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PART 2 OF THIS FEATURE IS IN OUR OCTOBER 2012 ISSUE, OUT NOW (link to PDF circuit diagram is at end of this article).





Several companies have offered an upgrade service for the legendary Leak Trough-Line FM tuner in the past, but what can be achieved by amateur enthusiasts? Neville Roberts describes his very own approach to doing it yourself...


Leak Trough-line II front

Leak Trough-line II front before upgrades


Upgrading the legendary Leak Trough-Line FM tuner was big business in the nineteen nineties, but with rumours of the demise of FM stereo broadcasting in the UK, interest dwindled in recent years. However, it now looks like good old FM has had a stay of execution and is set to continue for a good while. And with this in mind, what better time to perform a few well-judged mods? This special tuner lives on in people’s hearts and minds because of its exceptional sonics.
    The story starts way back in the nineteen fifties, when we were still listening to Medium Wave and Long Wave. Many budding audiophiles were waiting for the arrival of Frequency Modulation broadcasts, which promised greatly reduced background noise and far superior sound. The BBC, working in partnership with Standard Telephones and Cables Ltd (later STC plc), were developing this technology for the UK, and a certain Harold Joseph Leak and his British company H. J. Leak & Company Limited were one of the

first to develop their own tuner for this emerging market.


Leak Trough-line rear
Leak Trough-line II rear before upgrades


    Most FM tuners work on the superheterodyne principle and the Trough-Line is no exception. The radio frequency signal enters a mixer, along with the output of a local oscillator, in order to produce a so-called intermediate frequency (IF) signal, which in the case of the Trough-Line is 12.5MHz. Tuning the receiver involves changing the frequency of the local oscillator so for the Trough-Line to tune in Radio 3 at 90.7MHz, the local oscillator will be tuned to run at 103.2MHz. The output of the mixer will be the difference of these frequencies: 12.5MHz. Similarly, to tune in Radio 2 at 88.3MHz, the local oscillator will be tuned to run at 100.8MHz and the output of the mixer will again be 12.5MHz. This means that all further processing of the signal is conveniently done at a single frequency – the IF - thus no further tuning for different stations is required.
    One of the challenges of radio circuit design in the nineteen fifties and sixties was to design an oscillator that was stable. Many tuners of the time needed to be periodically retuned as they warmed up, otherwise they would distort. The problem was mainly caused by changes in inductor dimensions with temperature (a particular problem with valve equipment) that would lead to significant variations in their electrical properties at the 100MHz region chosen for FM broadcasts.Leak Trough-line underside

Leak Trough-line II underside before upgrades

Leak’s solution to this problem was to design a circuit that used a tapped transmission line as the main tuning element, rather than conventional wound coils or inductors. They developed a tuner that used a quarter-wavelength section of transmission line, shorted at one end, which behaved as a parallel resonant circuit of very high Q and electrical stability.  The mechanical construction   led to a device of great rigidity and electrical stability. Tuning across the frequency range was achieved with a conventional air-dielectric variable capacitor.



Leak Trough-line dusty interior
Inside dusty Trough-line II  as bought.



    The first Leak FM tuners utilised a U-shaped trough as the ‘concentric’ outer conductor for ease of production and as a result, the tuner was christened ‘The Trough-Line’. In the original model Trough-Line Mk.1 launched in 1955, the U-shaped trough had its open side placed facing downwards under the chassis. Subsequent models used a metal cylinder as the outer conductor.


Leak Trough-line after vacuuming
Inside Trough-line II after vacuuming



As with the later models, the original Trough-Line utilised a Foster-Seeley discriminator circuit and a ‘magic eye’ EM81 valve as a tuning indicator (incidentally, the EM81 and the later EM84 were often used as a level meter in early domestic tape recorders). There were two types of FM detectors that were popular at the time: the Ratio detector and the Foster-Seeley detector or discriminator. The Foster-Seeley discriminator has the disadvantage over the ratio detector of being affected by amplitude variations and therefore requires a limiter stage preceding it. It does, however, offer lower levels of distortion and that is why Leak chose to use the design.
    The Mk.1 only covered the range of 88-100 MHz, while the later models covered the full range of 88-108 MHz. A particular feature of all the Trough-Lines was that they were self-powered from the mains supply – unlike many other British tuners of the time that required HT and LT power from the power amplifier.



Chassis underside showing enlarged cutout for Mundorf HT capacitor cutout.

