Zenith Trans-Oceanic Theory of Operation

 

The following chapters were taken from the Department of the Army Technical Manual TM11-877 for the R-520/URR Radio Receiver published January 12, 1954. It is presented here because it represents a good explanation of the operation of all tube type Zenith Trans-Oceanic Radios. It was scanned and the text converted using OCR. Every effort was made to correct the inevitable mistakes. The text may refer to some figures not presented that are outside of Chapter 4 and 5. However, figure 40 is presented at the end of Chapter 4. The figures have high resolution, but (depending on your browser) you may have to save the files and view them with a separate image program.

 

A valuable resource for Trans-Oceanic collectors is the book: The Zenith Trans-Oceanic, the Royalty of Radios : The Royalty of Radios (Schiffer Book for Collectors) by John H. Bryant and Harold N Cones.

You can also buy a CD ROM with schematic and other information about every TransOceanic ever built!

Your comments and corrections are welcome: Click here to send me E-Mail

Before we start, here are some other useful links for TransOceanics:

http://www.transoceanic.nostalgiaair.org/

http://www.angelfire.com/ma2/AdamVon/tobatt.html

 

TM11-877

CHAPTER 4

THEORY

42.        Block Diagram

(fig. 15)

Radio Receiver R-520/URR is a portable superheterodyne receiver designed to receive am signals in the broadcast and shortwave bands. The frequency ranges are given in paragraph 4. Figure 40 is a complete schematic diagram of this receiver. The tuning system consists of an antenna section, an r-f stage which uses a 1U4 tube, and a 1L6 pentagrid converter which operates as a combination mixer and h-f (high-frequency) oscillator. A different mode of tuning and tracking is used on each of the three, groups of bands (broadcast band, continuous coverage shortwave bands, and the spread bands). The block diagram, which shows the signal path through the receiver, is discussed in a through e below.

a. First R-F amplifier. The signal is fed from the antenna to tuned r-f amplifier tube V1. The r-f amplifier serves to increase; the signal voltage, provides isolation between the oscillator section of tube V2 and the antenna, and also stops unwanted signals (at image frequencies) from entering the converter

b. Converter. The signal from the r-f amplifier is fed to the mixer section of combination mixer and oscillator tube V2. The oscillator section of tube V2 produces an h-f oscillator voltage that is 455 kc higher than the signal frequency on the broadcast and the two continuous coverage bands, and 455 kc lower on the four spread bands. This voltage is combined with the received signal in the mixer section of tube V2 to produce a difference beat frequency of 455 kc, which then is amplified by i-f amplifier tube V.3.

c. I-F Amplifier. The i-f amplifier is a high-gain circuit which is fixed-tubed to the frequency difference between the h-f oscillator and the incoming r-f signal, and is thus a constant, single-frequency amplifier that operates on 455 kc. Most of the signal amplification occurs in the i-f amplifier.

d. Detector and First A-F Amplifier. The amplified signal from the i-f amplifier is fed to detector tube V4 for demodulation. V4 is a dual tube, which serves as a detector and a first a-f (audio-frequency) amplifier. The detector stage also produces the avc (automatic volume control) voltage which automatically controls the gain of the receiver by regulating the bias voltage to the grids of tubes V1, V2, and V3.

e. Final Audio Power Amplifier. The audio signal from V4 is fed to audio-output stage V5 for power amplification. The output of power amplifier V5 is applied across an impedance-matching output transformer which permits the use of headsets (not supplied) or the loudspeaker.

43.        Tuning Circuits

Tuning for the receiver is provided by a three-gang variable capacitor in conjunction with other circuit components. The components required to tune the receiver through a desired band are selected when the correct band selector button is pressed. This action connects the correct tuned circuits to the antenna, r-f, and oscillator stages. Where practicable, switching is simplified by shunting the desired circuit components across the unused circuit components. Most of the unused coils are shorted to prevent any undesirable resonant effects. The tuning methods used for the three different types of bands are described in a through c below. Additional details on the receiver tuned circuits are given in paragraphs 44 through 48.

a. The tuning range of the broadcast band is from 540 to 1,600 kc. Tuning is provided by means of ganged tuning capacitor C2, sections A, C, and E. Figure 16 shows the r-f and converter stages for broadcast operation when BC pushbutton SlA is pressed. The switch sections actually are arranged in line (fig. 34), but are shown rearranged in figure 16 to facilitate circuit discussion (pars. 44, 45, and 46).

b. The tuning ranges of the two continuous coverage bands are from 2 to 4 mc and 4 to 8 mc, and tuning is provided by means of the same gang tuning capacitor as used on the broadcast band. Both continuous coverage bands have similar antenna, r-f, and converter circuits. Figure 17, which is used to facilitate discussion (par. 47), shows the r-f and converter stages for 4- to 8-mc operation (pushbutton S1B pressed). Figure 35 shows the circuit with the sections of S1B drawn in line.

c. The tuning ranges of the four spread bands are from 9.4 to 9.8 mc (31M), 11.6 to 12 mc (25M), 14.9 to 15.5 mc (19M), and 17.5 to 18.1 mc (16M). Tuning is provided by means of the same ganged tuning capacitor used on the broadcast band. All four spread bands have similar antenna, r-f, and converter circuits. Figure 18, which is used to facilitate discussion (par. 48), shows the r-f and converter stages for 16 M operation (pushbutton S1D pressed). Figure 36 shows the circuit with the sections of S1D drawn in line.

