HOW TO ADJUST THE RECEIVERS
To correctly adjust the RX17, RX18, RX19, RX21 and RX22 FM receivers, we
recommend the following method which requires only an oscilloscope capable
of working with direct current.
With the transmitter off :
1. For the RX19 or the RX21, connect the scope to the test point Ld of the 145170. Turn the bobbin core of L3 to have negative impulses as small as possible. The resting line of the oscillogram being at the high level. The synthesis is then locked.
2. Set the scope on "Direct Input", 0.5 V/div, connect it to point BF, bring
the oscillogram, showing FM noise, to its maximum level and adjust
the symmetry of the shape. The vertical position should be adjusted to have
this oscillogram correctly centered in the middle of the reference axis.
--> do not modify this adjustment while going to the next step.
--> the adjustment of the bobbin of the FM demodulator (L4 in general) will be considered as final, if the receiver is in its casing.
3. Power up the transmitter, set on the frequency
of the receiver (72250 or 41100, at the beginning for RX19 and RX21). Replace
the antenna by a 12V/0.1 bulb returning to ground. The signal should
appear. It will undoubtedly have an insufficient force and will probably
be higher or lower than the center line of the screen. It is with the adjustment
of the oscillator that we will have to correct that. Either by the bobbin
L3 of a crystal-driven Rx (RX17, RX18), (L2 in the case of the RX22). Or
by the adjustment of 10245 (or 11155) of the RX19 and RX21.
Check by powering down and up again. The BF signal must be placed exactly on the same vertical level as the oscillogram on the screen of the oscilloscope, the noise being always at its maximum amplitude while NOT emitting.
Do not cheat by trying to fine tune the amplitude of the BF signal by changing the previously set L4 (L3 of the RX22). It would lead to possible operating problems: erratic and not easily foreseeable operation in certain configurations of flight.
4. Move the transmitter with its bulb away to have a becoming noisy signal. Then adjust the HF input bobbins to reduce this noise as far as possible.
5. Finish by checking the action of the AGC. Without a signal, the
tension should be about 4V on J310. This tension falls to 0 if the transmitter
Do not forget that these HF adjustments can only be regarded as final if the Rx is in its casing. Permanently set the bobbin cores with candle wax.
Let us point out the need for having the appropriate tool for adjusting the bobbin cores which are very fragile. Never screw them down completely.