    In 1960, Leak launched the Trough-Line Mk.2, which sported a distinctive ‘Art Deco’ front panel made of Diakon (a form of acrylic plastic made by Lucite International Inc.) in brown and gold, rather than the gold enameled steel finish of the Mk.1. This matched their range of Varislope amplifiers. Apart from the wider tuning range, the valve complement was changed to accommodate a switchable Automatic Frequency Control (AFC) and Local/Distance sensitivity control on the front panel, as well as some changes to the design of the line to reduce weight and costs.
    At that time, no decision had been made nationally about the standard for FM stereo, so a separate output on the back panel prior to the de-emphasis circuit would allow the mono unit to be upgraded in the future to stereo by connecting an external decoder.
    In 1964 the Trough-Line Mk.2 was phased out for the Trough-Line Mk.3, which adopted a new visual style. Silver and black was now the order of the day with the passing of the Art Deco style of the Mk.2. Apart from the exterior appearance, it was identical to the Mk.2 electrically.
    Then, in 1966, the Trough-Line Stereo was launched as a result of the BBC adopting the same standard as in America, the GE-Zenith multiplex system. Some changes to the Mk.3 circuit and valve line-up were required as Leak did not want to change the Foster-Seeley discriminator, which has a limited bandwidth, and compromises were reached between increased bandwidth and lower sensitivity. This was necessary in order to feed the internal stereo decoder that, incidentally, used three AF126 germanium PNP transistors. Alas, this decoder falls short of the mark in terms of quality and is best replaced with a modern Phase Lock Loop (PLL) decoder. By 1969, many companies looked towards the new solid-state technology and Leak was no exception. The Trough-Line Stereo was phased out in favour of a new semiconductor-based design called the Stereofetic.


Leak Trough-line, new capacitor & resistor

Mundorf HT capacitor and Mills series resistor fitted.

    However, in the decades since the Trough-Line era, it became clear that no one had ever bettered the sound produced by a Trough-Line.  Consequently, a unit that has been fully serviced and upgraded with modern components is considered to be one of the finest sounding tuners ever made, and therefore highly sought-after.

As GT Audio, one of the companies who used to offer a vintage restoration service puts it, “the restorations are not cheap,” and with the rising cost of labour, it is becoming increasingly expensive. This got me thinking – I wondered what could be achieved by a DIYer with inadequate test equipment and limited FM tuner alignment experience?
    There are, once again, some real bargains to be had on eBay and I was fortunate enough to pick up a Trough-Line Mk.2 for £60.  When my prize arrived (which was described rather oxymoronically as ‘working but untested’), it was not in bad condition externally, but upon removal of the lid, it appeared to have spent most of its life stored in a vacuum cleaner dust bag!


Leak Trough-line underside
Chassis underside showing 3rd HT and bypass capacitors fitted.



    Further investigation revealed that, with my unit, part of the mains fuse was missing and the on/off switch on the volume control had failed.  Another fortuitous eBay purchase enabled the volume control to be replaced with a New Old Stock (NOS) one and the fuse holder, together with the mains lead, was also replaced. This was followed by a thorough vacuuming and an overall clean using isopropyl alcohol before powering it up to see how well it worked, if at all. Well, I’m pleased to say that it worked like a dream and the sound quality from this mono tuner was nothing short of breathtaking.
    Another thing I noted, which is likely to be an issue with any unit of this age, was that the little rubber feet had perished and weren’t supporting the tuner properly. New feet were acquired via, you guessed it, eBay – black polyurethane 20.5mm square x 13.2mm high self-adhesive feet fitted the bill perfectly! Obviously, getting the vintage tuner working is an important starting point. From there, one can determine what needs to be done, what is desirable to do and what should be left alone...

NEXT MONTH (October 2012 issue, out now)  -
What you can and cannot do at home to renovate the Trough-Line. Differences between models, and fitting a stereo decoder.

All valve (tube) equipment contain dangerous voltages and old equipment is especially dangerous because of decay. Before switching on check that the chassis is earthed using a continuity meter (Maplins sell them) and check again by using a neon screwdriver that it isn't live after switch on.
    A common practice is to wind up mains slowly using a Variac, to avoid a sudden bang and possible damage to the mains transformer, or fire from an overheating component.
    After switching off, high voltages will be maintained by the electrolytic power supply capacitors, unless they are fitted with bypass resistors of around 100k.
    Farnell and RS Components can supply parts, as can Maplins.


Click on the link for circuit diagram PDF –>       Circuit diagram PDF



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