This NEMS-Clarke 2501A receiver (bottom) and associated spectrum display unit (top) was purchased and modified for me by Richard S. Flagg back in 1974. He added a narrow-band FM detector. The receiver tunes continuously over the range 55-260 MHz. I have used this receiver since 1975 to discover wideband telemetry transmissions and to receive narrowband FM voice and telemetry. I still use it to check on the presence of telemetry on 166.0 MHz from Soyuz and Progress spacecraft. The typical signal spectrum is easily recognized on the spectrum display unit.
In its original configuration this receiver could receive AM and CW. As you can see from the picture of the controls it has a variable frequency BFO (Beat-frequency oscillator). One wonders what the receiver was for!
If I need really good fidelity voice from a Russians manned spacecraft on 121.75 MHz or 143.625 MHz I use this old receiver, bougfht as a surplus item from the Swedish armed forces. It has a 40 kHz wide IF strip for FM, so the reciver demoudlates the voice without distortion! I use it rarely, but it is a nice old item.
I used it until the LCD display faded. What an experience to get the frequency right from the beginning!
I built this single channel receiver for the Soyuz voice channel at 121.75 MHz in the early months of 1974. It used a Vanguard Labs model 407 converter to bring the 121.75 MHz signal down to 10.7 MHz. That signal was fed into a Philips LP1185 IF strip that had had a 150-180 kHz bandwidth. The output of the IF strip was fed into a little AF amplifier to driver the loudspeaker and earphones. the tape recorder took its signal straight from the IF strip. I picked up some great siganls from Soyuzes with this equipment, but as interference on the air band grew worse, I don't use it any more. Here is a great recording from Soyuz 16, voice on 121.75 MHz, Anatoli Filipchenko calling "Ya Buran,.." at 1335 UT, Dec 5, 1974 (30 kB, RA). I did the testing of this device at the radio lab of the Stockholm power company.
To get a really powerful receiving system, Dick Flagg built me the preamp below that was mounted directly at the receiver input (it should have been at the antenna, but that was difficult to do).
