The Collins 51S-1 with it's metal cabinet painted in grey is a very smart set; with it's dimensions of 37,5 x 16,7 x 33,2 cm and it's weight of 11,8 kg, the receiver is only slightly bigger then JRC's top of the range receivers - a hollow state double conversion set in such a small cabinet is a suprising bit of engineering.
The 51S-1 is prepared to be powered by 115 or 230V AC, the power consumption of the 17 valves is 125W. The 51S-1A(F(=rack mount)) can be operated from 28V DC.

The different rear connectorsmake it obvious, that the 51S-1 has it's roots among the professional receivers: there are not only connectors for low and high impedance antennas but also for an external VFO, a 500 kHz IF output, and a connector for a long- and mediumwave converter.

The front panel gives a very basic first impression: in contrast to several older professional receivers from Collins, the 51S-1 comes with only nine frontpanel controls: The analog dial has an accuracy of better then 1 kHz, the MHz-digit is moved by the MHz-bandswitch and the 0.1 MHz digit is moved by the main tuning knob like an odometer, You hear a click when You go passed a ...99 / ...101 kHz step. You can adjust the zero line of the main dial with the "Zero set"-knob using the internal crystal calibrator signals or the signal of a frequency standard or time signal after hafing tuned to zero beat.
The mains switch in the left upper corner has a "Standby" setting (in this setting, the valves are heated and frequency stability is reached immediately after switching the set to "On", there is also a position to activate the crystal calibrator.
The RF gain and AF gain / Volume controles are situated at the left and the right of the main tuning knob. Between this and the AF gain control, You find the modes switch - upper and lower sideband filters can be switched independantly - and a control called "Rejection Tune" slightly above. This activates the Crystal Filter used to notch out unwanted signals.
The If bandwidth filters switching is coupled with reception modes: in AM reception a LC or optional a much more expensive 6 kHz mechanical filter is in use, for USB /LSB a 2,4 kHz or an optional 2,75 kHz mechanical filter is switched in the circuit, for CW reception a 800 Hz crystal filter or an optional narrow 200 Hz filter.
The S-meter in the right upper corner of the front panel can be switched to display the RF or the audio frequency level.

This is the place to watch out for the variants: earlier models up to production year of 1967 carry the "winged Collins logo", the later production models the "round logo"; in earlier sets, the S-meter is white, in the later variants it's darker brown coloured.

The high frequency signal from the antenna input will first pass a bandpass filter and the mechanically coupled automatic preselection stage. In the 0,2 - 2 MHz ranges, the signal is internally up-converted to 28-30 MHz.
In the 2 - 7 MHz band segments, the signal will - after the first RF amplification stage - be converted to 14,5-15,5 MHz, in the second mixer to 2 - 3 MHz and in the third mixer to the last IF of 500 kHz - thus triple conversion operation.
In the 7 - 30 MHz segments, the signal will be directly converted into 2 - 3 MHz in the first mixer and to 500 kHz IF in the second mixer stage double conversion operation in these ranges.
The 500 kHz IF will have to pass the 6 kHz LC filter in AM mode, the mechanical 2,4 kHZ uSB or LSB filter in SSB mode or the 800 kHz crystal filter in CW mode. After the Q-multiplier circuit with HF notch and two more RF amplification stages, the signal is sent for demodulation to a diode demodulator in AM and a product detector in SSB modes. Two more stages are used for AF amplification.

In practical use, the 51S-1 is still a superior AM and SSB receiver. It's sensitivity is good, thanks to the mechanically coupled preselection, the receiver can be fed from long wire antennas without unwanted signals from intermodulation. A minor drawback is the fact, that the choice of bandwidth filters is coupled to the reception modes, so You have to use ECSS tuning technique to make use of the excellent 2,4 kHz mechanical filter in crowded bands also for tuning in AM stations.
The mechanically tuned 51S-1 is well suited for searching signals within one shortwave bands, for checking several parallel frequencies of one broadcaster in different broadcast bands, retuning the receiver to all these frequencies results in turning the main tuning knob many many times... but this "problem" You will find an any older receiver - it took over 25 years until the first microprocessor controlled shortwave receivers providing electronic memories were available to the public. Using the optional external VFO gave You the possibility to switch to a second frequency within seconds, but these optional VFOs seem to be scarce.

Compared to other professional shortwave receivers built in the same years, operating a 51S-1 is really straightforward, the shortwave performance perfect, the design of the receiver in the desktop really smart - many points to explain why the prices for these sets on the used marked sre still above average.