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====== Century-21 ======
Manufactured by Seiki Electronics Inc, Japan.

In the 1980s, a Far East made receiver appeared in the shops, which with it's designation Century-21, did not evoke any familiar associations, but the set is technically similar to the [[Drake]] [[SSR-1]] and probably practically identical to the Standard [[C-6500]]. The set, which is easy to service and modify, was a quite popular entry-level receiver for DXing or used as a secondary receiver by shortwave amateurs and occasionally turns up in the second-hand market.

{{:images:divers-century-21.jpg?600|Seiki Century-21}}
===== Technical data =====
  * [[Principle]]: [[Double conversion superhet]], [[IF]] 2500 / 455 kHz
  * [[Operation modes]]: [[A1]] with BFO, [[A3|AM (A3)]]
  * [[Frequency range]]: 0.5 - 30 MHz
  * [[Frequency display]]: Analogue display, linear, approx. 5 kHz
  * [[Frequency memory]]: none
  * [[Signal strength indicator]]: [[S-meter]]
  * [[Signal processing]]: [[RF gain]], [[preselector]]
  * [[Features]] 
  * [[Sensitivity]]: [[SSB]] < 0.5 μV, [[A3|AM (A3)]] < 1 μV / [[Selectivity]]: AM 7/13 kHz, SSB 4/8 kHz (-6/-30 dB).

==== Power supply ====
  * [[Mains operation]]: 220 V
  * [[Batteries]]: 8 x 1.5 V ([[UM-1]]), 12 V DC

==== Dimensions ====
  * 335 x 152 x 250 mm, weight 6.4 kg

==== Accessories ====

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{{:images:divers-century-21-front.jpg?600|Seiki Century-21}}
===== Operation =====
With it's dimensions of 340 x 290 x 156 mm, the receiver has the size of a typical tabletop receiver. The metal case is very easy to open and leaves enough room for modifications to an experienced hobbyist; working on a Century-21 is a relief compared to reassembling an [[ICF-6800W]].

The set can be operated from eight [[UM-1]] mono cells, from 12 volts DC from a car battery or from 220 V mains voltage. The dimensions and the weight of 6.4 kg make the set portable, but not more. 

It has found a good use as a secondary receiver in our mountain cottage, where it is running on a 12V solar power supply; otherwise I would rather not take it along on holidays.

The **front panel** is simply structured for a set from the 1980s, but the operation is not without problems if you never have had the experience to operate a [[Wadley loop]] circuit.

In the upper half of the front panel, the loudspeaker is located on the left and the tuning elements on the right. With the MHZ - TUNE control the desired MHZ - band is selected, the receiver must be set to the noise maximum point within the corresponding MHz marking of the inner round dial. With the main tuning knob, you set the kHz - digit on the outside 000 - 999 dial with an accuracy of 5 - 10 KHz. In comparison with other amateur radio receivers from the same time period (such as the Trio-Kenwood [[9R-59DS]] or [[R-300]], the early Grundig Satellit without digital display), the improved frequency dial accuracy compared to conventional sets becomes apparent, but newer sets with PLL digital frequency synthesis usually outshine the [[Wadley Loop]] technology. For good reception, the PRE-SELECTOR must be tuned to maximum signal.

The lower half of the front panel contains, from the left, the headphone jack, the [[RF gain]] and [[AF gain]] controls, the band selector which switches the different preselector ranges, and the mode switch that can be used to select one of the two sidebands in single-sideband mode. The "Clarify" control is a fine-tuning function that is useful for SSB and CW reception.

The connectors on the rear panel are easily understood, next to the mains cable entry (no socket) and the telescopic antenna mounted at the rear, the jacks for the tape output, the mute switch (MUTE) and the antenna and earth connections, as well as the fuse holder are found from the left to the right.

The operation is in itself unproblematic if you know the philosophy behind the receivers based on the [[Wadley Loop]] circuitry, but otherwise it can make you quite nervous and might lead to the false suspicion that the set is faulty.

