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~ LEAK Trough Line VHF FM Tuner ~


The LEAK Trough Line Tuner was advertised as the answer to drift in VHF receivers ~ The component used to control the local oscillator and thus the tuned frequency was originally a square section open sided transmission line in the form of 'a trough' where the centre conductor could be tapped for coupling and or tuning

H J Leaks first Trough Line was a simple design which used a 2 section variable tuning capacitor so had only one stage of RF selectivity ~ The troughlines in all models are short compared to 1⁄4 wavelength of the lowest oscillator frequency 88MHz + 12.5MHz intermediate frequency and so appear as an inductor at the open end

The trough line inductor in the Mk2 and later versions is a round tube transmission line tapped at two points to feed the oscillator ~ Not being a true trough line like that in the first model it is not easily adjusted either at manufacture or later during repair where the high frequency performance of the local oscillator may be improved

If you download the advert link above you will see that the drift is quoted as 15kHz from switch on without AFC but this is optimistic ~ The drift with AFC on is quoted as 3kHz or 30ppm when referred to 100MHz ~ Mid band is 98MHz but it is unlikely that an original British market LEAK TL ever actually worked well beyond 98MHz

If the local oscillator were tuned with a variable inductor L and a fixed capacitor C of a particular temperature coefficient as in the QUAD FM valve tuners and some others ~ the positive temperature coefficient of the L could be be cancelled with a negative temperature coefficient C and the drift without AFC could be very low indeed and the tuned circuits could also be so much smaller than the trough line or other circuits using variable capacitors

The reason so much emphasis was placed on temperature drift is because the temperature variations inside most small valve receivers like the LEAK Trough Line is very high due to the number of valves used in a confined space ~ Inductors tend to increase in value as temperature rises and the length of the winding wire increases

The Trough Line is tuned with an air spaced variable capacitor so any inductance change with temperature can only partially be offset with fixed capacitor (C11) temperature coefficients ~ The troughline element does however have a reasonably low change with temperature and the results are acceptable when tuned and AFC switched on

With the Local–Distant switch set correctly for accurate tuning indication the AFC should hold the station from switch on as it warms up ~ The only problem is that over the years the AFC switch and the Local–Distant switch have probably stopped working correctly and in the case of the AFC switch this may not be obvious

The Local–Distant switch and the AFC switch are pop riveted to the front panel and although the switches can be drilled out they can often be repaired without having to remove the body from the chassis

It is very easy to unsolder the wires to the switches and then remove the back of each switch by levering up 2 tabs on one side only ~ you can then remove the sliding contacts for cleaning ~ you may have to loosen the "flywheel" and move it away from the Local– Distant switch as shown

If you decided to remove the back of an intermittent switch you should see the problem ~ The silver plated contacts have tarnish and are very black

The copper strips on the rubber block (be careful with these) where fitted may also be dirty and most likely bent back where users have found that pushing on the switch sometimes works but in the long run it only becomes worse

The contacting surfaces are best cleaned with a fibreglass pencil if you can find one nowadays ~ if not use very fine emery paper ~ straighten the copper strips and clean and tin the solder tags before reassembling as this will help prevent damage to the rubber block as the connections can be quickly soldered back with minimum heating ~ You may also want to remove and clean around the switch button while it's all apart
To ensure that it works for many years the Local–Distant switch contacts can be doubled up as shown ~ Reassemble the back to the switch body and bend the tabs down with a light tap of a metal "punch" to make sure they hold tight ~ You could also replace the switches ~ fitting new parts like those from Rapid electronics using small head countersunk screws with the heads covered by tape to prevent scraping the front of the perspex where fitted on the MkII

The AFC switch contacts cannot be doubled up and sometimes one end may have become damaged from arcing ~ if the contacts do not clean well then try reverse the parts to use the best connecting bits ~ MkII cream coloured switches may only have the active contacts fitted