44.        Broadcast Antenna Stage

(fig. 16)

The tuned antenna circuit consists of loop antenna E2, antenna loading coil T4, and ganged tuning capacitor C2A. The high r-f end of the loop antenna (junction of J1 and L4) is connected to the control grid of the 1U4 r-f tube through capacitor C30 which isolates the control grid of the 1U4 r-f tube for avc action. Antenna loading coil L4 increases the tuning range of the circuit. R-f coupling from the lower end of loop antenna E2 to the filament of the r-f amplifier is provided by capacitor C3. Similarly, r-f coupling from the lower end of tuning capacitor C2A to the filament of the r-f amplifier is provided by capacitor C10. Capacitor C3, resistor R1, and capacitor Cl0 form an impedance-matching network for any external long-wire antenna and serve to minimize any change of loop antenna tracking caused by different lengths of external antennas. Trimmer capacitor C2B is mounted on top of ganged tuning capacitor C2A and is set to track at the h-f end of the broadcast hand. Capacitor C1 isolates the power line from a direct ground connection when operating from a power line source.

45.        Broadcast R-F Stage

(fig. 16)

Plate voltage for the r-f stage is supplied from the high B+ bus through primary winding CD of coil L1 and damping resistor R3. Screen voltage for the r-f stage is supplied directly from the low B+ bus. Primary winding CD of coil L1, in series with R3, forms the plate load for V1. Secondary winding AB of coil L1 is tuned by ganged capacitor C2C. The filament of V1 is returned to B- for r-f through capacitor C9. Avc voltage to control the gain of the r-f stage is supplied through isolating resistor R2. Trimmer capacitor C2D, mounted on top of ganged tuning capacitor C2C, is set to track at the h-f end of the broadcast band. The filament circuit is discussed in paragraph 59. The plate and screen voltage distribution system is discussed in paragraph 58. The avc voltage distribution system is discussed in paragraph 53.

46.        Broadcast Converter Stage

(fig. 16)

The combined functions of the mixer and h-f oscillator are performed by pentagrid converter V2, which uses a type 1L6 tube. The filament and first two grids act as a triode to comprise the oscillator section. Grid 1 (pin 4) functions as the oscillator grid, while grid 2 (pin 3) functions as the oscillator plate. The mixer section consists of the filament, mixer grid (pin 6), screen grid (pin 5), and the plate (pin 2). Plate voltage for the oscillator section is supplied from the low B+ bus through primary winding EF of coil L2. Screen voltage also is supplied from the low B+ bus, but through dropping resistor R7. The screen is bypassed for r-f by capacitor C11. Grid leak bias is provided by the combination of C12 and R6. Avc voltage to control the gain of the converter stage is supplied through isolating resistor R26 (par. 53). The filament is returned to B- for r-f through capacitor C9. Paragraph 59 contains a discussion of the filament circuit.

a. The oscillator is the tuned-grid, plate-tickler, feedback type and its operating frequency on the broadcast band is 455 kc higher than the incoming signal frequency. Section EF of coil L2 is the oscillator plate feedback winding and section DB is the tuned-grid winding. Windings GH and AC of coil L2 and coil L3 are not used on the broadcast band. Ganged tuning capacitor C2E is in series with 600-kc padder capacitor Cl3, and both capacitors shunt grid winding DB. Temperature compensating capacitor Cl5 and trimmer capacitor C2F are both in parallel with ganged tuning capacitor C2E. Trimmer capacitor C2F is mounted on top of capacitor C2E and is set to calibrate the oscillator at the h-f end of the broadcast band. Coupling from winding DB of coil L2 to the oscillator grid is provided by capacitor C14. Windings GH and AC of coil L2 are not used on the broadcast band. To prevent undesirable resonance effects, winding GH of coil L2 is shunted by capacitor C42 through a section of switch SlA, and winding AC is shunted by capacitors C38 and C14, in series, through another section of switch S1A.

b. The signal from the r-f amplifier is applied through capacitor C7 to the mixer grid (pin 6). A portion of the r-f amplifier output also is applied through capacitor C8 to the oscillator plate (pin 3) which effectively neutralizes the interelectrode capacity between the oscillator plate and the mixer grid so that oscillator pulling is minimized. Pulling in a pentagrid converter, such as the 1L6, is the tendency of the oscillator frequency to shift toward the applied signal frequency, and it produces an incorrect i-f frequency in the output of the converter. Mixing action between the oscillator and r-f signals occurs within the electron stream of the converter tube. The difference frequency (i-f) is taken from the plate (pin 2) and applied to the i-f transformer T1.

47.        Continuous Coverage Bands

(fig. 17)

The two continuous coverage tuning ranges are from 2 to 4 mc and 4 to 8 mc. The antenna, r-f, and converter stages are tuned by ganged tuning capacitor C2. Both tuning ranges have similar antenna r-f, and converter circuits; therefore, only the 4- to 8-mc circuit will be discussed.

a. Antenna Stage (4- to 8-me). The telescopic whip antenna is coupled to antenna coil L5 through capacitor C27, and an external antenna (when used) is coupled to antenna coil L5 through capacitor C28 The antenna circuit consists of coil L5 shunted by tuning capacitor C2A and trimmer C2B. Coupling to the control grid of the 1U4 r-f tube V1 is provided by capacitor C30. Coil L5 is set to track at the i-f (low-frequency) end of the tuning range. Trimmer capacitor C31A in series with parasitic suppressor R24 is set to track at the h-f end of the tuning range. Capacitor C1 isolates the chassis from direct ground connection when operating from a power line source.

b. R-F Stage (4- to 8-me). Plate, screen, and avc voltages for the r-f amplifier are supplied as described in paragraph 45. The r-f stage is coupled inductively to the converter stage by r-f coil L1 and is coupled capacitively by capacitor C6. R-f coil L12 is effectively shunted across the secondary of broadcast coil L1 through a section of band selector switch S1B and capacitors C34 and C10. Tuning is provided by gang tuning capacitor C2C. Coil L12 is set to track at the l-f end of the tuning range and trimmer capacitor C35A is set to track at the h-f end of the tuning range. The filament circuit is discussed in paragraph 59.

