The article that follows was supposed to be published in the HRSA (Historical Radio Society of Australia) "Radio Waves" magazine after I wrote it in 1997. The response was enthusiastic, but nothing was ever forthcoming, despite being informed it would be published sometime in 1998.   Later, I was requested to write an FM receiver article, for home construction, to go in Radio Waves sometime in 2004!
This request, I might add, was after mention of my valve FM receiver construction in Silicon Chip, which incidentally published a transistor superregen circuit of mine in the April 2003 issue. Silicon Chip liked my contribution enough to reward me with a nice true RMS multimeter.
Meanwhile, with HRSA Radio Waves it's waiting....waiting....waiting....

Needless to say, after the amount of work I'd gone to for my Fremodyne article, and never having it published, I haven't contributed anything since.  Well, I got tired of waiting for hell to freeze over; so here's my article on my site for all to see. I figured that the 10Mb of space I get with my ISP would be a better place for my electronic work to be viewed, rather than the excessively conservative " if it's not a 4 valve AM superhet working at less than 30Mc/s we don't want to know" attitude of the HRSA Radio Waves magazine. I know I sound like I'm pissed off with the whole thing, but it was nine handwritten pages of A4, with photos taken as well. Fortunately I did keep a copy which is what I'm reproducing here.  In view of the additional information and actual receivers which have come into my possession since my original article, I've spread it over several pages.

  The above heading will undoubtedly bring back memories to readers of Electronics Australia
during the 1960's. The Fremodyne was basically a superregenerative superhet which was used
as a simple VHF communications receiver [in the EA version]and it was a popular project. However, the Fremodyne has its origins as a low cost FM receiver well before this as the following article will explain.
  Where does the name come from? FREquency MODulation, and DYNE being a generic sort of suffix added to the names of various radio circuits since broadcasting began.
   My own interest in the Fremodyne came about nearly ten years ago (1987) as a result of the
AM music stations going to FM. Being the type of person who likes
constructing regenerative receivers in preferences to superhets, I looked for a similarly simple
way to receive FM. The first course of action was to look at the articles in Electronics Australia.
From here on I started my fascinating research into, and construction of, superregenerative
receivers.

Background

   During WW2 there was intensive research done into superregenerative receivers, to turn them
from unpredictable critical devices, to stable and reproduceable designs used for the war. The
IFF (Identify Friend or Foe) receivers are one example. Around this time FM stations were being set up in the U.S.A. and with the results of this wartime research, the Fremodyne was born by the Hazeltine Corporation as a way of getting FM receivers to the public at minimum cost.
   Fremodyne receivers came in two forms; as a stand alone "FM converter" for feeding into the
P.U [Pick Up; ie. gramophone cartridge]terminals on an existing AM set, or as part of a combined AM/FM mantel radio. At this point it might be wondered why an ordinary super regen receiver was unsuitable for this application.
Firstly, a super regen set will work on the FM band; this 6C4 design or my 12AT7 design being typical. There are several drawbacks, however. To start with, one has to adjust a regeneration control each time a different station is tuned in, a lot like an AM regenerative
receiver. Obviously this is unacceptable for mass production for non technical users. Secondly,
super regen sets radiate interference on the frequency to which they are tuned. While this isn't a
problem for a few home made units, when you want to have thousands of them across the U.S.A
it is clearly unacceptable. Even if these two problems could be overcome, there is a third. The
receiver sensitivity and bandwidth varies from one end of the band to the other.
In Europe, with the introduction of FM after WW2, a similar situation occured where low cost FM receivers needed to be made. Here, conventional super regenerative circuits were used, but with an RF amplifier to reduce radiation and aerial loading. Variable inductance tuning was used to provide relatively constant performance across the band without requiring regeneration adjustment. Some known sets are Philips 7758 and Grundig 840. There was also a retrofittable converter, Telefunken UKW1C which was mounted inside an AM radio. However, variable inductance tuning was never popular in the U.S.
   The obvious way out of the radiation  problem is to use a superhet circuit. In addition, with a fixed IF,  the bandwidth and sensitivity would be constant across the band. And with a superregenerative IF of 21.75Mc/s the radiation would be confined to one unimportant frequency.  The regeneration would no longer need to be adjustable and most signifigantly, there is only the tuning control left for the user to operate.
   The first Fremodynes came off the production line in 1947 and the design was popular for only
about three years. My only knowledge of the circuit being used in Australia at the time was as an
option for the "Telecond" car radio, for reception of Sydney's new FM transmissions in 1947.
Apparently, space was left in the radio for a Fremodyne converter to be installed. However, to
my knowledge there were no sets so fitted.

