Table of Contents
Advertisement
receiver is frequently quite critical.
For accurate calibration of home built receivers or eq iip-
ment, pi *ce?d as follows.
Make a rough calibration with the signal generator.
Then, with a
receiver, tu le in W VV (Bureau of Standards) at 2.5, 5, or 10 me. Set the signal generator to a
suitable sub-harmo lie, such as 500 or 1000 kc, and adjust the generator for zero beat.
Now
lurmonics of the sh nal generator occur every 500 kc or 1 megacycle, and these harmonics may
be used to give acci rate calibrations at points 500 or 1000 kc apart, such as 2500 kc, 3000 kc ,
3500kc, 4000 kc, etc. These known frequency points can be marked on the dial of the equipment
being calibrated. T le object of the rough calibration is merely to furnish a means of identifying
for example, the 3(00 kc point from the 2500 kc or 3500 kc points.
For calibration of higher
frequency equipmei t, a choice of higher sub-harmonic will reduce the confusion between the
n ultitude of harmoi ics and will also insure adequate signal strength.
When checking the cali¬
bration accuracy of the Heathkit Signal Generator, the most convenient standards of comparison
of sufficient accurac / are broadcast stations of known frequency. Crystal oscillators of standard
frequency when zero beat against WWV, are also convenient to use if available. The use of re-
c :iver dial calibrations is frequently not of sufficient accuracy to warrant consideration.
Output Voltage: Tha RF signal strength going into the output control depends upon the strength
oi oscillation of the 12.AU7 oscillator.
In all variable frequency oscillators the amplitude will
vary with the tunini condenser setting.
With careful design the variation may be minimized.
In theHeathkit Signal Generator, the variation is kept down to a ratio of about 2 1/2 to 1 on each
band except band E, where the L/C (inductance to capacitance) ratio becomes sufficiently un¬
favorable that oscillation may drop off rapidly in strength when the condenser is near maxi¬
mum capacity.
Hov ever, even on this band, the output of the SG-8 is in excess of 100,000 mi¬
crovolts which is mare than sufficient for the average application in which this generator, will
be used.
IN CASE OF DIFFICULTY
1.
Recheck the en ire wiring.
Follow each lead and color it on the pictorial with a colored
pencil.
If possible, have a friend recheck the wiring for you.
Most cases of difficulty re¬
sult from wrong or reversed connections. In this unit, there is also the possibility of shorts
to the chassis o xurringon some of the bare wire connections from the coils. These wires
should be carefi lly spaced away from the chassis when the instrument is assembled.
2.
Check the test load with an ohmmeter to be sure there are no shorts between the inner and
outer conductor caused by overheating during the soldering process.
3.
If the wiring is found to be correct and the signal generator still refuses to work, try chang¬
ing tubes.
It is possible that one of the tubes is defective.
4.
Check the voltages.
A voltage chart is included showing the normal voltage to be expected
at the pins of the tube sockets.
These voltages; were measured with an 11 megohm input
vacuum tube voltmeter.
A normal variation of ±15% is to be expected.
With regular volt¬
meters, readings may be very much lower. All voltages are DC unless otherwise indicated .
5.
If only one band on the signal generator is inoperative, it is very likely that the coil associ¬
ated with that band has become damaged. An ohrr meter connected between the terminals of
the coil should show continuity.
If it does not, one section of the winding is open.
VOLTAGE CHART
TUBE
Pin 1
Pin 2
Pin 3
Pin 4
Pin 5
Pin 6
Pin 7
Pin 8
Pin 9
6C4
65
NC
6.3 AC
0
65
-0.5
1
12AU7
78
*- 2 to -15
3.5
6.3 AC
6 3 AC
*75-85
0
0
♦Dependent upon frequency.
NC - no connection.
BIBLIOGRAPHY
Marcus and Levy; E lements of Radio Servicing
Kiver, Milton S.; How to Understand and Use TV Test Instruments
Johnson, J. Richard; How to Use Signal and Sweep Generators
Page 16
Advertisement
Table of Contents