c. Converter Stage (4- to 8-me). When the 4-8 mc continuous coverage band is selected by section S1B of the band selector switch, oscillator coil winding L18A is shunted effectively across the primary of broadcast oscillator coil EF through series capacitors C37 and C39. At the same time oscillator coil winding Ll8B is shunted across tile secondary of broadcast oscillator coil DB through fixed oscillator padder capacitor C39. Windings GH and AC of coil L2 are not used on the 4-8 mc band. To prevent undesirable resonance effects, winding GH of coil L2 is shunted by. capacitor C42 through a section of switch S1B, and winding AC is shunted by capacitors C38 and C14, in series, through another section of switch S1B. Coil L3 is a high-impedance loading coil that improves the operation of the h-f oscillator on shortwave. Oscillator plate voltage is supplied through winding EF of coil L2. Screen voltage is supplied through dropping resistor R7. The screen is bypassed for r-f by capacitor C11. The filament circuit is discussed in paragraph 59. Tuning is provided by ganged tuning capacitor C2E. The core of coil L18A, Ll8B is set to calibrate at the 1-f end of the 4-8 mc band. Trimmer capacitor C40A is set to calibrate at the h-f end of the band. The signal from the r f amplifier is coupled to the mixer grid (pin 6) through capacitor C6 and also through coil L1 and capacitor C7. A portion of the r-f amplifier is applied through capacitor C8 to the oscillator plate (pin 3) which effectively neutralizes the interelectrode capacity between the oscillator plate and the mixer grid so that oscillator pulling is minimized. The oscillator frequency is 455 kc higher than the incoming signal frequency. The oscillator and r-f signals are mixed within the electron stream of the converter. The difference frequency (i-f) is taken from the plate (pin 2) and applied to the i-f transformer T1.

48.        Spread Bands

(fig. 18)

The four spread bands tune as follows: 31M (9.4 to 9.8 mc), 25M (11.6 to 12 mc), 19M (14.9 to 15.5 mc), and 16M (17.5 to 18.1 mc). The antenna, r-f, and converter stages are similar in operation on each of the four spread bands and therefore only the 16 M circuit will be discussed.

a. Antenna Stage (16 M). The antenna circuit is made up of antenna coil L7 shunted by two series capacitors C32 and C33. Tuning capacitor C2A with its trimmer, C2B, is shunted across capacitor C32 to provide band spreading over a narrow frequency range. This circuit is coupled to the control grid of 1U4 r-f amplifier tube V1 through capacitor C30. Antenna coil L7 is set to track at the center of the tuning range. The telescopic whip antenna is connected through coupling capacitor C27 to the high side of antenna coil L7. When an external antenna is used, coupling to the high side of the antenna coil is provided by capacitor C28. Capacitor Cl isolates the chassis from direct ground connection when operating from a powerline source.

b. R-F Stage (16M). Plate, screen, and avc voltages for the r-f amplifier are supplied as described in paragraph 45. The r-f circuit consists of r-f coil L14 in series with isolating capacitors C34 and C36. The coil is set to track at the center of the band. The circuit is not tuned by ganged capacitor C2C. The plate of the 1U4 r-f tube is coupled to the control grid of converter tube V2 through capacitor C6 and the high side of r-f coil L14 is tied to this same point through capacitor C36. L11 is an r-f choke coil which acts as a fixed grid on spread-band operation. It is shunted across r-f coil L14 through capacitors C34 and C36. Since it has a low d-c resistance and a high impedance to shortwave frequencies, it creates a high-loading effect to the shortwave frequencies, and its low d-c resistance prevents the control grid of the converter tube from accumulating any grid charge. The filament circuit is discussed in paragraph 59.

c. Converter Stage (16 M)

(1) When the 16 M spread band is selected by section S1D of the band selector switch, no avc voltage is applied to the converter stage, since the mixer grid (pin 6) is returned directly to the filament through coil L11. This allows the converter to operate at the point of maximum sensitivity and avoids the possibility of h-f oscillator shift which might result when avc voltage is applied to a converter stage. Oscillator plate voltage is supplied through winding EF of coil L2. Screen voltage is supplied through dropping resistor R7. The screen is bypassed for r-f by capacitor C11. The filament circuit is discussed in paragraph 59.

(2) The basic oscillator circuit is a modified tuned-grid, plate-tickler, feedback type. Oscillator coil L20 is in series with shortwave secondary winding AC of coil L2. Both coils are shunted across broadcast coil secondary BD through fixed padder capacitors C14 and C42. Coupling to the oscillator grid is provided by capacitor C12. The series shunt combination of capacitors C14, C15, C38, C42, and gang tuning capacitor C2E with trimmer C2F results in band spreading over a narrow frequency range. Coil L3 is a high-impedance loading coil that improves the operation of the h-f oscillator on shortwave. The broadcast oscillator coil secondary BD is left in the circuit to simplify band switching and does not affect circuit operation. On shortwave frequencies, broadcast primary winding EF acts as a plate choke for the oscillator section. Oscillator coil L20 is set to track at 17.8 mc. The signal from the r-f amplifier is coupled to the mixer grid (pin 6) through capacitor C6. A portion of the r-f amplifier output is applied through capacitor C8 to the oscillator plate (pin 3) which effectively neutralizes the interelectrode capacity between the oscillator plate and the mixer grid so that oscillator pulling is minimized. On the spread bands, the oscillator frequency is 455 kc lower than the incoming signal frequency. The oscillator and r4 signals are mixed within the electron stream of the converter. The difference frequency (i-f) is taken from the plate (pin 2) and is applied to i-f transformer T1.

