Some surplus receivers, such as the BC-454, whose frequency coverage is 3 to 6 Mc, will not cover the 7-Mc band. Figure 1 is the diagram of a simple crystal-controlled, 7-Mc converter for such receivers. It uses a 12AU7, dual-triode tube and a 3.7-Mc band crystal from your transmitter (you won't be using it in the transmitter, while you are on the 7.2-Mc band), plus a couple of resistors and condensers.
The converter may be built in a small aluminum utility box and mounted on top of the receiver near the antenna terminal. It obtains its power from the receiver. The combination becomes a dual-conversion superheterodyne, and all tuning is done on the BC-454,
The frequency to which the converter responds is equal to the sum of the crystal frequency and the BC-454 frequency. For example, with a 3700-kc crystal, to receive a 7175-kc signal, the BC-454 is tuned to 3475 kc. And to tune to 7200 kc., the BC-454 is tuned to 3500 kc. Similarly, to cover the entire 7-Mc amateur band, the BC-454 is tuned between 3300 and 3600 kc. C1 is merely tuned for the loudest signal. For the Novice band, it may be peaked up in the center of the band and forgotten.
Other crystal frequencies will work equally well, but, as the 3.7-Mc Novice-band crystal is available, it might as well be used.
Fig. 1. The schematic and parts list for a simple 7-Mc crystal-control converter for BC-454 (3-6 Mc.) receiver.
Build in small aluminum box and mount on top or side of receiver near antenna terminal.
| Dual Triode Converter | |
|---|---|
| C1 | 50 μμfd., midget variable |
| C2 | 100 μμfd., mica |
| C3 | 270 μμfd., mica |
| C4 | 47 μμfd., mica |
| R1 | 2 megohm, 1/2w. |
| R2 | 47,000 ohm, 1/2w. |
| R3 | 27,000 ohm, 1/2w. |
| R4 | 47,000 ohm, 1w. |
| L1 | 18 t. #22 enam., close wound on 1" form. |
| L2 | 4 t. #22 enam., close wound, 1/4" below L1. |
| Tube | 12AU7 |