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Leak Trough Line Tuner: An Improved Stereo Decoder |
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Leak Trough Line Tuner: An Improved Stereo Decoder |
The original stereo decoder
supplied by Leak fitted to the "Trough Line Stereo", or factory
retrofitted to the "Trough-Line II" or "Trough-Line 3"
can be much improved on by using a modern integrated circuit to perform
the function. It is recommended that the original decoder be replaced.
The original circuit uses just 3 transistors to perform the demultiplexing. Two of these transistors, plus two diodes extract the 19kHz pilot tone on stereo transmissions, and generate a local 38kHz signal. The technique used relies on an L/C resonant circuit at 19kHz followed by full-wave rectification to double the frequency. A single transistor acts as a demodulator and matrix decoder. The design is clever in that it uses very few components, however it was found that the stereo decoding performance is not good. In particular, the 38kHz signal has a considerable amount of jitter present on it - it can be modulated by signals present in the programme material. This causes annoying, easily heard distortions in the stereo output.
A
replacement circuit
for the decoder was designed and built using the National Semiconductor
LM1800 integrated circuit. It was possible to build this on Eurocard matrix
board and fit it inside the sheilded box of the original decoder. In addition,
the "stereo-LED" was arranged to be visible through an existing
hole in the box (or this could be mounted in the dial scale if preferred).
The edge connector pins from the original Leak circuit board were removed
and used on the new circuit board so that it plugged into place in the same
manner as the original board.
The circuit diagram follows that recommended by National, with some additional components to provide power using the 6.3V filament supply. The decoder circuit requires one inductor which forms part of a tuned circuit that is resonant at 19kHz. The inductor used in the new circuit was removed from the circuit board of the original Leak circuit.
The time constant of the phase-locked-loop was made considerably larger than that recommended by National. This reduces the jitter present in the 38kHz signal generated by the chip, but tends to make the time taken to lock on to a stereo sub-carrier signal longer. The two 47uF back-to-back capacitors in the circuit can be reduced in size if a shorter time constant is required.
Addendum
1. LM1800
The Natuional Semiconductor IC used above appears to now be hard to obtain.
Motorola also make a suitable IC, but it would be necessary to use their
application notes to construct a suitable decoder.
2. "Studio 12" Stereo Decoder:
The "Studio 12" is another alternative for fitting a stereo decoder to a Leak Troughline. The decoder is availble in kit form - either as parts, or already assembled on a printed circuit board. The assembled board costs 18.50 UKsterling. Alternatively, the unit can be bought as a stand-alone, self power decoder, in a box, for 56.50 UKsterling. A supplier of this decoder in the UK can be contatcted at phone: +44 (0) 1736 798 393. The Studio 12 decoder is not available from this web site. (This information was updated in Oct 1998.)
