and the back resistance should r e a d approximately 3 megohms or more. To get a true reading of the forward resistance an ohmmeter equipped with approximately a 100 volt battery should be used. This arrangement will give a low forward resistance reading and essentially the same back resistance, i f the unit is in good condition.
4 -4
ADJUSTING THE OSCILLOSCOPE VERTICAL AND HORIZONTAL POSITIONING The trace on the front panel oscilloscope is adjusted to center by horizontal positioning control R32 and vertical positioning control R34 (see Figure 18.)
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4 -5
CALIBRATING THE MAIN TUNING DIAL Following replacement of r - f oscillator tube V 1 and V2, the high frequency end of the main tuning dial may be slightly out of calibration and the oscillator circuit may be capacitively out of balance. To recalibrate the tuning dial and bring the circuit back into balance, proceed as follows: a. Using a 20,000 ohm/volt d-c meter measure the voltage obtained at the MIXER OUTPUT jack. This voltage should range from approximately 1.75 vdc at 100 megacycles to 2.2 vdc at 200 megacycles. If this voltage i s too low, t r i m m e r capacitors C3 and C28 on the top of the r-f oscillator housing must be adjusted to peak this voltage. Front panel patch cords must be in position for this measurement. Connect the front panel FREQ. METER jack to the 525B input to measure the frequency of the 540A oscillator. Check the total frequency range of the oscillator: should be 1 0 0 to 220 megacycles. Set the main tuning dial to 200 and note frequency indication on the Counter. If the frequency i s not 200 megacycles, readjust C3 and C28 to bring frequency to 200 megacycles, making sure that balance of the two t r i m m e r capacitors is such that the d-c voltage r e a d on the voltmeter is peaked for any particular frequency setting. Check major calibration on frequency dial and refine the above adjustments for best overall accuracy.
