WhitakerAudio AM/FM Stereo Tuner 2012, Сборка и руководство пользователя, Страница: 118 / 166

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tuner, some response will usually be heard even with an AM signal, due to certain frequency
modulation effects that are usually present. The use of such an instrument is highly
recommended, if possible, but the test probe disturbance tests will often suffice.
In order for the receiver to work, it is necessary for the local oscillator (LO) to be functioning.
Due to the relatively low operating frequency of the AM tuner, LO startup problems are typically
rare. For the FM tuner, however, LO issues are sometimes observed. The triode section of V52
functions as the oscillator for the FM tuner. Determination of oscillator operation can be
identified by measuring the voltage at the plate (pin 1) of V52 (6U8). A voltage in the range of 90
to 110 V dc is typical for proper operation. Values higher than this are an indication of problems.
Measurement of the grid (pin 9) of V52 is unreliable as an indication of proper operation due
to the detuning effect of the test lead. The value given in Table 6.3 (–0.3 V dc) will typically be
measured with a test lead of 3 ft. or more. The detuning effect of the lead may cause the oscillator
to stop, or at least to operate well off the desired frequency. Were it possible to measure pin 9
without capacitive and inductive loading, the voltage would be in the range of –3 to –6 V dc.
An effective way of checking for proper operation of the FM LO is to use a frequency counter
capable of measuring at least 110 MHz. Most frequency counters in this range have sufficient
sensitivity to make measurements with a simple pickup coil. In the case of the FM receiver, a
three-turn coil of #22 wire will usually suffice. By placing the coil near the C55 tuning capacitor
assembly (in particular L52), measurements can be taken on the oscillator. With this setup, it is
possible to adjust the operating frequency of the oscillator using the frequency counter. Adjust
C57 to read 10.7 MHz higher than the front panel tuning dial. For example, with a setting of 88
MHz, C57 should be adjusted so the counter reads 98.7 MHz.
The variable tuning function of the oscillator can be checked with the frequency counter and
pickup coil by taking measurements while tuning up the band. When operating properly, the
measured oscillator frequency will vary from a low of about 98.7 MHz to a high of about 118.7
MHz.
In extreme cases, it is possible the oscillator will function correctly at the low end of the band,
but stop working at higher frequencies. It is then necessary to move the tuner frequency down to
the low end of the band to restart the oscillator. This problem can usually be solved by
repositioning the tap on L52, the oscillator coil. Coil L52 consists of three turns of heavy gauge
wire. The tap is typically made at about one turn from the ground side. If the oscillator stops at