All American Mohawk Corp

Service Schematic LYRIC Model SA-91

Note: All voltage readings listed are taken with all controls turned on full and no signal - usee 1000 ohms per volt-voltmeter

Tube operating voltages are shown at the respective tube elements.

IF Peak 175 KC

All American Mohawk Corp

Model SA-91 Data

Lyric Model SA 91 Receiver

The Lyric type SA 91 Receiver is a 9 tube superheterodyne, embodying the following circuits:

1	1 stage RF amplification	1 Type 58 Tube
2	First detector	1 Type 57 Tube
3	Oscillator	1 Type 56 Tube
4	1 stage IF amplification	1 Type 58 Tube
5	Second detector, AVC and NOISE suppressor	1 Type 55 Tube
6	First Audio Stage	1 Type 57 Tube
7	Second Audio Stage	1 Type 47 Tube
8	High voltage rectifier	1 Type 280 Tube

Inasmuch as the operation of the set up to the second detector follows conventional principles, no detailed discussion will be given here. The action of the automatic volume control and noise suppressor, however, will be described in detail. To make the operation of these circuits more evident they are shown isolated from the rest of the set in diagram #59 A.

The type 55 tube used in the second detector position consists of a standard three element tube, similar to the type 56 with the addition of two small diode plate elements placed at the lower end of the cathode.

Automatic Volume Control

The detector and automatic gain control functions are performed by the diode section of the type 55 tube which rectifies the energy sent to it by the intermediate frequency amplifier. The DC component of this energy passes through a network of high resistances and by-pass condensers to the control grids of the RF and IF tubes to control the amount of amplification in these stages.

An increase in signal strangth results in an opposite action increasing the amount of RF and IF amplification. The audio component of the signal rectified by the diode, is passed through the maual volume control which also serves as part of the diode resistance net work. The adjustment of this control sets the amount of energy passed on to the audio amplifier for furhter amplification.

Nosie Suppressor

The noise suppression system operates by blocking the first audio amplifier tube and is controlled by the AVC system. Reference to figure 59A will show that the control grid of the 55 tube derives its bias from the AVC circuit being connected to the same point as the IF grid circuit. The palate circuit of this tube includes a resistance int he grid circuit of the 1st AF stage and the "noise" suppressor control which is one section of the voltage divider.

When no signal is being received, no voltage is developed in the AVC system and consequently there is no bias on the control grid of the 55 tube. This permits current to flow in its plate circuit which builds up a bias across resistor "A" overbiasing the #57 first audio tube and preventing it from amplifying static or noise which is being picked up.

When a signal is picked up the condition is reversed as the voltage developed in the AVC biases the suppressor tube grid and stops the flow of plate current. This removes the blocking bias from the first AF tube and permits the amplification of the signal.

The function of the noise suppressor control is to limit the minimum signal level, which the set will receive. This control varies the plate voltage on the control tube. As the voltage is increased (control turned to left) the amount of signal necessary to unlock the control system is increased and as the voltage is decreased (control turned to right) the signal required to unlock the noise suppressor is decreased until at the extreme clockwise position the noise control is inoperative.

To set the control the receiver should be tuned to a point where no stations are heard with the noise control turned to the extreme clockwise position. The noise control should then be turned to the left just far enough to silence static and other noises.

Technical Data

Drawing no 59 Attached gives the complete circuit diagram of this receiver, electrical constants of parts and operating voltages on all tubes.

All voltage measurements should be made with a meter having a resistance of at least 1000 ohms per volt. The volume and noise controls should be turned to the extreme clockwise position and the set tuned between stations so that no signal is received while measurements are being made.

Balancing

Caution: When balancing radio frequency or IF circuits, be sure that the volume control is turned to the full "on" position and the output of the test oscillator adjusted to give a very weak signal. This is necessary tom minimize the automatic volume control action and to permit the most accurate adjustment.

Intermediate Frequency Circuits

The intermediate frequency amplifier of this receiver operates at 175 Kc. and an accurately calibrated test oscillator generating this frequency is necessary for ganging.

Current from the test oscillator should be fed into the set by removing the control grid cap on the type 57 detector modulator tube, and connecting the oscillator output terminals between the chassis pan and the control grid cap of this tube.

The IF transformers are tuned by adjusting the screws under the removable name plate on the rear of the chassis.

To align the RF circuits the test oscillator should first be set to some known frequency between 1400 and 150 Kc. and the set tuned so that the dial pointer indicates this frequency. The trimmer condenser of the oscillator section of the variable condenser (front section) should then be tuned until the test signal is received with greatest output.

There are two possible adjustments on the trimmer condensers at which this signal may be received; the proper adjustment is that at which the trimmer is set to minimum capacity; that is, the adjustment at which the trimmer plate is farthest out. When this has been done the trimmer condensers of the second and third variable condensers are to be st to give maximum output.

The set should next be balanced at approximately 1250, 950, 700 and 550 Kc. in the order mentioned as follows: