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When I bought my first Trough Line 3 on Ebay a few weeks ago I did a little googling and found
this article on replacing the stereo decoder in the stereo version and
this one on an external decoder. It struck me that
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converting a mono tuner to stereo might be a fun project that could be achieved fairly cheaply with salvaged components. All I needed was a tuner with a stereo decoder, which I had. Fortunately for the Sony, before I ripped it apart an older manually tuned Chinon receiver came up on Ebay locally, so I grabbed it for a fiver. Here it is accepting the multiplex output from the Trough Line.
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Being a fairly ancient receiver, mid 70s I guess, the stereo demultiplexer chip was the only IC in the box, so fairly easy to spot, a mix of tracing the wiring, from mono/stereo switch, stereo indicator lamp (N.B. lamp, not LED), voltage checks and poking the pins with the CRT revealed what did what and the fact that it used a single 12V supply. I reckoned the easiest way to get it into the Trough Line would be to cut out what I needed from the circuit board using a hacksaw. So that's what I did! Here it is with a voltage doubler power supply taking 6.3V a.c. from the Trough Line heater supply.
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So far, so good. Now to try and fit it in the Trough Line. The obvious
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thing to do is to put it where the decoder is fitted in the stereo version. Wow, it fits! But it's a tight fit. Looking at the board more closely reveals that the tallest component is the smoothing cap for the regulated power supply. A modern cap of the same value taken from an old PC motherboard is half the size!
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That done the installation can continue. I decided that screening probably isn't necessary. Perhaps I'll be proved wrong on this later. The voltage doubler goes underneath. Done!
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