Standard Communications Corp., Japan

Standard C 6500 Communications Receiver

 
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Standard C-6500
 
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überarbeitet am 2.11.2010

The classic "boatanchor" maritime receiver RA-17L was the first set in 1959 using the Wadley Loop circuitry developed by Dr. T. L. Wadley. The main principle relies on tuning in to a harmonic multiple of a 1 MHz crystal oscillator and using this amplified signal for controlling the first mixer, in which usually a signal from a linear oscillator tuning over one MHz - range is mixed which will result in the desired reception frequency.
In many amateur receivers, this principle has been used to construct shortwave receivers with linear frequency response and good frequency accuracy in the whole shortwave spectrum. After the Yaesu FRG-7, the Japan made Drake SSR-1 and the portable set Barlow Wadley XCR-30, in the eighties, some Japanese sets appeared on the market and were available for a reasonable price. The Standard C6500 is of very similar appearence to the Century-21 with a nearly identical circuitry.

Double Conversion,

Analog dial +/- 5 kHz

AM, USB, LSB

Sensitivity
< 1,5 MHz AM 5 uV / SSB 1 uV
1,5 - 30 MHz AM < 1 uV / SSB < 0,5 uV

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

Attenuator, Preselektor

The Standard C-6500 is a desktop shortwave receiver built in Japan in the eighties using the Wadley Loop circuitry. A signal of an oscillator with a linear frequency response over one MHz band will be mixed with a harmonic multiple signal of a 1 MHz crystal oscillator, tuned by the MHz control knob. When the 1 MHz tuning control reading is combined with the setting of the 000 - 999 kHz dial, You can directly tune in to the desired reception frequency.
Due to this technology, a linear frequency readout will show You the reception frequency with an acceptable accuracy in the whole frequency coverage of the set. Unlike in receivers like the Trio - Kenwood 9R-59DS or the older Grundig Satellit receivers, You will find between two stations 10 kHz apart on the dial the same distance in mediamwave band as in the 29 MHz segment. In the older Trio set, it's easy to tune in a certain mediumwave channel, but it's nearly impossible to tune in a certain frequency in 10 meters, es the same 5 mm of distance will cover a frequency range of 2 MHz in the upper end of the dial.

The receiver with it's black metal cabinet has a physical size of 340 x 290 x 156 mm and is portable with it's weight of 6,4 kg, You can easily do an inspection of the inside after loosening 4 screws of the top cover and another four screws of the bottom cover.
from top from underneath

You can easily detect the triple tuning variable condensator at the right of the main tuning mechanism / the VFO and the double variable condensator of the MHz - bandswitch left to the middle of the main PCB.

The set can be powered by eight UM-1 cells, 12V DC from a car battery or using 220V AC from the mains. In the top cover, You find the internal telescopic antenna.

The front panel has a very straightforward design: At the left hand of the front panel, You will recognize the speaker and the headphones socket below. At the right, You find the tuning knobs on Your right. Select one of the MHz segments using the MHZ knob, tune within the selected MHz mark for maximum noise. Then use the main tuning knob to tune in 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. Use the PRE-SELECTOR knob to tune for maximal signal strength.
Below the main switch, find the Volume / AF gain control, when pulled out, an attenuator is activated. The bandswitch selector ist not used to switch the receiving frequency but the segments of the preselector circuit; next to this, You find 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 sockets for the tape recorder output and muting, 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 main switch to turn the set on and adjust the VOLUME for a light noise, MODE should be set to 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 upper circular dial and You should hear a slight noise. Then use the main tuning knob to set the inner 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, try to improve the reception by pulling out the volume knob and activating the attenuator. 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.

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. 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 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.
The Standard C6500 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. Some C6500s 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 Century-21.

 

© Martin Bösch 15.3.2008