Circuit

   Despite the apparent simplicity of a superregenerative detector, the circuit performs in a very
complex way and it is beyond this article to analyse it. However, for the purposes of
understanding the following circuit, it's operation is as follows: It is essentially an elaboration on an ordinary regenerative detector which has its operating point taken past the point of oscillation; that is, a beat would be heard. However, it is taken in and out of oscillation at a supersonic rate so the beat is not heard. The rate of this is known as the "quench frequency" and is typically 20-100Kc/s. The result is potentially a very sensitive [depending on design some detectors will receive a few uV, while others require considerably more than this. The amount of hiss when tuned off station is NOT an indication of sensitivity; it merely indicates that super regeneration is taking place] AM receiver capable of detecting a few microvolts of signal with a wide bandwidth of typically 200Kc/s. The fact that the receiver is going in and out of oscillation is what causes it to radiate interference.

For full size circuit click here

The lower triode is a conventional Colpitts oscillator operating 21.75Mc/s above the received frequency. The upper triode is the superregen detector tuned to an IF of 21.75Mc/s. This triode performs four functions: 1. Superhet converter, 2. Super regen IF amplifier, 3. FM to AM converter, 4. A detector providing audio output. FM to AM conversion occurs by tuning the receiver to the most linear part of the selectivity curve; ie. slope detection. A feature of the circuit is the automatic stabilising circuit which allows the regeneration control to be eliminated. It also allows a specially shaped quench waveform to be obtained which gives the right selectivity, high audio output and linear FM detection. It thus gives superior audio quality to a conventional super regen set for FM reception.
   The VHF signal is coupled to the grid of the upper triode via a 2pF condenser along with the
local oscillator signal and the difference frequency is developed in the plate circuit, the load of
which is the IF coil. This signal is amplified by a Colpitts oscillator type super regen detector
tuned by the two 30pF condensers and the inductance of the coil to 21.75Mc/s. The 15K resistor
lowers the Q of the IF coil to broaden its bandwidth. Why 21.75Mc/s you may ask? The IF is
chosen to avoid harmonics in the FM band - important if the IF amplifier is oscillating. For
example, 31Mc/s would be unsuitable as its 3rd harmonic is 93Mc/s. 21.75Mc/s has it's 4th and
5th harmonics just outside the band. 28Mc/s is also a suitable IF.
   The audio is developed across the 22K resistor in the cathode circuit and coupled to the
volume control via a de-emphasis/quench filter comprising the 100K resistor and .001uF
condenser. The 1500R resistor and .0025uF condenser control the quench waveform. The
stabilising voltage is developed by the 150K resistor and 10uF condenser. As grid current flows, the 10uF charges and discharges through the 150K. This time constant is chosen to give
maximum AF output with stable operation. The 150K also sets the quench frequency which is in
the region of 30Kc/s. Since a super regen receiver is sensitive for only short periods, noise
occuring between these periods is not received. In this way the receiver has good noise immunity.
   Sensitivity is quoted at 200uV. Although weaker signals can be heard, the signal to noise ratio
deteriorates. The circuit is designed to feed a 500K volume control and drive a standard triode
pentode audio amplifier. It will also drive Hi-Z headphones at a level similar to that of a crystal
set. Instructions for Fremodyne receivers suggest using a piece of wire 3 to 8 feet long as the aerial where the "power line" aerial is insufficient (ie. the aerial terminal is connected to the power line via an isolating condenser). Failing that, a proper dipole is suggested in weaker signal areas. No hints are given as to whether coaxial transmission line should be used to suit the unbalanced input, or if 300 ohm ribbon can be used. In reality we can guess that in most "dipole" connections would have been with balanced line. This may seem crude, but it saves the cost of a matching transformer. As interference is not usually a problem in the FM band, the unbalancing of the aerial system does not cause any practical ill effects.
   Other manufacturers of the Fremodyne performed slight alterations to the circuit but it
remained essentially the same. Some used a 14F8 or 7F8 valve which are the loctal versions of
the 12AT7. In some Fremodynes the IF coil damping resistor was of a higher value to suit their
coil charactersitics. In others, the oscillator was a Hartley type connecting the cathode to a tap on the oscillator coil, thus saving one choke. The aerial coupling also differed to either a tap on the input coil, or a separate primary coil to couple the aerial to the tuned circuit. More importantly was a modification of the VHF input circuit to the mixer. As can be seen, the grid choke serves to couple the low frequency quench voltage into the grid while also allowing the VHF signal into the grid unimpeded. The question may now be asked, what if the 500pF condenser was shorted out and the quench voltage fed into the bottom of the aerial coil? Indeed it can, and another choke and condenser are eliminated. About half the Fremodyne circuits took this approach. The circuit of such a receiver is here:

