By Sam Wilson, CET
Figure A
1. Which of the following would you expect to find in a digital storage oscilloscope?
A. A PIN diode
B. A D/A converter
C. A special CRT
D. A video enhancement circuit
2. The Z-axis input terminal of any oscilloscope-including the digital storage oscilloscope controls the
A. angular movement of the CRT beam.
B. vertical deflection It beam.
C. horizontal deflection of the beam
D. None these choices is correct.
3. What type of diode is used to convert infrared remote signals into electrical impulses?
A. Tunnel diode
B. Hot carrier diode
C. PIN diode
D. Four-layer diode
4. The gap in a VCR head is about
A. 14 thousandths of an inch wide.
B. 1.4 thousandths of an inch wide.
C. 25 millionths of an inch wide.
D. 217 millionths of an inch wide.
5. In a VHS VCR the adjacent tracks overlap.
To prevent signals from being affected by overlapping tracks,
A. a different index of modulation is used for each head,
B. the azimuth is different for each head.
C. the elevation is different for each head.
D. the direction of motion for each head, with respect to the tape, is reversed for each adjacent track.
6. The most common problem involving circuit board rework or repair is
A. removing and replacing components.
B. removing fungus protection.
C. reconnecting cracked foil.
D. re-applying fungus protection.
7. The resistor in Figure A is called a
A. swamping resistor
B. equalizing resistor.
C. temperature compensating resistor.
D. bleeder resistor.
8. Should one of the heads in a VCR system become inoperative, the resulting picture will he a combination of ___ and ___.
9. Is the following statement correct? The recommended way to clean the video heads in a VCR is to use a lintless swab, a recommended solvent, and a gentle up and down motion.
A. Correct
B. Not correct
10. An audio equalizer is really just
A. a variable-frequency active filter.
B. a bass boost tone control.
C. an op-amp with a linear roll off.
D. an elaborate set of tone controls.
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Answers to the quiz
1. R. Unlike the earlier storage oscilloscope, the digital storage oscilloscope does not require a special CRT. The waveform to be: stored is converted into digits by an analog/digital converter, and those digits are stored in a memory. To retrieve the waveform, a digital/analog converter is used.
(For more information, see "The Oscilloscope: The Eyes of the Technician" by Conrad Persson, in the April 1987 issue.)
2. D. A signal at the Z-axis input terminal controls the beam intensity.
3. C. Does anyone remember when diodes were just used for detecting and rectifying? Tunnel diodes are used as fast switches and oscillators. Hot carrier diodes have a low forward breakover. Infrared diodes are used to generate infrared signals. The PIN (positive intrinsic negative) diodes convert the infrared signals into electric impulses. (See "What Do You Know About Electronics?" in the April 1987 issue.)
4. C. By comparison, the thickness of a human hair is about 250 times the width of a VHS VCR head gap. (See the April 1987 Video Corner.)
5. B. One video head is installed with an azimuth of + 6°, and the other is installed with an azimuth of-6°. This ensures that alternate tracks are recorded at an angle of12° to each other, so little signal is generated by overlapping tracks.
6. A. A vacuum desoldering system reduces this problem considerably. (See "Desoldering Components" by James K. Bausell in the May 1987 issue.)
7. A. The swamping resistor lowers the Q of the tuned circuit so the bandwidth is greater. See "Test Your Electronics Knowledge" in the May 1987 issue.)
8. Bad image and lots of snow.
(See "Maintenance and Cleaning of VCRs" by Victor Meeldijk in the May 1987 issue.)
9. Absolutely NO! The swab and solvent parts of the statement are OK, but the up and down motion may damage. fragile heads!
10. D. The equalizer divides the audio spectrum into a large number of hands-usually ten or so.
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