 

49.        I-F Amplifier

(fig. 19)

The i-f amplifier consists of a single stage tuned to 455 kc. I-f amplifier V3 uses a 1U4 tube. I-f transformers T1 and T2 are iron-core tuned. Capacitors C16, C17, C19, and C20A are fixed capacitors across the primary and secondary windings of T1 and T2 and are used for resonating the input and output circuits to 455 kc. Capacitor C8 does not provide complete bypassing for the screen grid of the i-f amplifier. Instead, capacitor C18, in conjunction with isolation resistor R12, provides a feedback path to the screen grid to neutralize the effect of the capacity between the signal grid and the plate. With an optimum value for capacitor C18, the gain of the i-f amplifier can be increased considerably without danger of self-oscillation. The 455-kc i-f output from the plate of V3 is applied to the tuned circuit consisting of capacitor Ci9 and the primary of transformer T2. The i-f signal is coupled to the tuned circuit consisting of the secondary of transformer T2 and capacitor C20A, and then is applied to the diode section of V4 for detection and development of avc voltages

50.        Detector-Amplifier

(fig. 20)

Detector-amplifier stage V4 uses a 1U5 diode pentode. The 455-kc i-f signal from the secondary of i-f transformer T2 is applied between the diode plate and one side of the filament. The audio output appears across diode load resistors R16 and R32 and volume control R17. The audio signal is fed through coupling capacitor C21 to the control grid of the pentode amplifier section. Grid load resistor R18 is high enough in value to provide contact potential bias for class A operation. Screen voltage is supplied from the low B+ source through decoupling network R21 and C22. The amplified audio developed across plate load resistor R22 is fed through coupling capacitor C24 to the grid of the audio-output stage. Capacitor C23 bypasses the plate of the amplifier for radio frequencies. Plate voltage for the pentode amplifier section is supplied through voltage-dropping resistor R4. Additional filtering is provided by capacitor C25. The action of the avc circuit is discussed in paragraph 53.

51.        Audio Output

(fig. 20)

The audio signal from the detector-amplifier is applied to the grid of the 3V4 (V5) audio-output stage across grid resistor R20. Screen voltage is supplied directly from the high B+ bus. Plate voltage also is provided from the high B + bus through the primary winding of output transformer T3. Capacitor C45C bypasses the plate and screen voltage source for audio frequencies. The correct grid bias is obtained by returning R20 to a point on the series filament circuit (par. 54) capacitor C26 bypasses undesirable noise frequencies and prevent Oscillations in transformer T3. The secondary of transformer T3 is coupled to loudspeaker LSl through the normally closed contact of jack J3. For headset operation, the loudspeaker is disconnected automatically when the headset ping is inserted in jack J3.

 

52.        Tone Control Circuit

(fig. 21)

a. The operation of the tone control circuit depends on an inverse feedback voltage that is fed back to the volume control through a resistor-capacitor network from the tertiary winding on output transformer T3. The tone control resistor capacitor network consists of resistors R16, R17, R32, R33, R34, R35 and capacitors C47, C48, and C49. The function of the four-tone control switches is to vary the resistor-capacitor network frequency characteristic which results in a change of a-f response. For example, when BASS tone control switch 52B is open, only the higher audio frequencies are fed back. The high audio frequencies are effectively suppressed in the output circuit, because they are fed back out of phase; this results in a bass boost effect. Since capacitor C48 has a higher reactance to the low audio frequencies, the low frequencies are boosted effectively when BASS switch 52B is open. When switch S2B is closed, capacitor C48 is shorted out and eliminates the bass boost; this results in increased treble response.

b. Resistor R33 is a limiting resistor and is used to prevent regeneration at the very low audio frequencies. Resistor R35 limits the amplitude of the audio frequencies fed back to the volume control circuit. When the ALTO switch, S2D, is open, R35 is placed in series with the feedback voltage and the output level of the low boost is reduced effectively. Capacitor C47 bypasses the higher audio frequencies around R16 when TREBLE switch S2A is closed. Capacitor C49 passes the higher frequencies more readily and produces a bass boost when switch S2C is closed because of the inverse feedback voltage. When switch S2C is closed, capacitor C49 is shunted across resistor R34; this causes a greater inverse feedback voltage of the high and middle frequencies. The gain at these high and middle audio frequencies is reduced by this inverse feedback. Since the gain at the lower audio frequencies is not affected, the low audio frequencies have been boosted in relation to the middle and higher audio frequencies.

53.        Avc System

(fig. 22)

a. The avc system controls the gain of the r-f, converter, and i-f stages. Figure 22 shows the avc network for the broadcast and continuous coverage bands. On spread-band operation, the avc circuit for the r-f and i-f stages remains the same but no avc voltage is applied to the converter stage. The converter control grid is connected through the band selector switch from grid resistor R26 to r-f choke coil L11 (fig. 18). Under these conditions, 1L6 converter tube V2 operates at the point of maximum sensitivity.

b. Resistors R16, R17, and R32 form the diode load circuit across which the avc voltage is developed. Three different levels of avc voltages are obtained from a voltage-divider network connected between the diode load circuit and different points of the filament circuit to develop the correct level of avc voltage and correct operating bias for each stage.

c. To prevent r-f overload and distortion, the greatest portion of avc voltage is fed back to the control grid of r-f amplifier tube V1. The next greatest portion of avc voltage is fed back to the control grid of mixer tube V2, and the smallest portion of avc voltage is fed back to the i-f amplifier control grid of V3.

d. The maximum amount of avc voltage developed across the diode load is fed back to the control grid of V1 from the junction of R19 and R15 through isolation resistor R2. Resistors R14 and R10 are shunted by resistor R9 through part of the filament circuit. This branch is in series with resistor Ri3 to the diode load. This shunt divider network feeds avc voltage to the control grid of V2 through isolation resistor R26. From the junction of R14 and Rio the smallest portion of avc voltage is fed back to the control grid of V3. Filtering for the avc system is provided by capacitors C4, C43, and C44.

