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überarbeitet am 19.10.2010

Double conversion, Wadley Loop circuitry

Analog dial, linear, ca. 5 kHz

AM, USB / LSB

0,5 - 30 MHz

Selectivity -6/-30 dB
AM 7/13 kHz, SSB 4/8 kHz

Sensitivity
SSB < 0,5 uV, AM < 1 uV

RF-Gain, Preselector, S-Meter

In the eighties, a shortwave receiver from the Far East has been sold in several ham radio and electronics shops in Germany, but it's designation Century-21 made it difficult, to establish a relationship to a well known radio manufacturer in my mind. The set turned out to be of a similar contruction to the Drake SSR-1 and probably technically almost identical to the Standard C-6500. Maybe one of the readers might have a better understanding, which company was the actual manufacturer of the set.
This set was found to be easily modified and serviceable and could be encountered quite often in the radio shacks of newcomers to the DX hobby or as a secondary receiver in ham radio shacks. It can be found from time to time in the used market or in electronic auction platforms.

The set has a physical size of 340 x 290 x 156 mm and is portable with it's weight of 6,4 kg, but it's not really a delight to carry it around when going abroad for holidays. It's rugged metal cabinet is quite easily accessible and there is aufficient room for modifications in the inside. Fitting the parts of a Century-21 cabinet together is a real pleasure compared to doing this to a Sony ICF-6800W.
The set can be powered by eight UM-1 cells, 12V DC from a car battery or using 220V AC from the mains. I found the set very useful running it in our mountain cottage relying on solar power from an accumulator.

The front panel has a very straightforward design, but the operation might seem a bit tricky, if You're not familiar operating a set based on a Wadley loop circuitry.
In the upper half of the front panel, You will recognize the speaker at the left hand and the tuning knobs on Your right. Select one of the MHz segments using the MHZ - TUNE knob, tune within the selected MHz mark for maximum noise. Then use the main tuning knob to chose the exact receiving frequency on the 000 - 999 circular dial, You can read the desired frequency with an accuracy of better then 5 - 10 kHz. This is one of the main differences to receivers from the same period equipped with analog dials with good bandspread in the lower frequency bands and very poor frequency readout on the high frequency bands. Use the PRE-SELECTOR knob for maximal signal strength.
In the bottom half of the front panel, You find from the left the headphones socket, the RF gain (high frequency gain) and Volume / AF gain controls, the bandswitch selecting not the receiving frequency but the segments of the preselector circuit, the reception modes switch and the CLARIFY control which helps You tuning in a single sideband transmission from "Mickey mouse voice" to optimum readibility.

On the rear, You find the fixed mains lead, the telescopic antenna and the sockets for the tape recorder output, the muting socket, the antenna and earth terminals and the fuse.

Operating the set will be an easy task, when You get used to the Wadley loop receiver's philosophy - if not, You might consider a badly detuned set as defective.
Use the VOLUME knob to switch the set on and adjust for a light noise, the RF gain has to be set to maximum / fully clockwise, MODE for AM reception and CLARIFY in the center position. For tuning in the Austrian International Service on 6155 kHz, set the BAND switch to 5-12 MHz, the MHZ tune control to the [ 6 ] mark in the inner circular dial and You should hear a slight noise. Then use the main tuning knob to set the outer circular main tuning dial to 155 and You should already recognize the speaker from Vienna, use the PRE-SELECTOR for peaking the signal to a maximum reading in the S-Meter and try to tweak the MHZ-TUNE for maximum signal strength. If You encounter distortion, usually when You feed the set from a longer outdoor longwire antenna, turn the RF gain down to a reading of 7 - 9 on the S Meter. For the reception of SSB transmissions, set the MODE switch to USB or LSB and use the CLARIFIY control to tune to the matching pitch, quite often You might find it useful to manually control the RF gain.

The set uses the Wadley Loop principle developed by T.L.Wadley and used in Racal's RA-17L for the first time. From a main crstal oscillator, the receiver tunes in to harmonic multiples of the 1 MHz oscillator crystal in the range of 45,5 - 75,5 MHz. A 42,5 MHz oscillator will mix down this signal to an intermediate frequency of 2,5 MHz and this signal will be mixed with the one of a tuneable 2 - 3 MHz VFO, this will determine the 000 - 999 kHz digits of the reception frequency. The intermediate frequency will me mixed to 455 kHz and after passing the I.F. filters and amplification, the signal will be demodulated.

In practical use, the Century-21 will have an impressive sensitivity to wake signals, it works well with the telescopic antenna and a random wire antenna, long wire and active antennas might cause overloading and will require attenuation of the signal using the RF gain control. When a signal will appear very faint, the signal strength might really be poor due to receiving conditions, the maximum of the MHz - control might be out of tune, the preselector might be out of tune or switched to the wrong band, the RF gain control might be reduced - but the risk of forgetting one control in a bad setting is high. Quite often You might encounter the calibration of the main tuning control 1 - 3 kHz out of tune, You might correct this carefully adjusting the L21 coil.
The Century-21 is a collector's item, it might do a good job as a secondary receiver in a DXers shack and it's a well suited receiver to start Your own modification project. Quite a few of the Century-21s are found in quite bad shape at hamfests and online auctions, You can't compare the set's ruggedness and stability after many year's of use to the one of a classic commercial "boatanchor" receiver.

It might be of interest that the circuitry is very similar to the one of the Drake SSR-1 and the Standard C-6500.

further reading:
d: Kurztest Century - 21, Günther Schellhorn, wwh 10 / 1986
d: Modifikationen am Century - 21, Günther Schellhorn, wwh 11 / 1987

© Martin Boesch 15.3.2008