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~ LEAK TroughLine VHF FM Tuner ~


The LEAK TroughLine Tuner (LEAK TL) was advertised as the answer to drift in VHF receivers ~ The component that was 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 Troughline was a simple design which used a 2 section variable tuning capacitor so had only 1 stage of RF selectivity ~ The trough lines in all models are short compared to 1⁄4 wavelength of the lowest oscillator frequency 88MHz + 12.5MHz and so appears as an inductor at its open end

The trough line inductor is ~ in the Mk2 and later versions ~ 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 during repair where the high frequency performance of the local oscillator could be improved

If you download the 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 as some like to refer it 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 with variable capacitors

The reason so much emphasis is placed on temperature drift is because the temperature variation inside most small valve receivers like the LEAK TL 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 increases

The LEAK TL is tuned with an air spaced variable capacitor so any inductance change with temperature can only partially be offset with fixed capacitor temperature coefficients ~ The troughline element does however have a reasonably low change with temperature and the results are acceptable when tuned and AFC used

With the Local/Distance switch set correctly and the AFC on the tuner 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/Distance switch have probably stopped working correctly and in the case of the AFC switch this may not be obvious

The L/D 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 doing so

It is very easy to unsolder the wires to the switches and then remove the back 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 switch as shown

If you decided to remove the back of an intermittent switch you will 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 are also 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 fiberglass pencil if you can find one ~ 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 LOC/DIST 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 perpex 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 TroughLine Stereo and Troughline 3
You may find that whatever resistors were originally fitted that you have to change some as they are out of tolerence

It is a good idea to change the resistors R10 R14 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 ceramic variable C8 as pictured on the left ~ This is most likely to "pre-heat" these components to improve the temperature stability so take care when deciding to change R9 ~ C11 should not require changing

Many of the RF grounds are made to tags held in place by the valve base rivets and over the years these can become lose ~ With the old resistors (and some capacitors) removed it is easy to solder these tags to the chassis with a very hot iron as the chassis is tinned and solders well

Note I have changed some earth connections across the valve bases so that a single wire is grounded at both ends ~ See green areas on the left ~ And finally before you set about tuning make sure the IF 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 cans T2 and T3

Paper and electrolytic capacitors may need replacing and this is best done with modern "poly cap" replacements not NOS PIO or vintage electrolytics

Distortion and the tuning changing when AFC is switched could be due to failure of R28 R23 or the OA79 diodes which are inside the can of T4 as shown ~ Here the damaged OA79 diodes have been replaced but the resistors measured good when heated with a hair dryer with the old diodes removed

Click on the images for larger images showing component location

Beware the cores of the RF and IF coils can be stiff to turn and may break and lock in place ~ you should use the correct plastic or ceramic tool depending on the type of core fitted ~ before you attempt to turn them put a small amount of PTFE oil or WD40 down the thread of the coils as this should free the rubber band which may still do its joc holding the core in place but allowing adjustment if not . . .

TO BE CONTINUED OCT 2014

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