01/1972 html

Gimmicks and Gadgets A Crystal Filter for the Drake R-4B BY DAVID SUMNER,* K1ZND * P.O. Box 17401, West Hartford, CT 06117.
Heere IS an easy (though expensive) modification which may be performed on the Drake R-4 series of receivers to add a crystal ftïlter. No holes need be drilled and the receiver can be restored to its original condition in less than an hour. The modified receiver has an i-f system with a 6-dB bandpass of approximately 2.2 kHz and a 6 :60-dB shape factor of 1.5 : 1.The R-4 is a dual-conversion receiver with a four-pole, 6-kHz-wide crystal filter in the first i-f stage (5645 kHz). The purpose of this filter is to provide immunity to out-of-band signals and to assure adequate image rejection which might otherwise be a problem in a receiver with a 50-kHz second i-f. The companion T-4XB exciter/transmitter uses two 8-pole crystal filters at 5645 kHz -one for each sideband. If provision is made for impedance matching, it is possible to use the 8-pole transmitter filters in the receiver first i-f stage. The combination of the sharper crystal filter in the first i-f and the LC filter in the second i-f provides excellent selectivity for ssb operation. *Components* The 8-pole filters are available from the R. L. Drake Company at a cost of approximately $80. Included in the package are two	filters, a switch, and a mounting bracket. Additionally, a set of matching transformers is required (T2 and T3 in the T-4XB). These cost approximately $8 for the pair. The only other items needed are three feet of shielded table (RG174 or equiv.), a metal box, and three 1/4-inch-dia rubber grommets. Any aluminum box larger than 2 1/2 x 3 x 3 inches Will be satisfactory. *Step-by-Step Procedure* 1) Mount the switch, bracket, and filters in the enclosure as shown in the photograph. Include two grommets on the rear of the box. 2) Route a pair of 18-in. coaxial tables through the grommets on the rear of the enclosure and connect the center conductor of each cable to the center lug of each switch pole. Connect the shields of both cables to the nearest ground lug, respectively. 3) Remove the receiver top and bottom covers. 4) Locate T5 and T6 (the filter matching transformer and the existing crystal filter). Make a sketch of the connections to these transformers (for later reference).

Fig. 1 - Circuit diagram of the modified i-f transformer section of the R-4B receiver. Cl is a 27-pF diskceramic capacitor in the R-4A. ‘The R-4B uses a 21-pF capacitor. The filter assembly consists of two fi-ters, a ceramic switch, and a mounting bracket. T2 and T3 are the T-4XB transformer designations which replace the receiver T5 and T6 respectively.

The box is homemade. Two of the screws for securing the receiver top cover are used to hold the assembly in place. Press-on decals and a Drake knob add a final touch.
5) Disconnect all of the leads to T 5 and T6, then remove the transformers. It is not necessary to break or drill anything; the transformers are held in place with a pair of spring clips. 6) InstalI the new T2 and T3 (T-4XB nomenclature) transformers where T5 and T6 were mounted. T2 replaces T5 and T3 replaces T6. Note that in both cases the high-resistance windings (about 2 ohms) of both transformers are nearest the tube socket to which they are connected. See Fig. 1. The low-resistance windings of each transformer face each other. 7) Reconnect a11 of the wires and components to the lugs on both transformers. Exception: Omit the connections between the two transformer low-resistance windings. 8) Position a rubber grommet in the spare jack hole on the rear chassis apron. Route the shielded wires from the filter box through the grommet and along the chassis to the new i-f transformers. Connect these cables to the i-f transformers as shown in Fig. 2. The center lugs on the transformers are not used. The lugs to which the shields are connected are then grounded with a short piece of hookup wire. *Alignment* Set the receiver tuning dial to position one of the crystal calibrator markers in the middle of the passband. Select either USB or LSB with both the passband tuner and the crystal-filter switch. Adjust the PRESELECTOR , T2, and T3 for maximum S-meter reading. This condition occurs with the transformer slugs nearly all the way out. There should be no noticeable decrease in sensitivity or overall gain after the modification is completed. *Operation* The crystal-filter switch must be placed in the proper position for the mode of operation. The 4.8-kHz selectivity	position of the passband switch is limited by the crystal filter, but the receiver is still usable for a-m reception. The additional selectivity provided by the filter is noticed mostly while operating ssb during crowded band conditions. However, the cw performance is also improved since there is a substantial reduction of the audio image on the other side of zero beat. The entire modification requires roughly 2 1/2 hours. QST
	BOTTOM VIEW Fig. 2 - The two tables from the filters are routed through the grommet mounted on the rear chassis apron. The center lugs on the transformers are not used. Bottom view of the modified R-46. The leads from the filter assembly are routed through a grommet in the rear chassis apron.
(c) QST January 1972 - p 30