54.        Bias System

(fig. 22)

a. The avc and bias voltages for the receiver are tied together in a combination bridge network to supply the proper bias voltage to each tube. The common B- bus of the receiver is used as the point of zero voltage. Bias voltage for the various tubes is provided by returning the control grid to the appropriate point in the complex voltage divider formed by the series tube filaments and the avc resistor network. Filament and bias voltage are derived from the 8.4-volt dc filament supply (par. 59).

b. Avc voltage is developed across diode load resistors R16, R17, and R32, when an i-f signal is fed into the T2 secondary winding. The resistance network proportions the avc voltage to each control grid. Avc action reduces the gain of the r-f converter and i-f stage and prevents overload and distortion on strong input signals. Normally, a potential of -.25 volt is developed across diode load resistor R16. The control grid to B- voltage of each tube is determined by the resistance networks between the diode plate and the various control grids.

c. Because the filament voltage on direct heater-type tubes varies along the physical length of the filament, the center of the filament is used as a reference point when measuring the grid bias, EG. The filament-center to B- voltage is determined by the electrical position the tube occupies in the series filament circuit.

d. The difference between the control grid and filament center voltage is the operating grid bias, EG, of each tube.

e. On spread-band operation, no avc is applied to the control grid of V2 (par. 48). The grid return which normally goes to R26 is switched to r-f choke coil L11 and returns to the negative end of its own. filament (fig. 18).

f. The grid bias on the 3V4 power-amplifier tube is obtained by connecting the grid d-c return through resistor R20 to the junction of the 1U4 r-f and 1U5 detector-amplifier filaments. This junction is 1.4 volts positive with respect to B-. The 3V4 power-amplifier filament center is approximately 7.0 volts dc positive with respect to B-. This results in an operating bias EG of -5.6 volts on the grid of the 3V4 power-amplifier tube which is normal for class A audio operation for this type of pentode. The operating bias EG of 1.7 volts on the control grid of the 1U5 detector-amplifier tube is the sum of the internal contact bias created by normal filament emission plus the voltage existing between the center of the 1U5 filament and B-. The contact bias across resistor R18 is approximately -1 volt and the filament center-to-B- voltage is 0.7 volt. The resultant operating grid bias, EG, is therefore -1.7 volts. Figure 22 shows the manner in which the remainder of the tubes receive their operating bias, EG.

55.        Power Control Circuit

(fig. 24).

Power for Radio Set R-520/URR is applied through switches S3A-S3B, S4, fuse Fl, thermal resistor RT1, and power supply adapter Z1 (which is plugged permanently into J2 on the receiver chassis). Adapter Z1 is shown schematically in figure 23. Switch S4, located on the chassis and to the rear of the speaker, changes both the B+ and filament circuits from external power to battery power when the power plug is inserted in the socket of switch S4. Switch S3A-S3B, which is mounted on the volume control, controls the application of power to the radio set. It completes both the filament and B+ circuits to the receiver. The receiver requires an 8.4- to 9-volt d-c source for the series filament string and a 90- to 105-volt d-c source for the B+ line. The power circuit supplies these potentials whether the set is operating from battery or from an external power line source. With switch S4 set for power line operation, section A of switch S3 connects one side of the power line to the B- bus, while section B completes the B+ circuit through fuse Fl and R36. With switch S4 set for battery operation, section A of S3 connects the grounded negative terminal of the battery system to the B- bus, while section B completes the B+ circuit to the positive terminal of the B battery.

56.        External Power Source

(figs. 24 and 25)

a. Selenium rectifiers CR1 and CR2 change the a-c line voltage to d-c. Rectifier CR2, which is located in power supply adapter Z1 functions only when operation is from a 230-volt a-c or d-c source. The d-c output voltage from the cathode of rectifier CR1 is applied through surge limiting resistor R27, fuse Fl, and filter resistor R36 to capacitor C45C and the high B+ bus. Resistor R4 drops this voltage to a lower value and applies it to capacitor C25 and the low B+ bus. The d-c output voltage from the cathode of rectifier CR1 is also applied through resistor R27 to the filter circuit consisting of C45A, RT1, and C45B. The output of the filter supplies 8.4 volts dc to the filament string through R30 and a section of S4. Thermal resistor RT1 is in series with resistor R30 to the filament circuit of the receiver and tends to maintain a constant current flowing through the tube filaments despite variations in line voltage. Fuse Fl has a 3/16-ampere rating and protects the power supply from overload in the B+ and filament circuits. Resistor R28 is a bleeder resistor which discharges capacitor C45A when the power is turned off.

b. Power supply adapter Z1, when plugged into receptacle connector J2 at the rear of the chassis, is in series with the power line. It consists of a three-position switch, S5; a selenium rectifier, CR2; and resistor R38A-R38B (fig. 23). The complete assembly is housed in a perforated metal can. Receptacle J2 is a closed circuit-type connector. Therefore, if power supply adapter Z1 is removed, the receiver may be operated from a 110-volt a-c or d-c power source. When switch S5 is in the 110 V. AC-DC position, plug connector P2 is shorted out and the power line is connected directly to the receiver. When switch S5 is in the 220 V. DC position, resistor R38, sections A arid B, and selenium rectifier CR2 are placed in series with the power line to lower the voltage to 110 volts dc. When switch S5 is in the 220 V. AC position, resistor R38A is shorted out, leaving R38B and selenium rectifier CR2 in series with the line. Since filter capacitor C45A draws an appreciable amount of ripple current, the total current drawn through the line is greater on 230-volt a-c operation than on 230-volt d-c operation. Shorting-out resistor R38A compensates for the additional voltage drop that would occur through the power supply adapter and maintains the correct line voltage to the set.

Note. The receiver will not operate on dc if the polarity of the power line cord plug is reversed; however, electrolytic capacitor C45A, B, and C will not be harmed by reversed currents because selenium rectifiers CR1 and CR2 have a high back resistance.

