Restoring a Leak Trough-Line Mk1 'Hi-Fi' FM Tuner Gerry O'Hara VE7GUH 2009(?)

	Leak Trough-Line Mk1.

In the UK in the early to mid-1950's, several companies brought FM tuners to the market: Leak, QUAD and even Eddystone had all introduced these items into their product lines by mid-1955, and many found their way over to the US and Canada, either by immigrants bringing their prized-possessions with them, or Hi-Fi buffs wanting to buy what they considered to be the best available. In the UK, the boom in FM tuners came with opportunities opened with the introduction of new BBC services, and the spread of the 'Hi-Fi bug'. More and more homes were acquiring radio and audio equipment that was capable of providing very good quality reproduction and there was a fledgling 'high-end' market developing for the more 'well-heeled' listener.

The 'Leak' brand of Hi-Fi equipment had launched their 'Trough-Line' (MkI) tuner in the early-1950's. The Trough-Line's main design/selling feature was a very stable oscillator circuit using a tapped transmission line formed by a 4 inch long metal 'trough' surrounding the resonant element (hence the catchy name) - stability being a concern for free-running oscillators at VHF frequencies. This unit was supplied without a case, it being meant for inclusion in a home-made console as was the vogue. A good description of the tuner appeared in a contemporary issue of the UK magazine "Hi-Fi News":

Leak have adapted for their oscillator, a variation on the quarter-wavelength line type of resonator. It has been found in communications engineering that for frequencies in the 100Mc/s region and above, it becomes increasingly difficult, for a multitude of reasons, to make the conventional coil and condenser combination operate satisfactorily; the chief among these reasons are the capacitance inherent in the coil, and the inductance in the condenser. Now, a quarter-wavelength section of transmission line, shorted at one end, behaves as a parallel resonant circuit of very high Q and electrical stability, while the mechanical construction of such a device leads almost automatically to great rigidity (and hence stability). The Q being high, tapping down will still permit adequate voltage at the output; and as the coil is a straight piece of wire, selection of optimum tapping points is obviously much facilitated. Below is a diagram of the resonator in the Leak circuit, and also the bread and butter version. It is clear that the Leak resonator is not a true quarter-wavelength section, its length being under 6 inches, but its frequency in the Trough-Line is controlled by added (variable) capacitance.

• Frequency Range: 88-100MHz (note that current FM tuners cover up to 108MHz);
• Foster-Seeley discriminator circuit;
• Magic eye tuning indicator, 2kHz accuracy;
• 10lbs weight;
• Self powered - quite a selling feature as it meant less of an installation effort unlike many other British tuners of the time, that required HT and LT power to be supplied from the audio amplifier they were being used with; and
• Tube complement (8) was as follows: EM81 (magic-eye tuning indicator), EB 91, 2 x EF80, 3 x ECF80, EZ80 (rectifier).

The MkI Trough-Line was replaced by the squatter, though similar-looking MkII in late-1958. This model had a quoted drift of 15kHz without its AFC switched on and with the AFC circuit switched on this was reduced to 3kHz drift (or approx 0.003%). The published specification for the (very similar) MkII Trough-Line includes the following information:

• Frequency Range: 88-108 MHz (note the extended upper end of the band from 100MHz to 108MHz).
• 2 microvolt sensitivity for full limiting.
• 300 Ohms or 75 Ohms aerial input.
• Cathode follower audio output.
• Multiplex output for add-on stereo decoder.
• Tube complement (7) was as follows: 2x ECF80/6BL8, ECC84/6CW7, ECC85/6AQ8, EF80/6BX6, EM84/6FG6 (magic-eye tuning indicator), EZ80 (rectifier).

The build-quality of the Leak units was generally very good, with well-thought-out point to point wiring and high-quality components used throughout, such as TCC electrolytic capacitors, Mullard tubes and McMurdo tube sockets. The Leak Mk1 circuit design was rather sophisticated for its day, featuring a cascode RF amplifier and a cathode-follower AF stage, the use of the trough-line tuning element and adjustable quieting ('squelch') to eliminate inter-station noise.

Restoring the SPARC Leak Trough-line MkI

The SPARC museum has a collection of equipment from the 'Golden-Age' of Hi-Fi, including a MkI Leak Trough-Line tuner. Gerry O'Hara was restoring an Eddystone S.820 FM tuner from his own collection in December, 2007 and it was an opportune time to also restore the Leak tuner and undertake some comparision testing. More complete articles on these restorations can be downloaded from links below.

The top of the Leak tuner chassis needed a thorough clean using a de-greaser - soapy automotive hand cleanser worked best on this chassis. Underneath, all that was necessary by way of cleaning was the careful use of a dry paintbrush and a vacuum cleaner to remove cobwebs and dust. The MkI tuner is designed for 220v to 240v, 50-60Hz power. It was decided to retain the original circuitry and components if possible, so the tuner was powered through a variac coupled to a 120v/240v step-up transformer. Following visual inspection for faulty wiring and obviously distressed components (eg. mechanical damage, baked or burned resistors), simple continuity checks on the power lead, power switch and transformer windings were undertaken. All the tubes were removed and the transformer tested by gradually increasing the primary voltage up to 240v - all secondary windings were giving correct ac voltages.

The front panel was very grubby and needed a thorough clean behind the transparent section where the tuning scale is located. Dismantling of the front panel was straightforward: removal of five retaining screws and the volume control and tuning bushing nuts and off it came. The acrylic panel had originally been affixed to the underlying metal plate with a black compound (tar?) that had lost its adhesion. Following careful cleaning of the rear of the panel with warm soapy water and the front with anti-static acrylic plastic cleaner, the acrylic panel was re-mounted onto the underlying metal plate with 'Goop' and the retaining screws and nuts re-fitted. The tuning dial needed re-stringing after this operation, though it is a simple mechanism and this only took a few minutes.

Hardened oil/grease was cleaned from the tuning shaft bearings and these were re-lubricated with suitable grease (sparing amount of molybdenum-disulphide grease). The tuning pointer guide rail was also lightly smeared with this grease.

Initial Power-up and Electrical Checks

	Stack for listening test.

All the tubes tested good apart from two ECF80's: one had a cathode-heater short, the other had a low-emission triode section. The power supply electrolytic capacitors were re-formed in-situ. Basic leakage and capacitance checks on a few of the 'Hunts' (black) plastic encapsulated paper capacitors indicated (surprisingly) that they were OK. Several resistors were also checked and they were also within tolerance. The ECF80 tubes were replaced from the SPARC stock and a short aerial was connected to the FM aerial connection and a (powered) computer speaker set was attached to the (mono phono) AF output. The tuner was powered-up on the variac over around 15 minutes, monitoring the HT voltage - it was found to work well. Some alignment checking indicated that the tuner was pretty-well aligned.

'Hi-Fi' Listening Test

Conclusion

For more reading:

• Longer article about the Leak Mk1 restoratiion (PDF).
• For comparison, restoration of an Eddystone S.820 FM tube tuner (PDF).
• Postscript to above (PDF).