— LEAK Troughline Repair and Tuning

The LEAK TroughLine Tuner ( LEAK TL) was advertised as the answer to drift in FM receivers but the component that is supposed to control the local oscillator and thus the tuned frequency drift — The trough line inductor — is in the later version a short circuit transmission line which appears as an inductor and capacitor at its open end — OK it can be considered a trough line because it is a transmission line which is tapped at several points but it is not a true trough line like that in the first model and not so readily adjustable 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 beyond 97MHz as they were never required to ) 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 much smaller than the trough line The reason so much emphasis is placed on temperature drift is because the temperature range inside most small valve receivers like the LEAK TL is very high due to the number of valves used in a confined space — The inductors and troughline physically increase in length as the temperature rises and not only does the tuned frequency therefore change but the tuning of the intermediate frequency IF stages would also change if not compensated for and tuned when the set is warm The LEAK TL is tuned with a 3 gang air spaced variable capacitor so the 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 with AFC off and then switched to AFC on — The only problem with this is that over the years the AFC switch and the LOCAL/DISTANCE (L/D) switch have probably stopped working
		The L/D switch and the AFC switch are pop riveted to the front panel and although the facia can be removed and the switches drilled out you are unlikely to find a replacement that works It is very easy to unsolder the wires to the switches and then remove the switch back by levering up 2 tabs on one side
To access the L/D switch you may have to loosen the "flywheel" and move it away from the switch as shown above
	If you decided to remove the back of an intermittant 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) 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 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 to come the LOCAL/DISTANCE switch can be doubled up as shown — Clean and tin the tabs and leave those you are going to connect to clear — Reassemble the back to the switch body and bend the tabs down with a light tap to make sure they hold tight The AFC switch cannot be doubled up and sometimes one end may have become damaged from acring — if it does not clean well then simply turn over the rubber block as you replace it


So with the switches working correctly you should next ensure all the voltages indicated on the schematic are correct — LEAK used small brown 0.25W resistors at one time and these almost always need to be changed — Later they went back to ceramic body 0.5W carbon resistors for the TroughLine 3 and Troughline Stereo (shown above) but still you may find you are changing all the resistors and capacitors apart from the tubular ceramic and mica ones . . . MORE TO COME Beware the cores of the RF and IF coils can be stiff to turn and may break and lock in place — before you attempt to turn them put a small amount of PTFE oil or WD40 down the thread of the coils and this way the rubber band locking them may free and still be useable if not . . .










		Many RF grounds are made to the 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 with a very hot iron as the chassis is tinned and solders well with a large hot iron — Also 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 can crack

"Up on the hill — they think I'm ok — or so they say"

— LEAK Troughline Repair and Tuning —