During the life of an FM tuner it can become de-tuned or out of alignment
due to components such as capacitors drifting in value.The procedure for
aligning an FM tuner requires:
- skilled knowledge of UHF circuits and IF-strip alignment
- understanding of the Foster-Seeley discriminator
- some high-powered test gear: with a UFH sweep generator and a good
oscilloscope being mandatory
(see also note below)
In general, alignment of an FM tuner of the calibre of the Leak Trough-Line
should not be undertaken by the keen amateur. However, I get repeated requests
for any such information, so presented below are some notes I obtained regarding
the alignment of the Trough-Line. The procedure presented has not been verified
as to its validity (it is not from official Leak publications). Furthemore,
to attempt such an alignment, the person undertaking it should have sufficient
knowledge not to actually require the notes below!
However - good luck!
NB: These notes have appeared from a number of sources. The most recent
copy appeared via Terry Bateman of the UK. His source stated that these
notes were those used in the factory for aligment of the tuner, but this
has not been confirmed.
Aligning a Troughline Tuner with no
It is possible to perform an alignment of a Troughline tuner with just
the following items of equipment:
- A second, operational Troughline (in addition to the one to
- An audio oscillator.
- An audio CRO using X & Y inputs.
Detailed notes on the procedure will not be given here (but I may add
these details later). The outline procedure is as follows:
- Remove the AFC DC-control voltage from the AFC control triode in the
second Troughline (i.e. the one not being aligned). This means disconnecting
L1 from the AFC control switch (in the circuit diagram).
- Connect the audio oscillator output to the AFC control triode input
grid at the end of L1 where it was previously connected. The oscillator
in the "second Troughline" will now act as an RF signal generator,
modulated at a frequency determoned by the audio oscillator. Choose a low
frequency between 10 and 20 Hz (as this will allow for use of the grid-leak
bias voltages to "see" the shape of the tuning alignment in the
"Troughline to be aligned" ).
- Drape a single wire connected to the the antenna input of the "Troughline
to be aligned" across or near the circuit of the RF oscillator in
the "second Troughline".
- From memory, the Troughline oscillator runs 12.5MHz on the "high-side"
of the tuned frequency. So, to put the "second Troughline's"
oscillator within the 88 to 108Mhz band, you will need to set the dial
of the "second Troughline" to a frequency below 96MHz. If set
to 88MHz, the RF oscillator will run at 100.5MHz., so search for a strong
signal with the "Troughline to be aligned" near 100MHz.
- Connect the audio oscillator output to the X-input of the CRO and adjust
the CRO for appropriate deflection.
- You will need to determine the amount of audio drive voltage into the
AFC control valve by inpecting the CRO traces for full excursion.
- The Y-input of the CRO is then connected to various test-points in
the "Troughline to be aligned". In particular
- Pin 7, V3A will display the first-RF alignment, by adjusting L5
- The junction of R19 & C29 will display the first-IF alignment,
by adjusting T2
- The junction of R25 & C36 will display the second-IF alignment,
by adjusting T3
- The junction of R30 & C47 will display the discrimainator alignment,
by adjusting T4
Again - such an aligment requires some backgound knowledge! So I apologise
for the sketchy nature of these notes.
H. J. LEAK & CO. LTD.
Designers and Manufacturers of Specialised
Electronic Instruments for the Communications Industries
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