— —
18
Section IV
CALIBRATION
The upgrade to PWM processing has greatly simplified calibration of the
DAVID-II
when compared with its predecessor, the Model 715
DAVID
.
The number of trimmer adjustments under the top cover has been
reduced from
nineteen
(!) to
only eight.
“Routine” calibration of the
DAVID-II
is never required. The PWM
processing and digital-synthesis circuitry is simple and stable. The only
justification for confirming any of the internal adjustments is out-of-
specification operation, and only after all other possible faults have been
considered and eliminated. It’s worth a call to the factory to discuss any
anomalies before charging-ahead and possibly making things worse.
Equipment
Required
•
Dual-Trace Oscilloscope —
5mV sensitivity, 20Mhz bandwidth,
with two matched 10:1 probes.
•
Audio Generator —
10Hz–1MHz, +20dBm output capability.
•
Digital Multimeter
•
Frequency Counter —
capable of accurate frequency
measurement from 1kHz to 2Mhz.
•
“Precision” FM-Stereo Demodulator —
station Mod-Monitor
with input for composite baseband signal.
•
Spectrum Analyzer
(optional*)
—
must have good resolution in
the 100Hz to 100kHz (FM composite baseband) display range.
* The Spectrum Analyzer is
not
required if the “Precision” FM-
Stereo Demodulator is capable of resolving crosstalk, stereo
separation and 38kHz “residual” measurements to a figure of
70dB or better.
Power Supply
Check
1. Apply power to the
DAVID-II
.
2. Check that the positive and negative 9-volt regulated supplies are
between 8.5V and 9.0V. You can check these on pin 8 (positive) and
pin 4 (negative) of any 8-pin IC.
Clock Set
3. Use one of the low-capacitance ‘scope probes to connect to the input
of the frequency counter. Check the clock frequency on pin 11 of
IC8.
4. Adjust C16, the trimmer capacitor to the right of the crystal, for
exactly 1,216,000Hz.