LEAK used small brown 0.25W resistors at one time and these almost always need to be changed ~ Later they went back to ERIE ceramic cased 0.5W carbon resistors for the Trough Line Stereo and Trough Line 3
You may find whatever resistors were originally fitted that you have to change some because they will be out of tolerance ~ It is a good idea to change the resistors R10 R14 and C24 and check R13 if the tuning does not 'hold' as these often drift high in value when the chassis warms up

Note that R9 is fitted close to C11 and the orange ceramic variable C8 as pictured on the right which is most likely to "pre-heat" C11 to improve the temperature stability during warm-up so take care when deciding to change R9 ~ C11 should not require changing

Pictured below C33 has been changed for a 0.1µF mustard polycap and R22 for a metal film 10MΩ and R24 on the EM84 valve base not shown to 680kΩ to get the magic eye tuning indicator to work correctly ~ If you intend to align your Trough Line make sure the tuning indicator works correctly as it will be useful during the process
Some of the RF grounds are made to tags held in place by the valve base rivets and over the years these can become loose ~ With the old resistors and some capacitors removed it is easy to solder these tags to the chassis with a very hot iron because the chassis is tinned and solders well

Note I have also changed some earth connections across the valve bases so that a single wire is now grounded at both ends ~ See green areas on the left ~ And finally before you set about tuning make sure the coil can screws are tight but do not overtighten or the plastic may crack

It is very unlikely that you will need to change any of the original Ceramic or Mica capacitors or any components in the IF transformer cans T2 and T3 ~ Most of the paper and electrolytic capacitors may need replacing and this is best done with modern "poly cap" replacements not NOS PIO or vintage electrolytics

Distorted audio and tuning changing when AFC is switched may be due to failure of R28 R23 or the OA79 diodes inside the can of T4 as shown ~ The d.c. voltage at the MPX output should swing positive and negative equal amounts as you tune through a station ~ Here damaged OA79 diodes have been replaced but the resistors measured good when heated with a hair dryer with the old diodes removed

If you really need to align your troughline the pictures below show the positions of the RF coils and intermediate frequency amplifier IF coils L5 T2 T3 and FM detector or 'discriminator' T4 ~ There are also links below to marked up pictures and schematic that may be useful ~ but as the footnote at the bottom of the LEAK Trough Line Stereo 'manual' states :—

On no account should the pre–set trimmer capacitor C8 or the pre–set tuning slugs be tampered with or adjusted in any way.

It is not possible for an FM tuner of this calibre to be accurately aligned without using a double beam oscilloscope, FM signal generator complete with accurate 'marker' oscillator. a d.c. valve voltmeter and a multiplex generator.

These requirements should be emphasised to a potential service engineer, and if all the above equipment is not available, work should not be started.

Other manufacturers were not so protective and supplied good information for service engineers ~ LEAK did provide limited information for alignment of the Trough Line but it was full of mistakes and made many assumptions some of which would have been known to engineers back in day but not obvious nowadays

Click on the images for the bigger picture

or for

picture of the chassis

and alignment schematic

Beware the cores of the RF and IF coils may be stiff to turn where the rubber bands have hardened and may break and lock in place

Use the correct plastic or ceramic tool depending on the type of core fitted which may have a flat slot as seen above or a hollow hexagon used on later models as pictured here

Before attempting to turn the cores put a small amount of PTFE oil or WD40 or knitting machine silicone spray down the thread of the coils as this should free the rubber band which may still do its job holding the core in place while allowing adjustment

If the rubber is bad you can remove all the cores and wrap them with PTFE tape as shown ~ how much tape is a matter of judgment but when you have it right the core will turn easily but not freely allowing fine adjustment

A broken core often locks in place and even the correct tool will not move it ~ Applying PTFE oil etc. after the event should free the rubber band and it may be possible to remove the core by turning it from the other end after removing the 2nd core where fitted

It may be possible to still use the core by turning it around and using PTFE tape which centres the core better and prevents the sharp broken edge from binding ~ Cores with an end missing should work as well as full ones and if you can ~ use the original ~ Hexagonal cores can be superglued back together and still work okay


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