a modern
Neosid 5mm former and slug with complete success. It may be possible
to use a slug tuned
455Kc/s transformer but any ferrite cups or rings will need to be removed.
For the 5mm or 1/4"
formers, 12 turns of 36 B&S enamelled copper wire is wound on.
As there is only one winding it is easy to alter the number of turns if
resonance if the IF cannot be obtained. As for the RF
chokes, all three of them consist of 100-120T of 32 B&S E.C.W on
a 5/16" former such as part of a ball point pen casing, a Rawl plug, or
a high value (>100K) carbon resistor. For those who
don't like winding coils, it is possible to use commercially available
15uH chokes. The value of
inductance calculated from the choke dimensions turned out to be very
close to their measure
value and operation of the prewound chokes has been confirmed in practice.
A receiver
constructed with these minature RFC's is also a lot neater.
Next we come to the aerial and oscillator coils. They
are both 7mm inside diameter, wound
with 3.5 turns and 3 turns of 18 B&S tinned copper wire, for the
aerial and oscillator coils
respectively. The trimmers across the two coils should have a cpacitiance
range in the region of
2-20pF and are only necessary if not provided for on the tuning gang,
which incidentally is likely
to be the most obscure item. It is a dual gang unit with a capacitance
range of 2-15pF. Higher
capacitance values are useable but will give a greater tuning range.
The gang in my latest
receiver was obtained at a HRSA meeting but there are several other
solutions to obtaining this
item. One can take the approach of using separate single tuning condensers
as per 1920's
technology. A pair of reaction condensers would be a good start but
the capacitance is likely to
be around 100pF and therefore series condensers [ try 33pf]
will be required to prevent the FM band being crowded together at one end
of the tuning range. One butcherous method used in some home made post
war sets for VHF reception was to remove plates from a standard broadcast
gang. Typically two or three plates would be left on the rotor section.
It is not something I recommend these days with vintage components. I have
also used with success a modern plastic unit for replacement in AM/FM radios.
The FM sections are about 20pF and have trimmers. These units require an
extension shaft to allow use of a normal 1/4" knob. This consists of a
piece of pot shaft with a 2.5mm screw through the middle attached to the
tuner shaft. Panel mounting is by two short 2.5mm screws and care must
be taken not to let them protrude inside the case and damage the plates.
Having assembled the receiver it should be turned on and
adjustments made to the power
supply voltages if necessary, ensuring the Fremodyne part has 100-110V.
A rushing sound
should be audible at this stage to indicate the super regenerator is
working. The next step is
alignment, which is done the same way as with any other superhet. First,
feed an AM signal of
21.75Mc/s into the aerial terminals and adjust the IF coil for a peak
in signal, reducing the signal
generator output as necessary. If the coil won't peak at 21.75Mc/s
this would indicate that turns
would have to be added or subtracted from the IF coil. If the coil
peakes up at a higher frequency
it has insufficient turns and vice versa.
The local oscillator is aligned next. By means of the
local oscillator trimmer at the high end of
the band (108Mc/s) and by contracting or expanding the turns spacing
of the oscillator coil at the
88Mc/s end of the band, ensure that the reciever will tune from 88-108Mc/s.
Again, turns may
need to be added or subtracted if there is not enough adjustment available.
Finally the aerial coil is set up. With the receiver and
signal generator on 88Mc/s, expand or
contract the aerial coil for maximum sensitivity. Tune to 108Mc/s and
use the aerial trimmer to
peak up for sensitivity again. Both the oscillator and aerial stages
should be aligned once more
due to interactive effects. Note that the aerial tuning is very broad
and adjustment is not critical.
In fact, one
version of the Fremodyne eliminated the tuning circuit altogether,
but this resulted in
poor image rejection.
For those without a signal generator, adjustment is still
possible. Tune a shortwave receiver to
21.75Mc/s and place it near the Fremodyne. Adjust the IF coil for maximum
noise on the
shortwave receiver. Then, use off air stations to set the local oscillator.
Eg; for Sydney check
that 2RDJ (88.1) is receivable at the low end and 2SER (107.3) is receivable
a the top end.
Adjust the aerial coil as before.
The easiest way I found to align the aerial and oscillator
coils was to use a G.D.O. Simply
ensure that the oscillator coils resonates betwen 110-130 Mc/s while
at the same time the aerial
coil resonates at 88-108Mc/s.
Performance
First thing evident with a Fremodyne, or any other superregenerative
set on the FM band, is
the whistle audible in the background on some stations. This is a result
of the stereo and SCA
subcarriers beating with the quench frequency. It can be annoying or
not noticeable depending
on the station. Obviously the problems does not occur when receiving
mono stations. It is
important to realise that stereo FM did not exist prior to the 1960's
and therefore this problem did not exist with the original Fremodynes.
Some improvement can be made by increasing the
quench frequency, by reducing the 150K resistor, at the expense of
sensitivity. Also reducing the
.0047uF across the 150K can make a big improvement. Filtering the audio
with a notch filter
seems to have some promise, though I have not fully investigated this.
Interestingly, this
problem is not evident with Hi-Z phones, probably due to limited bandwidth.
Secondly, sensitivity is poor. At least 100uV of signal
is required for good reception. The
original Hazeltine specification was 200uV. So, this is not a set for
DX, but will bring in all the
local stations without any problems. An outdoor FM aerial (not TV aerial
that was designed for
Ch 2,7,9,10) is essential for peak performance - the receiver performing
very well for what it is.
Some low power community stations are then receivable. I have even
tried the set in a car
powered off a vibrator inverter, and using the existing broadcast aerial.
It certainly worked
around Sydney quite well, but I wouldn't recommend replacing the existing
car radio with it! An
RF amplifier stage could help in regards to sensitivity. Experiments
with a VHF TV distribution
amplifier seemed to indicate potential here.
Where this set really does lead over ordinary superregenerative
receivers on FM, is that the
sound quality is superior and remains constant across the band, and
with weak and strong
signals. Tuning is also less critical.
For its day, the Fremodyne certainly acheived its
aim as a low cost FM receiver of acceptable
quality. These days its practicality is limited to some degree by the
addition of subcarriers to FM signals, but where this isn't a problem it
makes an interesting and practical receiver.
If the Fremodyne's limitations seem too severe, all is
not lost, for there is another kind of simple
FM receiver, possibly even less well known. Sound is true Hi Fi and
sensitivity is a few microvolts.
Take a look at the Pulse
Counting Receiver.
cablehack@yahoo.com