57.        Battery Power Source

(fig. 24)

a. For battery operation, a B supply voltage of 90 volts and an A supply voltage of 9 volts are required for operation of the receiver. The battery supply voltages are fed to the receiver through both sections of switch S3, and through switch S4. The negative side of the B battery is connected to the positive side of the A battery. This connection boosts the effective B+ voltage by the amount of the A supply voltage (9 volts). When the power line plug is inserted into socket switch S4 for battery operation, all the switch contacts of S4 may not make or break simultaneously. If the B+ and A+ contacts close before the A- contact, the B battery will charge electrolytic capacitor C45C through the tube filaments and the closed contacts of S3A-S3B. When this occurs, resistor R37 will limit the initial surge of charging current and thus prevent filament burnouts.

b. Resistor R29, which is connected from the B- bus of the receiver to chassis, forms a high-impedance path to the external power line. When the receiver is operating on batteries, the B- bus is shorted to the chassis through switches S3A and S4. Capacitor C46 minimizes signal and noise pickup from the power line when the receiver is operating from an external power line.

58.        Plate and Screen voltage Distribution

(fig. 24)

Plate and screen voltage for the receiver is supplied either from the batteries or from the power line through the rectifier-filter circuits, and is present at switch S3B (par. 55). When operating from batteries, 90 volts is applied to the high B+ bus through S3B. When operating from the power line, 105 volts is applied to the high B + bus through S3B. The low B+ A bus (approximately 80 volts) is supplied through dropping resistor R4 and is filtered by capacitor C25.

a. The high B+ bus feeds voltage to the plate and screen grid of V3 (the 1U4 i-f amplifier) through dropping resistor R12. It also feeds voltage to the plate of V2 through the primary winding of i-f transformer T1, to the plate of V1 through resistor R3 and the primary of i-f coil L1, to the plate of V3 through resistor R12 and the primary of transformer T2, to the plate of V5 through the primary of transformer T3 and to the screen of V5.

b. The low B+ bus feeds voltage to the oscillator anode grid of 1L6 converter tube V2 through winding EF of oscillator coil L2. It also feeds voltage to the screen grid of V2 through dropping resistor R7. The screen grid of r-f tube V1 receives voltage from the low B+ bus directly, and the screen grid of detector-amplifier tube V4 receives voltage through dropping resistor R2 1. Voltage from the low B+ bus also is supplied to the plate of V4 through plate load resistor R22. Capacitor C25 is a filter for the low B+ bus and eliminates any hum voltage that may be present at that point.

59.        Filament Circuit

(fig. 25)

a. Radio Receiver R-520/URR uses four 1.4-volt and one 2.8-volt, 50-ma (milliampere) filament type tubes. The tube filaments are connected across the voltage source in a series arrangement. On power line circuit operation, the 105-volt d-c rectifier output is dropped down to 8.4 volts dc through thermal resistor RT1 and resistors R27 and R30. On battery operation, 9 volts from the A battery are applied across the filament circuit. During normal operation, the plate and screen grid currents of a d-c filament type tube pass through the filament and add to the current flowing through the filament. This current accumulates toward the negative end of the filament. To prevent this extra current from burning out the filaments, shunting resistors R8, R5, R11, and R23 are incorporated in the circuit. Resistor R23 is placed across half of the filament of the 3V4 audio-output tube because most of the plate and screen grid current ac-cumulates near the negative end (pin 1). Resistors R5 and R11 are shunted across the filaments of V2 and V3, respectively, to shunt the relatively heavy plate current for the 3V4 output tube around the filaments of V2 and V3. One resistor, R8, is used across the filaments of V4 and V1 because their plate and screen grid currents are small. Filament r-f bypassing for the detector-amplifier and r-f amplifier stages is provided by capacitors C5 and C9.

b. Thermal resistor RT1 is a self-regulating resistor that helps to drop the 105-volt d-c power supply voltage down to the required 8.4 volts dc. Its internal resistance varies from 600 ohms to 1,300 ohms with power-line voltage variations of 90 volts to 127 volts; a constant 50-ma current in the filament circuit is maintained. Capacitor C45D is connected from pin 1 of V5 to the B- bus. Thus, any audio voltage present in the output tube filament is prevented from entering and modulating the other tube filament circuits.

 

CHAPTER 5

FIELD MAINTENANCE INSTRUCTIONS

Section III. ALIGNMENT PROCEDURES

80. Test Equipment Required for Alignment

a. Signal Generator. The signal generator (such as RF Signal Generator Set AN/URM-25) must be an accurately calibrated instrument capable of producing modulated and un-modulated r-f signals. The frequency range must extend from 400 kc to 25 mc. The alignment frequencies are shown in the alignment chart (par. 93). If necessary, the second harmonic generally can be used when the fundamental is not available. Thus, a signal generator with a top frequency of 12.5 mc also is suitable. The generator should have an output of at least 5,000 uV for best results in aligning the r-f, h-f oscillator, and i-f circuits. Accurate frequency calibration of the signal generator is extremely important to insure that the receiver dial calibration will be correct.

b. Output Meter. The output meter should respond to audio frequencies and should provide readings at 50 mw and have a maximum range of 500 mw. For correct readings, the impedance of the meter must match the 3-ohm voice coil winding. Use Output Meter T-585/U (or equivalent), set for a 3 ohm load.

c. Frequency Meter. When a highly accurate signal generator is not available, a frequency meter can be used to check the accuracy of the generator. Frequency Meter BC-221-(*) is suitable for this purpose.

d. Headset or Permanent Magnet Loudspeaker. The receiver loudspeaker or a headset (such as Headset HS-30) can be used for rough alignment.

e Alignment Tools. The alignment tool, O 14, supplied with the receiver and an insulated screwdriver are required.

Caution: Alignment tool O 14 is intended for alignment of the i-f and r-f transformers. Do not attempt to use it for turning trimmer Capacitors or as a screwdriver.

