HEWLETT PACKARD (HP) 240A
Service Manual

Section III Paragraphs 3-18 t o 3-35 3-18. Set the SWEEP WIDTH control about half way t o maximum, turn the CRYSTAL MARKER switch on, and advance the CRYSTAL MARKER AMPLITUDE. Birdie-type m a r k e r s should appear on the oscilloscope. The number of m a r k e r s depends on the m a r k e r separation and the SWEEP WIDTH setting. 3-19. TEST SIGNAL AMPLIFIER.
TYPE 240-A

Model 240A

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R F OUTPUT-

1

1 5 0 1 - 8 CABLE

1

OSClLLoSCOPE
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1

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IIF

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3-20. An audio frequency signal of approximately 1/2 volt applied t o the TEST SIGNAL IN jack should appear at the COMPOSITE SIGNAL OUT jack as approximately 4 volts with the TEST SIGNALAMPLITUDE control turned full on. 3-21. AMPLITUDE MODULATION.

Figure 3-3.

Test Setup Using 501B Output Cable

3-22. Connect a diode detector circuit a c r o s s the terminals of the 501B Output Cable and connect the rectified output to the vertical input of the oscilloscope. Turn the R F LEVEL controls for full output, the function selector knob to CW, and the AM knob to ON. Adjust the internal linear sweep of the oscilloscope f o r about 1 0 0 cps. The rectified 1000 cps amplitude modulation should appear on the oscilloscope. 3-23. R F BLANKING AND FLATNESS.

Figure 3-4. Test Setup Using 501B Output Cable and 509B Output Cable

3-29.

R F OUTPUT.

Use 3-24. the diode detector and oscilloscope connections of paragraph 3-22 with the oscilloscope set t o 20 cps. Turn the CW-SWEEP sweep rate selector knob t o SWEEP and turn the SWEEP WIDTH control up about half way.
3-25. The oscilloscope waveform is shown in Fig. 3-2. The top line (demodulated R F ) and bottom line (base line) should be parallel and flat.

3-30. The R F output is obtained from the terminals of the 501B cable, which presents a source impedance of 25 ohms. The open-circuit voltage, acting in series with 25 ohms, is indicated by the combination of the meter and the RF LEVEL FULL SCALE dial. An additional 25-ohm series r e s i s t o r must be placed between t h e high terminal of the 501B cable and the input to a 50-ohm receiver. 3-31. 3-32. TEST SIGNAL IN. The detected output voltage of the circuit under

test is fed to t h e TEST SIGNAL IN jack.
3-33. SWEEP OUT

3-34. The SWEEP OUT and GND terminals are connected to the horizontal input of the oscilloscope. 3-35. The SWEEP WIDTH control determines the frequency deviation about the indicated center frequency. (See Sweep Range in Table 1-1.) NOTE: The system used for deviating an R F signal about a center frequency depends upon changing magnetic flux acting upon ferrite c o r e s in the R F oscillator coils. The center frequency, as indicated by the frequency dial, is determined by the current in a coil which is magnetically coupled to the ferrite core. Residual magnetism in the f e r r i t e and iron c o r e s can cause the center frequency calibration to be slightly off. Rotating the SWEEP WIDTH control fully clockwise and then counter-clockwise will effect the degaussing of the c o r e s and return the instrument t o proper calibration.

Figure 3-2.

R F Sweep Flatness

3-26. U S I N G THE SWEEP SIGNAL
GENERATOR.
3-27. For use as a conventional CW o r Amplitude Modulated Signal Generator, the only connection required is the 501B R F Output Cable. 3-28. Figures 3-3 and 3-4 show typical arrangements of t e s t equipment for measuring the pass-band of a radio frequency amplifier having its own detector. 3 -4