 

It was made by Meck as a stand alone converter.
Note the lethal audio output and aerial connections, typical of American circuitry of this period.
Also note that it's highly likely the above circuit contains a mistake where the chassis is shown to be connected  My early version of this converter
has the chassis isolated from the negative rail via a condenser.  However,  the third aerial terminal is still not isolated.

   Late 1947 saw a number of manufacturers jump on the Fremodyne bandwagon, only to withdraw from using the design a few years later. Here's some examples:

Meck FM Converter.

This is the Meck FM Converter; the first version circuit is here.

Inside the Meck. Valves are 7F8 and 6H6. Note that 14F8 and 35W4 valves are shown on the circuit, which would have been a later modification to reduce heater current and line cord resistor dissipation.
The safety aspect was also upgraded in a later version. The aerial connections were properly isolated and the audio output included an isolation transformer.
This is a Meck AM radio which uses the identical cabinet to the FM converters. This was undoubtedly a further cost cutting feature.
Here's the first Meck FM converter I acquired.

A similar FM converter to the Meck which appears to be a clone was made by Telvar but housed in a wood and leatherette cabinet.
Read about my Meck FM converter here.

Circuit for later model with transformer output.

From the UK came this variation:

This is a British version of the Fremodyne, sold in kit form by Osmor. Note the C1/L1 trap to reduce the super regen radiation. Also note the RF input is not tuned. This was acceptable as the three BBC VHF stations were always grouped close in frequency in each locality, thus image rejection was not an issue. No pics of the finished product are available; only this circuit.

Read about my Heathkit here

This is the well known Howard 474; a complete AM/FM mantel set. This was the most common of
the Fremodynes and examples of it still turn up. Like the Meck converter, the Howard 474 uses the same cabinet as an AM set of the same period; the 901-A. I acquired one of these sets in September 2004.
Read about my restoration of a 474.

For the  manual, go here.

Service manual is here.

Read about my Gilfillan 68F here

Read about mine here.

    There was also another kit from the Rollin company which used a super regen superhet design, but it was not of the Fremodyne type. Instead, a 6BE6 converter was used to feed a 6J6 operating as a super regenerative detector tuned to 31Mc/s.

    It all seems a delightfully easy way to receive FM signals and the reader may wonder if there's
a catch to this. Why did manufacturers still produce expensive ten valve FM superhets after
this simple alternative was developed? Well, yes there is a catch and it will be revealed in the
remainder of this article.

This is my home made Fremodyne which I'd built late 1996. Read about it here.
 
 
 
 

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