81. Calibration of Signal Generator

The procedure below is used only when the signal generator calibration is not acceptable.

a. Accurate alignment of the h-f oscillator in the receiver requires the use of the frequency meter to check the signal generator setting, as follows:

(1) Place the generator and frequency meter near each other. Turn them on and allow them to warm up for at least 15 minutes.

(2) Calibrate the frequency meter according to the instructions furnished with the unit.

(3) Attach a piece of wire to the signal generator output connection and place the wire near the frequency meter antenna.

(4) Set the meter to the exact frequency to which the generator is to be used.

(5) While listening to the headset connected to the frequency meter, tune the generator to zero beat with the meter. The signal generator now is set for the frequency desired.

b. Turn off the frequency meter and remove the wire attached to the signal generator output connection.

82. Preparation for Alignment

a. Remove the chassis from the cabinet in accordance with instructions in paragraph 84.

b. If available, use an isolation transformer between the power line and the receiver.

c. Be sure the power supply adapter switch is in the correct position to conform to the power source that is available.

d. Connect the power cord to the external power line source. Turn the receiver on and let it warm up for 5 minutes.

(1) Turn the volume control to its maximum clockwise position.

(2) Set the receiver for broadcast operation.

(3) Connect a 3 ohm output meter to Plug P~55 and insert it into jack J3.

(4) Align the various sections of the receiver in the following order:

I-F stage.

Broadcast band.

4-8 MC continuous coverage band.

2-4 MC continuous coverage band.

31 M band.

25 M band.

19 M band.

16 M band.

Note. With the signal generator modulated 30 percent at 400 cycles, the final sensitivity measurements are taken at standard output of 50 mw across a 3 ohm load.

83. I-F Alignment Procedure

(fig. 28)

a. Adjust the if transformer cores with alignment wrench O 14 supplied with the receiver. This wrench should be inserted through the hole in the top of the can to adjust the top core of the transformer, then lowered through the top core to adjust the bottom core. Be careful when aligning these transformers to keep each core approximately centered in relationship to its associated coil. If this is not done, it is possible to advance the top core beyond, and the bottom core above, its associated coil. This would result in an incorrect coefficient of coupling and unstable and improper alignment.

Note. Alignment wrench O 14 is a special purpose tool designed primarily for adjusting the i-f and r-f transformers. Do not use the alignment wrench as a screwdriver or to turn trimmer capacitors.

b. Set the signal generator to 455 kc, modulated 30 percent at 400 cycles, and connect its output through a 0.1-uf blocking capacitor to the stator terminal of capacitor C2C. The i-f signal is effectively applied to the 1L6 converter signal grid (pin 6). Do not set the signal generator output any higher than is necessary to provide a usable reading on the output meter (about 10 mw). Connect the ground lead of the signal generator output cable to the B- bus of the receiver.

c. With the alignment wrench, adjust the top and bottom cores of i-f transformer T2 for maximum output indication on the output meter.

d. Adjust the top and bottom cores of transformer T1 for maximum output indication on the output meter.

e. Repeat the adjustments given in c and d above.

84. Broadcast Band Alignment

a. Couple the signal generator output lead through a 0.1-uf blocking capacitor to the grid (pin 6) of r-f amplifier tube V1. Leave the ground load of the signal generator connected to the B - bus of the receiver.

b. Set the signal generator to 1,500 kc, modulated 30 percent at 400 cycles.

c. Press the BC band selector switch.

d. With the ganged tuning capacitor completely meshed (1ow frequency end), mechanically set the dial pointer horizontally across the dial scale.

e. Turn the tuning control so that the dial pointer is set to 150 on the broadcast scale.

f. Adjust oscillator trimmer capacitor C2F (fig. 28) for resonance at 1,500 kc. Do not set the signal generator output any higher than is necessary.

g. Adjust r-f trimmer capacitor C2D (fig. 28) for maximum output indication on the output meter.

h. Set the signal generator to 600 kc, modulated 30 percent at 400 cycles.

i. Turn the tuning control so that the dial pointer is set to 60 on the broadcast scale.

j. Rock the gauged tuning capacitor while adjusting broadcast padder capacitor C13 (fig. 30) for maximum output indication on the output meter.

k. Repeat the operations outlined in b through j above.

1. Repeat operations outlined in b through g above.

m. Figure 34, which shows the switch connections for broadcast operation, is shown for reference purposes.

(Note: not presented on web page)

85. 4-8 MC Band Alignment Procedure

(fig. 31)

a. Disconnect the whip antenna and connect the signal generator output lead in series with a 20-uuf capacitor to the whip antenna tip jack. Connect the ground lead of the signal generator to ground terminal G.

b. Press the band selector switch marked 4-8 MC

c. Set the signal generator to 7.8 mc, modulated 30 percent at 400 cycles.

d. Turn the tuning control so that the dial pointer is set to 7.8 mc on the 4- to 8-mc scale.

e. Adjust oscillator trimmer capacitor C40A for resonance at 7.8 mc.

Note. C40A has two resonant points. The correct one is near the maximum capacity of the trimmer. The second point is the image frequency, and care must be taken to avoid selecting the image.

f. Adjust r-f trimmer C35A for maximum output indication on the output meter. If two resonant points are noted, use the one nearest maximum trimmer capacity.

g. Set the signal generator to 4.2 mc, modulated 30 percent at 400 cycles.

h. Turn the tuning control so that the dial pointer is set to 4.2 mc on the 4- to 8-mc scale.

Note. There may be two resonant points on the shortwave coils in Radio Receiver R-520/URR. The correct setting for the oscillator coil cores is the one farthest from the open end of the coil. Images are created when the core of the oscillator on a shortwave band is set to the wrong peak. It is very important that the core be set correctly. The correct resonant point for all other coil cores is the peak nearest the open end of the coil.

i. Adjust the core of oscillator coil L18A, L18B for resonance at 4.2 mc while rocking the ganged tuning capacitor.

j. Adjust the core of r-f coil L12 for maximum indication on the output meter.

k. Repeat the operations outlined in c through j above.

