Motorola Conelrad monitor/receiver model DS-9660B

Motorola DS-9660B Conelrad monitor/ receiver

This Motorola Conelrad receiver has 6 switchable broadcast band tune positions with provisions for 5 crystals. This example came with four crystals installed including the standard 640 and 1240 KHz positions. The sixth switch position is a tunable setting. The unit was manufactured for Motorola Inc by the Multi-Products Company, makers of Multi-Elmac ham equipment. Several of the tubes in the set bear the Multi-Elmac logo. For an example of Multi-Elmac ham equipment, see the PMR-8 receiver.

Conelrad stood for CONtrol of ELectronic RADiation. It was established by President Truman in 1951. According to a civil defense pamphlet of the era, "Conelrad was devised to provide radio communications in a national emergency while denying enemy bombers the use of radio beams as an aid in finding targets. This is accomplished by having television and FM stations cease their regular transmissions and selected AM stations to go to either 640 or 1240 KHz."

All broadcast stations and even ham radio operators were required to have an appropriate monitor and to cease normal transmission in the event of a Conelrad alert. This relatively expensive Motorola monitor was likely used in a commercial broadcast radio or TV station. The unit is designed for rack mounting. This one came with the original cabinet.

Radios beginning with model year 1953 are marked with little CD triangles at 640 and 1240 on the AM broadcast dial. The CD stood for Civil Defense. The public could tune to either frequency for further instructions in case of an emergency. Conelrad was replaced by the Emergency Broadcast System (EBS) in 1963. For more information on Conelrad, EBS, and subsequent systems, see Barry Mishkind's radio history site

The red Alert light comes on until the unit warms up.
After the monitor is operational, the alert light shuts off until activated.
The smaller neon light under it is lit indicating a weak carrier.
A control on the rear panel allows for adjustment of signal strength.

Motorola Conelrad DS-9660B(56k)

Repairs and circuit info

During initial power-up tests, discovered a bad power supply choke. Replaced it. All of the switches and controls needed cleaning with deoxit to solve intermittent contact problems. Also needed deoxit on some of the tube sockets and tube pins.

The alert and carrier detect pilot lights are working properly. This circuit uses a 2D21 thyratron to drive a relay for the alert pilot light. I have not tested the 1000 Hz tone Alert triggering circuit. Terminals on the rear panel allow connection to the relay for driving a remote warning alert. Terminals are also available for a remote speaker.

I will use the set to listen to my favorite AM broadcast stations using primarily the tunable switch setting. The dual stage IF in this set is 265KHz. The crystals are for the frequency of interest plus 265 KHz. For example, the 640 setting uses a 640+265= 905 KHz crystal. The 1240 setting uses a 1240+265= 1505 KHz crystal.

Update When I repaired this set, I did not have a schematic. I have recently received a copy of the manual courtesy of Tim White. Thanks Tim.

12-22-02; Update 2-9-10

The Hammarlund Super-Pro 210LX was the previous item on the bench.

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