Suggested modernistic cabinet for police call alarm is given above, left. Paint edges aluminum, sides glossy black enamel.
(right) Mounted in attractive cabinet atop regular receiver, this police call alarm automatically brings in local police calls over regular programs. One tube circuit is used.

Hear POLICE CALLS, Music, at Same Time
Modern Mechanix' Radio Builders' Manual, 1933

With this radio call alarm connected to your broadcast receiver you can listen in to broadcast programs at any time, yet hear every police call sent out from your local police station without touching the receiver controls. These calls are heard with enough volume to make them easily understood regardless of the type of program being received, provided the police radio station is within ten miles of your home.
   

The alarm is simply a regenerative detector circuit tuned to the police wave band and coupled into the audio circuit of your receiver. All power for the alarm is taken from your set.

For sets with 2-1/2 volt filament tubes, such as the '27, '24A, '35, '57, '2A5, etc., a '56 tube should be used. Tap off this filament power as meat to the power transformer as possible. For sets with 6.3 volt tubes, use a '37 tube in the alarm.

Suggested panel mounting for parts pictured above. Coil form can be 2-1/2" long. Make 2mh. choke by winding 200 turns of No. 32 d.c.c. wire, jumbled, on lead pencil core.

To operate, turn on the receiver and the police call alarm. With no broadcast program tuned in, advance the potentiometer on the alarm until the hissing sound of regeneration is heard. Slowly tune the .00035 mfd. condenser until you hear a call from your local police station. Readjust the potentiometer for clearest reception of this police signal. No further adjustments are necessary. The filament switch of the alarm will turn it off at any time when police calls are not wanted.

The coil, L1 is connected in series with the aerial of your receiver. Other connections to the receiver are made with a five wire cable. B plus voltage is taken from the plate supply of your receiver by connecting to the plate of the output tube. A radio tube chart or manual will be of great help to you in making the necessary tube socket connections here.

Potentiometer may be any value from 50,000 to 250,000 ohms. Ratings for .25 mfd condenser is 400 volts. Other condenser sizes not critical--use closest you have on hand.

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