1. Repeat operations outlined in c through f above.

m. Figure 35, which shows the switch connections for 4- to 8-mc operation, is shown for reference purposes.

(Note: not presented on web page)

86. 2-4 MC Band Alignment Procedure

(fig. 31)

a. Use the connections outlined in paragraph 85a.

b. Press the band selector switch marked 2-4 MC.

c. Set the signal generator to 3.9 mc, modulated 30 percent at 400 cycles.

d. Turn the tuning control dial so that the pointer is set to 3.9 mc on the 2- to 4-mc scale.

e. Adjust oscillator trimmer capacitor C40B for resonance at 3.9 mc. This is the resonant point nearest maximum trimmer capacity.

f. Adjust r-f trimmer capacitor C35B for maximum output indication on the output meter.

g. Turn the tuning control so that the dial pointer is set to 2.1 mc on the 2- to 4-mc scale.

h. Set the signal generator to 2.1 mc, modulated 30 percent at 400 cycles.

i. Adjust the core of oscillator coil L19 for resonance at 2.1 mc while rocking the ganged tuning capacitor.

j. Adjust the core of r-f coil L13 for maximum indication on the output meter.

k Repeat the operations outlined in c through i above.

1. Repeat the operations outlined in c through f above.

87. 31 M Band Alignment Procedure

(fig. 31)

a. Use the connections outlined in paragraph 85a.

b. Press the 31 M band selector switch (9.4-9.8 mc).

c. Set the signal generator to 9.6 mc, modulated 30 percent at 400 cycles.

d. Turn the tuning control dial so that the pointer is set to 9.6 mc on the 31 M scale.

e. Adjust the core of oscillator coil L23 for resonance at 9.6 mc.

f. Adjust the core of r-f coil L17 for maximum indication on the output meter while rocking the ganged tuning capacitor.

88. 25 M Band Alignment Procedure

(fig. 31)

a. Use the connections outlined in paragraph 85a.

b. Press the 25 M band selector switch (11.6-12.0 mc).

c. Set the signal generator to 11.8 mc, modulated 30 percent at 400 cycles.

d. Turn the tuning control so that the dial pointer is set to 11.8 mc on the 25 M scale.

e. Adjust the core of oscillator coil L22 for resonance at 11.8 mc.

f. Adjust the core of r-f coil L16 for maximum indication on the output meter while rocking the ganged tuning capacitor.

89. 19 M Band Alignment Procedure

a. Use the connections outlined in paragraph 85a.

b. Press the 19 M band selector switch (14.9-15.5 mc).

c. Set the signal generator to 15.2 mc, modulated 30 percent at 400 cycles.

d. Turn the tuning control so that the receiver dial pointer is set to 15.2 mc on the 19 M scale.

e. Adjust the core of oscillator coil L21 for resonance at 15.2 mc.

f. Adjust the core of r-f coil L15 for maximum indication on the output meter while rocking the ganged tuning capacitor.

90. 16 M Band Alignment Procedure

(fig. 31)

a. Use the connections outlined in paragraph 85a.

b. Press the 16 M band selector switch (17.5-18.1 mc).

c. Set the signal generator at 17.8 mc, modulated 30 percent at 400 cycles.

d. Turn the tuning control so that the dial pointer is set to 17.8 mc on the 16 M scale.

e. Adjust the core of oscillator coil L20 for resonance at 17.8 mc:

f. Adjust the core of r-f coil L14 for maximum indication on the output meter while rocking the ganged tuning capacitor.

g. Figure 36, which shows the switch connections for 16 M operation, is shown for reference purposes.

(Note: not presented on web page)

91. Chassis Installation

Install the chassis in the cabinet. Reverse the procedure outlined in paragraph 64 but do not install protective cover A3 (fig. 4) until after final adjustments are made (par. 92).

92. Final Adjustment

The following adjustments are made with the signal generator modulated 30 percent at 400 cycles.

a. Broadcast Band.

(1) Loop a turn of wire from the signal generator output lead around broadcast loop antenna E2.

(2) With the signal generator and receiver set to 1,500 kc, adjust antenna trimmer capacitor C2B (fig. 28) for maximum output indication on the output meter.

(3) With the signal generator and receiver set to 600 kc, rock the ganged tuning capacitor while adjusting C13 (fig. 31) for maximum output indication on the output meter.

(4) Repeat the operation described in (2) above.

b. 4-8 Mc Band.

(1) Connect the signal generator output lead to an antenna 3 feet long and place it at a distance of approximately 1 foot from extended whip antenna El.

(2) With the signal generator and receiver set to 7.8 mc, adjust antenna trimmer capacitor C31A (fig. 31) for maximum output as indicated on the output meter.

(3) With the signal generator and receiver set to 4.2 mc, adjust antenna tuning coil L5 (fig. 31) for maximum output as indicated on the output meter.

(4) Repeat the operation described in (2) above.

c. 2-4 Mc Band. Adjust antenna trimmer capacitor C31B (fig. 31) at 3.9 mc and antenna tuning coil L6 at 2.1 mc, following the general procedure outlined in b above.

d. 31 Meter Band. Adjust antenna tuning coil L10 (fig. 31) at 9.6 mc, following the general procedure outlined in 6 above.

e. 25 Meter Band. Adjust antenna tuning coil L9 (fig. 31) at 11.8 mc, following the general procedure outlined in 6 above.

f. 19 Meter Band. Adjust antenna tuning coil L8 (fig. 31) at 15.2 mc, following the general procedure outlined in 6 above.

g. 16 Meter Band. Adjust antenna tuning coil L7 (fig. 31) at 17.8 mc, following the general procedure outlined in 6 above.

h. Protective Cover. Install protective cover A3 (fig. 4).

(Note: not presented on web page)