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A crystal-controlled 144 Mc. converter for 75-A series receivers

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Simple easily adjusted design with 26 to 30 Mc. output.

Some owners of Collins receivers are kept from trying 2-meter operation because few of the published v.h.f. converter designs are usable in their original form with these receivers. The tuning range of the 75-A receivers on the 7 or 14 Mc. ranges, normally used with crystal-controlled converters, is not wide enough for covering a v.h.f. band. The writer had a band-switching converter with a tunable oscillator, but its stability and noise figure left much to be desired. Using it on 2 was a good deal like trying to work 75 meter phone with a 2 tube regenerative receiver: it can be done, but there is little pleasure in it.

With the high stability and dial calibration accuracy that are available with the 75-A, it is a shame to be unable to make use of these desirable characteristics in 2 meter listening. The obvious answer is alteration of the converter i.f. range to 26 to 30 Mc., allowing tuning of a four-megacycle band with one change of the bandswitch. If the crystal frequency is chosen properly, the same calibration accuracy is then available on the v.h.f. band as with the receiver alone. Use of the 26 to 30 Mc. i.f. range may be helpful with other receivers as well. Most two-dial receivers tune about four megacycles on their 10-meter bandspread ranges, and thus a better tuning rate is available than at 7 Mc., where the general-coverage range must be used.

The front-end design of a converter having 26 to 30 Mc. for its output range is, of course, not necessarily any different because of this. Only the oscillator and multiplier circuits need be changed. There are, however, some front-end features in the converter to be described that will be of interest to other converter builders, as they make for simplicity and ease of construction and adjustment.

Two r.f. stages are used. The first is a push-pull neutralized stage with a 6BQ7A (or, 6BZ or BK7) or 5670 dual triode. This is inductively coupled to a 6AM4 grounded-grid amplifier. All r.f. circuits are adjusted by varying turns spacing, except for the antenna input, which is made adjustable for different antenna systems. The mixer is the pentode section of a 6U8, in which the triode section is a cathode follower, coupling out the i.f. signal. Injection is supplied by another 6178, in which the triode is a crystal oscillator and the pentode a tripler. A 39.333 Mc, overtone crystal (James Knights H-173L) is used to provide crystal-controlled injection on 118 Mc.


Mechanically, the converter is about as inexpensively built as any you're likely to see. The cover is a piece of thin scrap aluminum, bent up by hand to fit a wooden cheese box that serves as a case. The inside of the box is coated with aluminum paint and the outside is painted black. Looking at the external view, the input end is at the left. The tubes are, left to right, the first r.f. amplifier, second r.f., the mixer-cathode follower, and the oscillator-tripler. Power is brought in through a 4-pin fitting near the center of the chassis.

Photo 1
The W8NOH 2-meter converter is built on a sheet of aluminum, with its edges bent over to make a cover for a wooden box.

A more photogenic layout could have been made by the use of tie points for power wiring, but as mechanical stability is of no great importance in a crystal-controlled converter the components were wired in the simplest way possible. The only important point to keep in mind is to use the shortest possible leads in all r.f. circuits. Clip resistor, coil and by-pass capacitor leads to the minimum usable length. Ceramic condensers are desirable because of their small size.


Power for the converter may be taken from the Collins receiver supply through the socket marked "NBFM Adapter." This has the advantage that plate voltage is removed from the converter during transmission periods when the receiver B-plus is cut off. The extra load seems to be quite within the capabilities of the receiver power system.

A grid-dip meter will save plenty of time in the adjustment of the converter. By this method, all r.f. circuits are adjusted by varying the turns spacing until they resonate at about 145 Mc. If the crystal oscillator and multiplier plate coils are adjusted to resonance at the frequencies indicated on the schematic diagram, that portion of the converter should work well enough to permit some reception at once. The mixer plate coil slug can be adjusted with the grid-dip meter first, and then later peaked for maximum noise.

Now tune the oscillator plate coil for maximum output on the crystal frequency. Use your grid-dip meter as an output indicator, or measure the voltage developed across the tripler grid leak with a high-impedance voltmeter or v.t.v.m. Tune the tripler plate circuit for maximum output on 118 Mc. The d.c. voltage developed at the mixer grid can be used for this check. The dropping resistor in the tripler screen lead can be increased in value, if the injection is more than is necessary for good mixer action. The oscillator grid leak can be increased to cut down the output from that stage as well, if it is more than is necessary. It is desirable to keep the oscillator and tripler operating levels at the lowest usable value, in the interest of stability and low spurious response.

A noise generator will be helpful at this point, though it is not absolutely necessary. Neither is it necessary to have signals on the band, though it is comforting to hear them at this stage of the game. Ignition or the noise from an electric razor can be used if a noise generator or signals are not available. All coils except the input circuit should be adjusted for maximum response near the middle of the band. Then the input circuit should be adjusted so that a signal (noise generator, amateur or whatever) produces maximum rise over the receiver noise level itself. If you tune for highest S-meter reading only, you may not hit the point that gives the best signal-to-noise ratio.

Photo 2
Bottom view of the 2-meter crystal-controlled converter. The antenna end is at the right.

The pin connections shown on the schematic diagram are for a 6BQ7A or similar tube in the r.f. stage. If a 5670 is used (and it is recommended) the pin connections are different. With a 5670, our converter showed no appreciable difference with or without the neutralizing capacitors, CN. With the 6BQ7A, 2 pF fixed ceramic capacitors improved the noise figure appreciably. The builder who wants the ultimate in noise figure may want to put in adjustable trimmers for neutralizing.

Fig 1
Fig. 1. Schematic diagram and parts information for the W8NOH 2-meter converter. Pin numbers for the r.f. stage tube are for a 6BQ7A, 6BZ7 or 6BB7 tube. Connections are different for a 5670.

C1Ceramic padder, about 2 to 6 pF
C2Ceramic padder, 3 to 30 pF
CN2 pµf. (see text).
L12 turns wound over L2.
L212 turns, center-tapped.
L36 turns, center-tapped.
L42 turns wound over L3.
L512 turns, center-tapped.
L618 turns spaced wire diameter on 3/8 inch brassslug form.
L73 turns.
L812 turns on 3/8 inch brass-slug form, tapped at 5 turns.
L9,L10Bifilar-wound, 12 turns.
L11,L12 Same as L9,L10. All coils No. 20 enam., ¼ inch diameter, unless otherwise specified.
J1,J2Coaxial chassis fittings, female.

With the r.f. circuits peaked near the middle of the band, the response should be fairly uniform across the whole band. If it turns out that it is not flat enough to satisfy you, the mixer and second r.f. plate windings can be stagger-tuned to even it up. There is no ill effect on the converter noise figure if these stages are detuned slightly either way to smooth out the response across the band. Adjustment of the coils is done most readily if an insulated tuning wand is used. Once the proper settings are found, the coils may be coated with coil dope and you are ready for some real 2-meter reception.

The writer wishes to thank WBBGY and W8GYU for their help in the designing of this converter.

Louis A. Gerbert, W8NOH.