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T. V. George VU2TV 3205 Sector 27D Chandigarh* India

Hams in most of the world are lucky enough to be able to select their receiver from a vast variety of factory-made equipment, but in places like India, where there are relatively few amateurs, it is difficult to obtain even surplus communications equipment As a result, you aren't apt to find many Indian hams with modern equipment. The only choice left is to make the most o the surplus equipment that is available.

Most of the surplus receivers available to us were quite suitable for operation during the 40*s, but with modern operating techniques and the number of stations on the air, they are not suitable for present day operation. In this article I will present a few ideas on how to make the old BC 348 a moderate receiver for present day use.

li is converted into a double-conversion receiver with selectable upper and lower sideband. The second if of 85 kHz (obtained from an old BC-453) maintains good selectivity for normal operation. The front panel controls include main tuning, band switch, rf, if and audio gain controls, upper or lower sideband selection> antenna trimmer, BFO, calibration check, ANL, AVC on/off-fast/ slow, AM or CW/SSB, tone and transmit/ receiver.

Circuit

The ECC189 in the first rf amplifier is connected in the cascode configuration to utilize its low noise and high gain properties. The second rf amplifier is a 6BA6 with an rf gain control in the cathode circuit. Delayed AVC is applied to the grids of these tubes rather than through the rf coils as in the original circuitry of the BC 348. The first mixer and local oscillator are the same as originally designed, but the mixer was rewired on a new rf module along with the rf amplifiers. The regulated voltage to the oscillator is from the original neon bulb regulator.

A 6BA6 is used in the first 915 kHz if stage. The cathode bias is varied along with tiie first 85 kHz if amplifier with the if gain control A 6BA6 in the second mixer converts the 915 kHz signal down to 85 kHz. The BC 453 if transformers are peaked at 85 kHz for maximum selectivity. These transformers are lightly loaded by tapping down on tlie winding, thereby maintaining the high 0 necessary for good selectivity. A 12AIT twin triode serves as two crvstal oscillators-one at 1000 kHz. the other at 830 kHz, Upper and lower sideband selection is obtained by switching the proper oscillator into the mixer circuit.

Two 6BA6 85 kHz if stages provide the necessary amplification, Also gain can be realized from these stages, their primary function is to provide good selectivity, Th^ir gain is controlled by varying the cathode biase with the if gain control.

The product detector in this dual conversion BC 348 consists of a heptode converter with a variable Hartley oscillator for the VFO. The incoming 85 kHz signal is applied to one grid and the local Hartley oscillator to the other—the product of the l wo signals is obtained across the plate load, if both signals are at nominal zero beat with each other, the oscillator signal replaces the missing carrier in a SSB signal. If the oscillation is off by 1 kHz, the result is a 1000 Hz audible note for CW reception.

In the AM mode a 6AL5 series rectifier demodulates the incoming AM signal* The detected AM signal is connected to either the series noise limiter, another 6AL5, or directly to the audio amplifier through the mode switch. The series noise limiter derives automatic bias from the incoming signal. The clipping level is controlled by the average carrier level, his type of limiter is only effective for AM signals and optimum operation is obtained at 30 to 40% modulation. It maintains best signal-to-noise ratio regardless of signal strength,

A 6AL5 provides shunt-type delayed AVC. The 6A1,5 cathode is maintained at a fixed level by a voltage divider; this voltage allows v3 bj7

VS 6ba6

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