M Solid State

The basic design for this 28 MHz, one watt, solid state transmitter, was developed on 50 MHz. By the time I had it working, the band had gone dead. So 1 shifted it to 28 MHz. With a tew minor changes, it will work on either band, using the same transistors and suitable inductances.

There are three unconventional features about the design, one being the neutralized buffer stage, because on 50 MHz I found if necessary to neutralize both buffer and finai. The neutralization is not to prevent self-oscillation as one might think. What it does do is to greatly improve the modulation characteristics. On 50 MHz it also cleaned up some TVI thai appeared during modulation peaks.

Another unusual feature is the final tank circuit. It is really a pair of tank circuits in parallel. The exact configuration was arrived at by trial and error. However, since any change that I make in it seems to cause the output to drop, the logical thing is to leave it the way it is.

I normally run the transmitter from the cigar lighter in my Volvo. The car battery voltage is 12.4V with a negative ground. At this voltage the final draws .060A for a dc input of 750 mW. The input can be raised to 1W by inserting an extra 1.5V dry cell in series with the 12.4V to the modulation transformer secondary. There is so little difference at the receiving end that I seldom bother to do it.

The audio section is a 12V, I W commer cial amplifier obtained from Radio Shack for S5.95. At that price, it's hardly worth while to build your own. The only trouble with this one and most low-priced audio amplifiers — is that it has a positive ground. Be careful what you do with the microphone shield ground and the volume control ground. Naturally you will insulate the amplifier printed circuit board from the metal box in which it resides. Don't do what I did, and hard wire the volume control io both the amplifier ground and the aluminum box at tiie same time. All it did was to wipe out a half-inch section of printed circuit trace leading to the power switch. Fortunately this was easily replaced.

There is always a problem in locating a suitable modulation transformer for the higher powered solid state transmitters. Thai is the reason for the 1W output transformer wired back to back with the output transformer of the regular amplifier. This system is not 100% efficient, but it does get around the problem. Since we have more audio than we really need, the loss of efficiency is not serious.

The most unique feature is the combination of the diode and audio bypass condenser in series with the buffer collector. This arrangement was evolved in the process of obtaining upward modulation of the output. The result was semi-spectacular. Since there is no splatter and no appreciable distortion at the receiving end, it appears to be a practical approach. The IK resistor (in the dc path) was chosen in order to provide the

LOW—PASS FILTER

LOW—PASS FILTER

L2 a L3 ARE TOfiOID WOUND

Fig. I. Schematic of the 28 to 50 MHz rig. Coil data: LI, 3/8 in. dia. 6T #18 ii 1in. length; L2, "Red" core toroid, 7T; L3, "Red" core toroid, 8T; L4, 2T wound over L3; (Note: Buy one Amitron package — use the "Red" core for L2 and the "Yellow" core for L3. Use 10T for L3 and 3T for L4. 28 to 30 MHz happens to be the area in the spectrum where the "red" and "yellow" cores overlap.) L5, 'A in. dia. 10T, #22, ¥t in. length;L6, 2T oyer cold end of L5; L7, 'A in. dia. 24T #24, close spaced (or J. W. Miller #4204 5—12 ¡JH choke); T2, 1W transistor output transformer 150i2 : Sfi, dc resistance 8^1 pri. : lilsec.

L2 a L3 ARE TOfiOID WOUND

Fig. I. Schematic of the 28 to 50 MHz rig. Coil data: LI, 3/8 in. dia. 6T #18 ii 1in. length; L2, "Red" core toroid, 7T; L3, "Red" core toroid, 8T; L4, 2T wound over L3; (Note: Buy one Amitron package — use the "Red" core for L2 and the "Yellow" core for L3. Use 10T for L3 and 3T for L4. 28 to 30 MHz happens to be the area in the spectrum where the "red" and "yellow" cores overlap.) L5, 'A in. dia. 10T, #22, ¥t in. length;L6, 2T oyer cold end of L5; L7, 'A in. dia. 24T #24, close spaced (or J. W. Miller #4204 5—12 ¡JH choke); T2, 1W transistor output transformer 150i2 : Sfi, dc resistance 8^1 pri. : lilsec.

proper amount of drive for the final.

The 2N1711 is a surprise, especially up on 50 MHz. It turns out that only certain brands of 171 l*s work at the higher frequencies. The Fairchild 2N1711 does not work. Apparently it is manufactured by a different process than the Motorola 2N171 l's which I am using. A friend who works at Fairchild refsued to believe this. He took my MOT 2N171 l's into the Fairchild lab and verified there was indeed a difference.

If you cannot find MOT 2N171 l's, then I recommend the little brother of Sprague 2N3404, which is the 2N3414. It can be used as oscillator and buffer. The RCA 2N3118 will work in the final, but it costs four times as much as the Sprague, and does not work any better.

The low-pass filter is pretty much a requirement. My 5W CB transceiver has one built in. Solid state finals are always rich in harmonic output. Be certain to measure the output power on the downstream side of the filter. Otherwise you won't know whether any improvements you make are increasing the output at the fundamental or merely increasing the harmonic output.

The neutralizing coil for the buffer is rather large at 29 MHz. What is needed is a combination of "C" in picofarads and "L" in microhenries that will equal 30 (for 29 MHz) when multiplied together. My approach is to take a 2 pF fixed capacitor and solder it across the coil. Then adjust the turns on the coil to get a dip at 29 MHz with the slug all the way inserted. That indicates a 15 ^iH coil (ignoring the distributed capacity of the winding). Of course if you prefer the chicken method, just take a National XR coil form and wind on 24 turns of #24 wire close spaced.

Some type of rf power and VSWR indicator is highly desirable for checking the output and also the modulation. There are two types of these gadgets sold for use in CB rigs for under $10. Either one works fine on

28 or 50 MHz. My rig with 12.4V input will provide 2.2V defection on the 3V scale. If I wanted full scale deflection for VSWR measuring purposes I would simply add a 6V battery in series witji the 12.4V car battery (but don't modulate it under these conditions),

It turns out that an NE-49 pilot light soldered across the end of a piece of RG-58 coax makes a fine dummy load and modulation indicator. The VSWR checks out about 1.1 to 1.0. I can light the bulb to full brilliance by blowing in the microphone with only 12.4V do input.

I am using the Hustler mobile loaded whip antenna mounted on the rear bumper of the Volvo. The VSWR checks 1.3 to 1, which is satisfactory. Solid state final amplifiers do not tolerate high VSWRs. Anything worse than 1.5 to 1 may cause trouble.

Just as I was about to mail this article to 73 I made an important discovery. It came about when I moved the modulation transformer T2 from its original position on top of the chassis to a neater looking spot near the crystal oscillator. Everything fell apart. The output fell way off and the modulator ceased to modulate.

Because I am a ham first and an engineer only from necessity, I stumbled over the answer. The bypass condenser across the secondary' of the modijjation transformer cured the problem. I first tried a .002 mF bypass which did not seem to help at ail. Next I tried the .1 mF capacitor that appears in the schematic and everything worked again. You may not need it. It apparently depends on the placement of parts. On the other hand, it doesn't seem to hurt. In fact, it seemed to help a little over the original configuration.

On the air the expanded positive peak modulation can be mistaken for double sideband with slightiy more carrier than normal. The only disadvantage is that some distortion products are perhaps generated which appear on Channel 5 if in ciose proximity to the TV receiver. The Drake 100W to-pass filter will take care of this problem if it is bothersome. Naturally, we don't really need a 100W filter, but Drake doesn't build a smaller one.

DIY Battery Repair

DIY Battery Repair

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