VOLUME is used to switch the radio on, turn it up until there is some noise from the loudspeaker, RF-GAIN has to be set to the right maximum stop, MODE to AM for radio reception and CLARIFY to the middle position. If, for example, you want to receive the Austrian Radio Intl. on 6155 kHz, set the frequency range from 5 - 12 MHZ on the BAND switch (which is in fact a [[preselector]] range switch). Use MHZ-TUNE to tune to about the middle of the [ 6 ] mark on the inner circular dial; a clear hiss should be audible. Use the main tuning knob to tune to 155 on the outside round dial, and the transmitter from Vienna should be audible. The PRE-SELECTOR is used to tune to maximum deflection on the S-meter, you might again carefully adjust the MHZ-TUNE control. If distortion and overloading occur, which is not uncommon especially when a long external antenna is connected, the RF-GAIN can be slightly reduced until the S-meter reads around 7 - 9. To receive SSB transmissions, the mode switch must be set to USB or LSB and the CLARIFY control is used for fine tuning until the speech no longer sounds like Mickey Mouse and the music sounds acceptable. In this case, the RF-GAIN must often be adjusted manually, a narrower IF filter is used for SSB reception.

Electronically, the circuitry is based on the [[Wadley Loop]] principle - this circuitry was used for the first time in the [[RA-17]] by [[Racal]]: stable harmonic oscillations are derived from a mother crystal, which lead to a crystal controlled high stability signal on each full MHz in the range between 45.5 - 75.5 MHz. The MHZ-TUNE variable capacitor is used to tune to specific desired harmonic, the signal is converted down to an [[intermediate frequency]] of 2.5 MHz by means of a 42.5 MHz oscillator, and the signal is mixed with a linear VFO covering a 1 MHz segment, in this set from 2 - 3 MHz. The dial of this VFO, labelled from 000 - 999, is used to tune the KHz digit of the operation frequency; the intermediate frequency is thus converted to 455 kHz and, after passing the IF filter and amplification, fed to the demodulator.

In practical use, the Century-21 impresses with its high sensitivity, it gets along with quite short auxiliary antennas or the built-in telescopic antenna, but it likes to overload if long-wire or active antennas are connected, so that attenuation is necessary. The IF filters give the receiver a useful selectivity, they can possibly be replaced by narrower band 455 kHz filters.

Tuning takes some getting used to, with the need to operate MHZ and kHz tuning, preselector and band switch, and rapid frequency changes are tedious. If a signal weakens, it may really be the antenna signal that is poor according to the propagation conditions, or the maximum of the MHz signal is not correctly tuned, the preselector is slightly maladjusted, the RF gain is still reduced, etc., etc., etc. - so something is forgotten easily! Often the calibration of the kHz dial is off by one to three kHz, which can be corrected by re-adjusting L21.

{{ :images:divers-century-21-back.jpg?400|Seiki Century-21}} All in all, the Century-21 is a collector's set, suitable as a secondary receiver for DXers and a grateful victim as a starting point for experiences with modifications. I have encountered several Century-21s in poor condition at radio flea markets. The ruggedness of commercial range receivers, which display still correct to one kHz after years of operation and perform almost like on the first day, cannot be achieved with the price compromise of circuitry and manufacturing technology.

It remains to mention that the schematic and circuit of the [[Drake]] [SSR-1]] and especially the standard [[C-6500]] are very, very similar to the Century-21.

===== Technical principle =====
Double conversion superhet, frequency processing with [[Wadley Loop]] circuitry

==== Components ====
The set is solid state.

===== Technical documentation =====

==== Development ====
{{gallery>:images?divers-century-21*.jpg&0&80x80&lightbox&showtitle&titlesort}}
==== Further information ====
  * [[https://www.radiomuseum.org/r/unknown_century_21century2.html|Seiki Century-21 on the website of www.radiomuseum.org]]