“A Modern Homodyne”

 

Last update: April 6, 2010

 

 

Last winter I have constructed a simple receiver,  called the “Modern Homodyne”. Full information

is provided in two articles by G.W.Short which were published in The Radio Constructor (March

1972, p.492-495, and April 1972, p.552-557). Thanks to the efforts of Maurice Woodhead, Shorts

articles can be downloaded from internet. Apparently, the Homodyne is a regenerative rx providing

a simple form of synchronous detection.

 

 

The circuit diagram of the R.F. section  is shown in Figure 1. There is only a single LC-circuit for

tuning, although several coils are shown in the diagram, allowing band switching. FET TR1

serves as a buffer providing a high input impedance. Therefore, the tuning coils do not need any

tap. R.F.gain is provided by TR2, a germanium transistor which is still available from several

sources (I purchased ten AF239s from a small company in Germany, through Ebay).

RV4 should initially be adjusted for 3 mA collector current of TR2. Its setting may later be varied

for optimal quality of reception (between 1 and 4 mA Icc). RV7 is the regen control. Cx is a gimmick

capacitor (a few turns of twisted insulated wire). It compensates for phase shift in TR2 at the

higher frequencies.  D1 and D2 limit the maximal amplitude of oscillation when the regen control

is turned up. D3 and D4 are the detector, and are followed by a simple low-pass filter (C6, R9, C7).

R8 provides a relatively heavy load to the detector, resulting in smooth regeneration. TR3 serves as

an audio pre-amplifier. 

 

 

 

 

 

 

 

 

                                   Figure 1. RF section of Shorts Homodyne receiver

 

 

The R.F.section of this “modern” design was reproduced as shown in Shorts articles, including

the circuit layout. All electronic parts are soldered between copper pins driven in a small piece of

plywood. The RF amplifier (TR2 and associated components) is shielded with copper foil. As a.f.

amplifier, I haven’t used the published circuit with discrete transistors (not shown here), but a

TBA800 which provides better audio (Figure 2).  C12 (from Figure 1) is missing in my version of the

rx, since the TBA800 amplifier has an input capacitor.   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

               Figure 2. Output section based on a TBA800 i.c. (driving an 8 ohm LS)

 

 

Thus far, I have only used the Homodyne on the mediumwave broadcast band, using a ferrite

antenna salvaged from an old valve receiver. A reasonably good earth is a pre (reduces hand

effects and increases the strength of the received signals). I am quite pleased with the results.

Audio quality of the receiver (fed from six penlight cells in series) is very good, better in fact

than any other MW receiver in my home! I have about 15 valve and transistor radios in good

working condition. The radio isn’t bothered much by computer-generated interference, in contrast

to portables with telescope antennas or valve receivers with inbuilt loops. Easy tuning requires a

reduction gear on C2  (or C1 for fine tuning) and some practice. A loud howl is heard when one

approaches the carrier of a station. When  the  point of exact tuning is almost reached, the howl

becomes a low-frequency grumble, then suddenly the oscillator is synchronized with the station

carrier and the station is heard with excellent audio quality. I  greatly enjoy listening to BBC

World Service, the local Dutch (Radio 5, Radio Maria, Groot Nieuws Radio) and German

(Deutschlandfunk, NDR, WDR) stations, Kossuth Radio (Hungary) and NRK1 (Norway).

Give this vintage circuit a try, I don’t think you will be disappointed.

 

 

 

 

 

 

 

 

 

 

 References

 

Maurice Woodheads website : For scans of G.W.Shorts 1972 articles

Another side with Homodyne article : Another site with these scans

Mike Tuggle’s Homodyne : A Homodyne version with basket coil

Dan Schmarder’s Homodyne : A Homodyne version with spiderweb coil

Jim Kearman’s Homodyne : A Homodyne version with silicon RF transistor 

 

 

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