the trouble source has been found. 1t has been divided into the following:
equipment required—See paragraph 34d
Weak or no signals on all bands
Modulated recepltion—See paragraph 34e
Weak or no sigllals on any one band
Modulated reception—See parasraph 34f
Weak or no signals on all bands
C-W reception (modulated reception
normal)— See paragraph 34g
d. EQUIPMENT REQUIRED. — Few instruments other
than those found in a standard set analyzer (Test Set 1-56-A) are required in locating the most probable troubles in this receiver. The individual instruments required are as follows:
( 1 ) A modulated test oscillator (standard sig-nal generator) with a frequency range from 200 to 18,000 kc with provision for calibration accuracy better than 0.1% at aligning frequencies.
(2) *Voltmeter — 1,000 ohms per volt, ranges: 0-10; 0-100; 0-250; 0-500 volts.
(3) *Continuity tester.
(4) *Output meter rectifier type, 0-15 volt, 4,000 ohms
(5) Microammeter, 0-200 Microamperes.
(6) Audio frequency oscillator
(8) Adapter FT-211 consisting of an 8-prong octal plug,
and an 8-prong octal socket connected together by a short length of s-conductor cable, to permit use of the Test Set 1-56-A analyzer on octal tubes.
e. WEAK OR NO SIGNALS ON ALL BANDS, MODULATED RECEPTION.
(1) CHECK OF DYNAMOTOR VOLTAGES.— When all signals on all bands are weak or no signals are heard even wheal known to be present, check the dynamotor voltages at the dynamotor terminal strip (Figure 12, Socket \'oltages). The voltages should approximate the values shown. Conditions of measurements are:
Input voltage 28V; AVC-OFF-MVC switch MVC; Crystal control OUT; C-W Osc. OFF; Volume Control maximum; Load 4,000 ohms non-inductive reresistance. If these voltage reading do not approximate the values shown, the fuse should be checked, as well as the dynamotor and filter circuits, wiring and components
*part of Test Set 1-56-A
(2) TUBE CHECK—If the voltages at the dynamotor
terminalal strip approximate the values given, proceed to check all tubes with the Model 685 tube checker for emission and characteristics or replace tubes with those of known average characteristics, if this has not been done in the “Quick Check".
(3) CHECK OF SOCKET VOLTAGES.—If tubes check satisfactorily, or if after replacing with tubes known to be good the sensitivity is still low, proceed to check all tube socket voltages as outlined under Paragraph 34b( 1 ) with Test Set 1-56-A. The average socket voltages for Radio Receiver BC-348 (*) are given in Table C Socket Voltages.
(4) CHECK CIRCUIT WIRING AND COMPONENTS—If the tube socket voltages do not approximate she values shown in Table C Socket Voltages, the associated circuits and components should be checked for grounds, shorts and similar defects using Test Set 1-56-A, the wiring diagram Figure 30, Table D Resistance Between Socket Terminals and~Ground, and Table E.
(5) TEST OF AUDIO FREQUENCY AMPLIFIER.—After checking socket voltages, circuit wiring and components, proceed to the test of the audio frequency amplifier. This can be checked by capacitively coupling a 400 cycle voltage of approximately 2 volts RMS. from the detector signal diode to ground using a capacitor of .5 mf. As an alternative, a modulated 915 kc signal of 2 volts may be coupled through a .1 mf capacitor from the plate of the 3rd i-f Tube VT-116 to ground.
Proper functioning of the audio amplifier will be indicated by an output of 50 milliwatts for the 2 volt audio
signal or 1 milliwatt output for the 915 kc signal input.
Circuits, wiring and components should be checked if this order of response is not obtained.
(6) TEST OF INTERMEDIATE FREQUENCY AMPLIFIER.—Following a satisfactory test of the audio amplifier, check the intermediate frequency amplifier by capacitively coupling the modulated signal generator to the control grid of the 1st detector tube and ground, through a 0.1 mf. capacitor, the frequency being adjusted to 915 kc. A rough check of the proper functioning of the i-f ampilfier is indicated by a comfortable headphone output level with low input from the signal generator. (Approximately 35 microvolts input for 10 milliwatts output.) See Table G Alignment Chart, 1st i-f column, for connections and detailed information.
(7) I-F AMPLIFIER CIRCUIT CHECK—If the i-f amplifier does not respond as above or lacks sensitivity