CIRCUIT DESCRIPTION
SECTION 4
CIRCUIT DESCRIPTION
4.1. CENERAL. - The following paragraphs have been written to enable the owner of a 32V-3 to understand the functioning of his transmitter more full . This section should be read and understood before an extensive servicing is attempted.

4.2. CIRCUIT DESCRIPTION. -

4.2.1. RF CIRCUITS.
      a. OSCILLATOR. - A type 6SJ7 tube is employed in a highy stabilized master oscillator circuit to generate the controlling radio frequency voltage. This frequency generating unit is a linearly - tuning permeability tuned oscillator with a range of 1.6 to 2 megacycles. Sixteen turns of the main tuning dial cover this range. This provides 50 KC per revolution of the second harmonic (3.2 to 4 mc band). With the end points properly set up, the tuning curve is linear within one dial division of the ideal tuning curve on any of the bands in the operating range. The oscillator circuit is compensated for temperature changes and is entirely enclosed in a heavy aluminum case.

      b. INTERMEDIATE STAGES. - Following the master oscillator a type 6AK6 is employed in an untuned, Class A amplifier stage. This stage completely isolates the master oscillator from the remaining tuned stages. The 6AK6 drives a series of three frequency multiplier tubes, the first of which is a type 6AG7. The operating frequencies at the plate of the multiplier tubes for the different bands is given in the following table.
 
1ST MULT.6AG7 2ND MULT.7C5 3RD MULT.7C5
80m 3.5 mc 3.5 mc 3.5 mc
40m 3.5 mc 3.5 mc 7 mc
20m 3.5 mc 7 mc 14 mc
15m 5.75 mc 10.5 mc 21 mc
11m 6.8 mc 13.6 mc 27 mc
10m 7 mc 14 mc 28 mc
        Plate screen and filament power for these stages is obtained from the low voltage power supply. Gang tuning of the multiplier stages is ob- tained by moving powdered iron cores, attached to a common platform, in and out of the plate coils which are wound to give linear tuning. This platform to which the iron cores are attached is also ganged to the master oscillator tuning for complete, single control tuning of the exciter stages. Band switching is accomplished by adding extra padding capacity across coils by means of the band switch in all cases except that of the 14-mc output of the third multiplier where an inductance is switched in parallel with the existing 40 meter inductor to lower the tuning inductance for 14 mc output.

      c. POWER AMPLIFIER STAGE. - A type 4D32 tetrode power amplifier tube is used in the PA stage. This tube always operates as a straight amplifier. The plate circuit is tuned by a combination pi-network and "L" network which is band-switched along with the multiplier stages. The combination network reduces the output impedance to around 50 ohms on all bands by means of inductance and capacitance switching. The output network will actually operate satisfactorily with antenna impedances in the range of 26 to 200 ohms. It is also affective in reducing harmonic output of the transmitter. During phone transmission, the screen grid and plate of the 4D32 are both modulated. Plate and screen voltage is obtained from the high voltage supply while filament power is obtained from the low voltage plate supply transformer. The tube is biased with 75 volts of fixed-bias plus some grid leak bias.

4.2.2. AUDIO CIRCUITS. - The first and second audio amplifier consists of a type 6SL7 tube operated as a cascade amplifier. A volume control, R205, is located in the grid circuit of the second amplifier stage. The driver stage employs a type 6SN7 tube with the two triode sections operated in parallel to drive the modulator stage. The modulator stage utilizes a pair of type 807 tubes connected in a push-pull circuit and operating class AB2. The output of the modulator is coupled to the final amplifier by transformer, T202, to modulate the plate and screen of that stage. During CW operation, the secondary of the modulation transformer is shorted out by S302A. Bias for the modulator tubes is adjustable by R305, is obtained from the low voltage supply and is regulated by the voltage regulator tube, V304, type VR-75. The secondary of the modulation transformer has a 500-ohm tap provided for supplying 60 watts of audio power to an external load.

4.2.3. HlGH VOLTAGE SUPPLY. - The high voltage transformer is energized when the contacts of relay, K301, are closed. The high voltage supply employs two type 5R4GY rectifier tubes connected in parallel in a full wave circuit. The output is filtered by a single section choke input filter. This supply furnishes voltage for the plate and screen of the final amplifier and plate voltage for the modulator tubes. The amount of output voltage from this supply may be either 600 volts or 700 volts, depending on the position of the tap switch, S305, in the primary winding of the high voltage transformer, T302. For the same power input, the efficiency of the final amplifier improves with the higher operating voltage. The tube manufacturer recommends no more than 600 plate volts for phone operation, but this is for CCS rating. A pair of 15-ohm resistors are connected in series with HV plate transformer primary for "tune-up". These are shorted out when operating.

4.2.4. LOW VOLTAGE 8UPPLY. - Transformer, T301 furnishes power for both the voltage plate supply and the filament of all tubes in the transmitter. T-301 is energized by closing the LV switch, S304, Three separate windings on transformer, T301, furnishes filament power to the tubes, The low volt- age plate supply employs a type 5Z4 rectifier tube in a full wave circuit with a two section choke input filter. This supply has a total output voltage of approximately 315 volts, 240 volts is supplied to the audio amplifier, oscillator, buffer and multiplier stages. Bias voltage for the modulator and final amplifier stage is furnished by this supply, It also supplies voltage for the operation of relay, K301.