Philharmonic Radio Corp. 448C
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|Manufacturer:||Philharmonic Radio Corp.|
A - M ALIGNMENT
1. TUNING CAPACITOR fully meshed. Adjust dial pointer to reference dot at the low-frequency end of the scale.
2. VOLUME CONTROL maximum clockwise.
3. TONE CONTROL maximum clockwise.
4. SELECTOR SWITCH on AM (left-hand position).
5. SIGNAL GENERATOR - Use standard A - M Signal Generator with approximately 30 per cent modulation at 400 cycles.
6. SIGNAL - GENERATOR COUPLING.- Low side grounded to chassis. High side connected through 0.01 mfd capacitor to coupling point.
7. LOOP COUPLING.- For loop coupling, use a Standard Signal Injection Loop according to specifications. If a standard loop is not available, make a loop with 5 or 6 turns of insulated wire, close-wound on a 3" to 4" diam form. Place the loop coaxially with and at least 10 inches back of the receiver loop. Connect to the signal generator through resistor of from 100 to 400 ohms.
8. RECEIVER OUTPUT,
(A) Use a d-c electronic voltmeter similar to the Voltohmyst, low side to chassis, high side to AVC terminal of loop.
(B) Use a rectifier-type a-c voltmeter or a standard output meter across the speaker voice coil.
F - M ALIGNMENT
Using Frequency - Modulated Signal Generator and Oscilloscope
ALIGNMENT OF I - F STAGES
GENERAL.- When the designated F - M signal from the signal generator is applied to the I - F amplifier or ratio detector, the output at point (A) viewed on an oscilloscope with a 60 - cycle linear horizontal sweep is represented by pattern A. Pattern B shows the out put at point (B) with the 8 mfd capacitor 33 disconnected. Patterns more useful for alignment purposes are obtained by operating the horizontal linear sweep of the scope at twice the modulation frequency or 120 cycles per second. This gives a double trace on each pattern, one trace representing the increasing frequency half of the modulation cycle and the other representing decreasing frequencies, pat terns I and V. When properly aligned, the two traces of pattern V coincide.
CENTER - FREQUENCY MARKER. - An additional requirement for proper alignment is that the signal generator must operate at the correct center frequency. The 10.7 Mc signal of the marker oscillator is used to check the center frequency. As the F - M signal sweeps its band, it produces a beat frequency with the marker signal, which decreases as the center point is approached and increases on the other side of center. These markers are shown properly centered in patterns II and VI. Because of the amplitude rejection of the ratio detector it is difficult to determine the center point of the markers in pattern II, but they can readily be located by temporarily shorting terminals 1 and 2 or 2 and 3 of ratio-detector transformer 113. The resulting effect is shown in patterns III and IV. It 1s advisable to remove the marker signal when adjusting for coincidence of patterns or straightness of crossover lines, but checks should be made with the marker to make sure that the signal generator has not drifted from the correct center frequency. Pattern VII shows the effect when the signal generator is off center. The markers may be entirely separated or partially overlap. To correct this condition, readjust the center frequency of the signal generator until the markers come together and the combined marker length is a minimum,
as in pattern VIII, Then realign to give pattern V or VI.
F - M SIGNAL GENERATOR, center frequency 10.7 Mc / sec, 225 Kc deviation, 450 Kc total sweep, at 60 cycles / sec. Use only enough output for satisfactory wave forms.
MARKER OSCILLATOR, 10.7 Mc / sec fixed, crystal controlled or accurately calibrated. Use no more output than necessary. Excessive amplitude will distort the patterns.
COUPLING OF SIGNAL GENERATOR AND MARKER OSCILLATOR. See Diagram I. LOW side to chassis. Combined output through 0.002 mfa to coupling point.
OSCILLOSCOPE. + Vertical amplifier at maximum gain. Linear horizontal sweep synchronized at 120 cycles per second by ripple voltage from pin 2 of rectifier 573GT of the receiver. not use internal Y - signal synchronization. This will result in off-center alignment.
SELECTOR SWITCH on FM (center position).
VOLUME AND TONE CONTROLS, maximum clockwise. HOOKUP, as in Diagram I. NOTE. Unless receiver is seriously misaligned, omit steps 1 and 2.
F - X ALIGNMENT (Continued)
Alignment of R - F Section
HOOKUP, as in Diagram II.
SELECTOR SWITCH on FH.
VOLUME AND TONE CONTROLS, maximum clockwise.
F - M SIGNAL GENERATOR, 50 Kc deviation, 100 Kc total sweep at any rate from 60 to 400 cycles per second.
OSCILLOSCOPE. Adjust horizontal sweep to the modulation frequency of the signal generator and lock it into step with the internal (Y - signal) synchronizing control. TUNING. Patterns IX through XIII are Wave forms that will show on the oscilloscope as the tuning control is tuned through the F - M signal. Pattern XI represents the correct tuning position; the pattern is a pure sine wave of greater amplitude than the side patterns.
Patterns IX and XIII represent the two side positions where the signal is also received, but distorted and with less amplitude.
When aligning, do not confuse the correct position, Pattern XI, with either side position, Patterns IX and XIII. When tuned correctly, a slight movement of the tuning control to either side will give the highly distorted double-frequency patterns X and XII.
F - M ALIGNMENT. MILLER - RESISTOR METHOD
Using An Unmodulated Signal Generator and D - C Electronic Voltmeter.
GENERAL. For this receiver, the Miller-resistor method, which takes its name from Dr. John M. Miller, is the most satisfactory of the alignment procedures which do not require the use of an F - M signal generator. With this method, resistance loading is applied to all the secondary circuits in the amplifier while the primary circuits are tuned to the desired center frequency. Then the primary circuits are loaded with the proper resistors while the secondary circuits are tuned.
The resistor across the primary reduces the Ref the transformer sufficiently to produce a single-peak response curve so that the secondary can be tuned to frequency. moval does not detune the secondary circuit appreciably. A similar effect is produced when the resistor is across the secondary, permit ting accurate tuning of the primary. Small half-watt carbon resistors must be used with the shortest possible leads, to avoid over-all regeneration Solder-tack the resistor across the transformer terminals.
SELECTOR SWITCH, on F - M (center position).
VOLUME AND TONE CONTROLS, maximum clockwise.
SIGNAL GENERATOR, unmodulated, accurately calibrated. Ranges 10 to 11 Mc and 87.5 to 108 Mc. Output adjustable from 100 to 100,000 microvolts. Connect low side to chassis, for steps 1-5.
OUTPUT INDICATOR. D - C electronic voltmeter, preferably zero center, with input resistance of at least one megohm on low range, which should not exceed five volts full scale.
OUTPUT METER CONNECTIONS. Probe to point (A), ratio-detector output, zero volts to chassis when correctly adjusted. Probe to point (B), F - M AVC source, reading the increase in negative voltage above the value obtained when no signal is applied. The reading with no signal is caused by the diode and amplifier-tube contact potentials, and will usually be about - 0.5 to 0.7 volts.