Mo Residents Call Collect

Seventy-three, March 1980 57

Frank Butscher H '6SZS 6850 Westmoor Way San Jose CA 95129

CB to 10

— part XXIII: the Sears Roadtalker 40

Affectmg all things, change brings us mto the future, where a vener able old name like Sears, Roebuck and Co is simpli-tied to Sears, and where amateur radio sports new terminology such as microprocessor, phase locked loop, etc The ham hands have < hanged their makeup m the past 2r> years T he AM heterodyne alleys of 20 and 40 meters now ring with the cacophony of sideband and automatic computerized CW IDers. Even the IOmeter band (the one revered by most hams because of its uncon-gested spectrum, leisurely operating pace, and surprisingly low-power DX capabilities) now bustles with new activity incessant DX beacons, fully-quieted FM signals passing through intercontinental repeaters, the doppfer shift of CW tones as satellites whiz by( and the chatter ot < hannel-i/ed QRP ngs rising in a crescendo as sunspot activity moves steaddy toward peak 21 and beyond

When Sears and Roebuck together were a household name in the ear lv fifties (and before), operating on the 10-meter band was a lot simpler A ham in those days could afford to build his own rigs, or he could modify, for his needs, a wide variety ot

WWII surplus hardware Working all over \ urope in one morning with SQAVatts AM was, as remembered 25 years later, quite a surprise, as well as a very rewarding experiem r

Recently, several sur prises and rewarding experiences accompanied my modification and use ot a 40-channel phase-lof kod loop CB transceiver pur chased from that venerable old company, Sears In an age where building from scratch is cost-prohibitive, the Sears Roadtalker 40, model 3826, AM-SSB rig was purchased new and sight-unseen for under $80.00 during a recent sale Upon opening the box, the manual and accompanying schematic were immediately perused for the elec tncal details of conversion. Also, the bottom cover was removed to examine the physical problems concerned with the changeover—see Fig 1.

The "doghouse' con-taming the reference osc nl lator is in the upper center of the chassis This os< ilia-tor is the key to the ( onver sion The articles listed in the References are el-

Photo by Daniel B. Smith, Jr. K6PRK

Photo by Daniel B. Smith, Jr. K6PRK

Sears Roadtalker

Fig i. Bottom view of the Sears Roadtalker 40 show ing component layout lent in their statement and solution to the conversion problem. Fortunately, the Sears rig has an almost identical PLL circuit to the rig in Reference i

Fig, 2 shows the Sears model 3826 block diagram; the key to the conversion is the vcxo, Q701. The vcxo's third harmonic is mixed with the vco (Q707) frequency in Q703, is filtered b\ LPF1. and *s fed into the PLL controller For channel 1, this frequency is 1.28 MHz. In this PLL, the vco will automatically seek the correct frequency to achieve a 1 28 MHz difference with the vcxo refer en< e oscillator.

I n order to move the rig s operation into the 10-meter band, the X701 frequency has to be in creased. For example, to move the CB channel 1 operating frequency from 26 965 to 28 r>10, the vco has to be increased by 1.545 MHz lo achieve thrs, the X701 frequency should be increased from 12 320 to 12 835 MHz. This increase of .515 Hz is one third the vco frequency increase required The reason for only the 1/3 increase is that the 3rd harmonic is mixed with the vco.

So much for theory; now let s discuss thr actual conversion details A 13-MHz HC-18/U-styfe crystal was purchased at a local surplus parts house to get the rig tuned to the lO-meter band. With this crystal, channel 1 is 29 005 MHz. It was decided to tune the receiver and transmitter to channel 1 while hoping that the tuned circuitry was broadbanded enough to cover 40 channels above, as well as below, 29,005 MHz. As it turns out, this was a good choice of a tune-up Irequency The crystal and socket (shown m Fig 1) are taped to the outside of the metal doghouse because of its

Outdoor Metal Dog House
Fig. 2, Sears Road talker 40. model 3826, simplified functional block diagram.

large size I he crystal socket allowed easy crystal changes to check the bandwidth of operation for a single vi.o-luned circuit setting.

In order to unsolder the 12.320 crystal, the bottom of the doghouse has to be unsoldered from the sides A solder wick was used with a 45-Watt soldering iron to remove the solder bonds. It was easier to unsolder the circuit board ends of the three doghouse posts to gain easy access to the bottom cover than to unscrew the three doghouse flange attachment screws These soft metal screws were cemented in place and I stripped the Phillips head slots while attempting to remove them.

After the crystal and socket were added, the doghouse was set in place and one post was soldered to the PC board The doghouse assembly was pressed into the bottom cover Since it was a tight fit. no sides-to-bottom solder bonding was applied. This would facilitate easy removal for future performance improvements which require changes to this reference oscillator circuit These changes will be discussed later.

The next step was to remove the sealing wax from the top of all the transformer tuning slugs This was accomplished easily by scraping the wax out with the chisel end of a small jeweler's screwdriver All transformer slugs shown in Fig 2, except T103 and T406, were cleaned T701, the tank-circuit inductor (located in the small PC-board-mounted metal enc Insure to the left of the reference ost i J la tor doghouse —see Fig 1), had the most sealing wax applied to the top of the transformer With the vco cover removed, it was discovered that the circuit ry inside was completely covered with sealing wax, probably to prevent parts movement during vibration,

Alter securing a suitable alignment tool for the small square hole in the slugs, the moment of truth had arrived It was time to tune up on 10 meters. With the channel selector set on channel 1, a three-toot piece of antenna wire at-tac lied to the antenna jack ( enter conductor, and 12-V dc power applied, the T702 slug was turned slowly clockwise In about a half turn, a sideband signal was heard signing a W9 call Pin 2 on IC701 was checked with a Heathkit® frequency counter A stable 1 28-MHz frequency readout was observed. Next, T102 was turned clockwise, and in less than one turn, the level of the noise was peaked T705 andT102 were easily noise-peaked in less than one turn That's it for receiver adjustment

Next, an inline, inexpensive CB power/vswr meter and dummy load were attached to the rf output connector Transformers were peaked with the rig in the AM position on channel 1 for max rt output in the following order T7Q3, 704, 402r 403, 404, 408, 407, and 405. The reason lor this broken sequence is that T405 was not easily located Although the transformer numbers are silk-screen printed on the top of the PC board, the parts are tightly packaged, making identification difficult wit^hmjibnglht lighting, ihe numbers are more easily read if a flashlight is used to locate and decipher them.

By the time T405 was peaked, the power output read 5 Watts Not bad. With the mode switch in ;he USB position, a whistle was directed into the mi crophone The wattmeter read 7.5 Watts. With the three-toot piece of wire connected to the antenna lack, a beat note was heard in the speaker, indicating an AM signal With the mode switch in AM, ] H7XJ J was heard! Shades of the "40 r sunspot peak: "A! I you need is a piece of wire hanging out of the window!" Probably, if the three-foot wire was matched, 1 would have tried to answer his CQ; instead, I not-so-calmiy connected up to a two-element beam DX fever was coming on

He came back with a 5/6 signal on an FT-101 He was running 15-Watts output This was beginning to feel like that one spring morning 25 years ago! The next contact was with a W1 in Boston on AM I thought I would try sideband. Bingo! A WB4 in South Carolina was heard. Adjusting the

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