C - 6500 Communications Receiver
Manufactured by Standard Communications Corp., Japan.
The heavyweight RA-17L was the first communications receiver introduced in 1959 which used the principle of the „Wadley Loop“ developed by Dr. T. L. Wadley. The tuning is done by a linear oscillator covering 1 MHz, to which harmonics of a crystal oscillator at intervals of 1 MHz are mixed and to generate the desired reception frequency. In the amateur sector, the concept of the Wadley Loop was taken up in several sets, since it was made possible to construct a receiver with good frequency dial accuracy with simple means even without the use of a frequency counter, which was expensive at the time. After the Yaesu FRG-7, the Drake SSR-1 and the Barlow Wadley XCR-30 portable receiver, relatively inexpensive sets from Japan came onto the market in the 1980s, they were sold under various labels. The standard C6500 seems to have quite close relations to the Century-21, the set was robustly built and could be easily modified.
- Principle: Double conversion superhet, IF 2500 / 455 kHz
- Operation modes: A1 with BFO, USB /LSB, AM (A3)
- Frequency Range: MW, SW (0.5 - 30 MHz)
- Frequency display: Analogue dial, +/- 5 kHz
- Frequency memory: none
- Signal processing: Attenuator, preselector
- Sensitivity SSB < 0.5 μV, AM (A3) < 1 μV / Selectivity: AM 7 / 13 kHz; SSB 4 / 8 kHz kHz (-6/-30 dB).
- Mains operation: 117, 220 V
- 340 x 150 x 240 mm, weight 6.4 kg
The Standard C-6500 is a station receiver built in Japan in the 1980s, designed according to the Wadley Loop principle. In these sets, a 1 MHz range is covered by an oscillator with linear frequency response, and a stable MHz signal derived from the harmonics of a quartz oscillator ist mixed with it, thus generating the reception frequency. By combining a MHz dial and a dial with marks 0 - 1000 for the kHz digit, a desired reception frequency can be tuned in with quite high accuracy.
The Wadley Loop technology permits a linear dial reading in the entire reception range, so in contrast to receivers such as the Trio-Kenwood 9R59 or even the older Grundig Satellit sets, the 10 kHz dial markings in the medium-wave range and at 29.900 MHz have the same distance. With the Trio, for example, a medium-wave frequency can be found on the dial with quite good accuracy, but the same 5 mm on the dial overlap a range of 2 MHz when it's tuned in the 10 m - short-wave band.
The receiver with its black sheet steel housing is 340 x 150 x 240 mm wide (wxhxt), the set has about the same size as the FRG-7700 from Yaesu. By loosening four screws each, the metal cover with the side panels and also the base plate with the four rubber feet can be lifted off, and you have acces to the PCBs inside.
From above, the triple variable capacitor to tune the VFO and the variable capacitor to tune the MHz harmonics of the MHz oscillator are clearly visible. On the top left the telescopic antenna is protruding from the set, at the back of the top the cover the battery compartment for the 8 UM-2 batteries is hiding. At the front, on the left, there is the speaker grill and the headphone socket, which are mounted on the front panel.
In the bottom row of controls from the left are the volume control (located directly below the main power switch); when pulled, an attenuator (to prevent overloading of the input stage) is switched on. In contrast to the Century-21, there is no control for the RF gain provided. The band switch to the right of it selects the preselector band segments 0.5 - 1.5 MHz, 1.5 - 5 MHz, 5 - 12 MHz and 12 - 30 MHz. To the left below the large tuning knob is the operating mode switch for AM, USB and LSB, the sidebands with corresponding frequency offset can be selected here. On the right, below the tuning knob, the fine-tuning knob for CW and SSB is marked as „clarifier“.
In the middle row of controls, next to the loudspeaker, is the main switch. The rotary control next to the right is used to set the desired MHz band and to operate the outer MHz dial, the large main tuning knob is used to tune the reception frequency for the kHz digit. Between the MHz band and the main tuning knob, the preselector is located, which is important in a Wadley Loop design. If the input signals of the entire shortwave range were to reach the mixer stage, unwanted mixing products would occur and strong signals from the next MHz range would punch through. The preselector, which is quite steep-sided and easy to operate, only allows a small range of the band spectrum to pass through; there is no coarse frequency dial as found on the Sony ICF-6800W or the Barlow Wadley XCR-30.
In the top row on the front panel, from the left, are the pilot lamp and the push button switch to acitave the dial illumination in battery operation, on the right the large S-meter and the two circular dials for reading the MHz band and the kHz digits of the operation frequency.
On the back of the unit there is a connection for an external DC power supply, a mute connector (when the short-circuited plug is removed or the contact is disconnected, the receiver is muted, e.g. for operation together with a transmitter). The AF is connected to a record output for recording the programme with a tape recorder.
To operate the radio, switch on the set, turn up the volume control until a hiss is heard. Then select the appropriate band, for example 5 - 12 MHz to receive the BBC on 6.195 MHz. Now tune to 6 MHz with the MHz knob, near the middle of the marker the noise becomes noticeably louder. With the main tuning knob now adjust the kHz digits to 195 kHz, now something should already become audible. Finally, set the preselector to maximum. If the set was last operated in a completely different frequency range or the preselector was set to a completely different band range, the receiver may have remained mute until this step. If there is an incoming signal, set the MHz knob to maximum volume and S-meter deflection, retune the preselector and, if necessary, use the main tuning knob and the fine tuning „Clarify“ to optimally adjust the signal. For frequency searching in a band section or searching for a weak DX station, I first set the receiver to signal maximum using a strong station transmitting nearby, the MHz and also the preselector setting then no longer need to be adjusted when searching for the wanted signal.
As with all receivers using the Wadley loop technology, setting a frequency is somewhat more demanding; if the receiver is out of tune or operated incorrectly, you will hear nothing at all and the receiver will be prematurely judged to be defective. Since incorrect operation can quickly spoil the pleasure of using the receiver, I have gone into more detail about the operation.
As soon as you have mastered the technology to a certain extent, you can get good performance from this slightly outdated and simply constructed set and the frequency dial accuracy is much more precise than with older sets with a simple band spread or even with modern cheap „world band radios“ from a discount store.
The uncomplicatedly constructed set with the service-friendly cabinet and sufficient space for own developments invites those who know something about it to modifications.
Double conversion, frequency processing based on the Wadley Loop circuit.
The set is solid state.