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Dick Smith Electronics Function Generator Kit

Dick Smith Electronics PO Box 2249 Redwood City CA 94063 Price class: $70

by Thomas S- Row inski KA1MDA

As an electronics technician, I'm always on the lookout for test gear for the workbench or shack When my forly-year-old audio signal generator finally bit the dust, I decided it was lime for a new piece of gear to take its place Luckily. I found exactly what \ was looking for m a friend's Dick Smith Electronics catalog ihe K-3520 function generator kit it was inexpensive ($69.95), attractive, and had pretty decent specs.

The DSE K-3520 boasts a four-digit LEO frequency display and covers 10 Hz to 170 kHz rn three ranges. ~he specs aiso list an outpu: of 2 5 volts peak to peak into 600 Ohms and an amplitude stability of .1 dB The output waveform can be switched between sine, square, and triangle waves. Considering the price of the unit, these are impressive specs. \ couldn't wait to test the assembled unit!

Assembly

The kit was relatively straightforward and wen; together in one very long night. \ would definitely call this a two-evening kit! I don't recommend it to beginners, although anyone who has ever home-brewed a project or built a kit before should have no problems if he follows the instructions carefully

Keep in mfnd that this is not a Heath-type kit. There are no pans layouts screened on the PC boards, nor are there any step-by-step instructions for assembling every piece that comes with the kit. Instead, you are instructed to mount groups of components per each step. You are tofd to install all the jumpers first, then the capacitors, then the resistors, etc. As Jong as you keep a close eye on the parts layout in the manual and highlight each component afler it's installed, you should experience no problems during assembly.

The kn itself consists of three PC boards, the case assembly, and assorted hardware. The large main board is assembled first, the display board second, and Ihe small timebase board lasl. Finally, the boards and hardware are installed in the case. Since the timebase board is used only in the U.S. version, it is not mentioned in the manual—it comes with its own data sheet.

encountered only a few assembty quirks with my unit. DSE does not provide sockets for the chips: You are instructed to solder the iCs directly to the board. Play it safe: USE SOCKETS and do not install the chips until everything else is finished and ready to be powered up!

DSE also uses a strange way to label resistors: 2k5 means 2,5k Ohms. For whatever rea-

"The K-3520 function generator kit offers amazing performance for an even more amazing price!"

son, the drilling template supplied with the timebase board was incorrect Put everything else together first, then mount the board in a clear spot on the rear panel, On the front panel the baton on the range-selector switch was shorter than the other two switches.

When mounting the board, insert the switch leads only far enough into the PC board to solder the tips of the leads to the foil pads This will just about even out the length of the frontpanel switches. The trickiest pan of the kit involves soldering the display board to the main board at a90-degree angle. An extra pair of hands here makes the job much easier. Tack-solder ihe two outside traces first and test-fit the assembly by sliding it into the case. If all is well, finish soldering the rest of the traces.

Finally, the unit is powered by a 120-volt transformer, but provides no line fusing. I in* stalled a fuse holder on the rear panel and used a 5-A fuse in the hot line (black wire on the line cord) before the power switch for safety.

Aside from these problems, the kit went together patnlessly. DSE provides more than enough solder and hookup wire to complete the kit. I want to stress that the above problems were all minor in nature, and I really enjoyed putting this kit together.. Six hours into the project, I was ready to insert the chips and run the smoke test.

Test Results

My unit fired up and ran on the first try-There were a few bugs present, bul it ran. The first step in calibrating the unit involved setting the timebase osciflator to 3579.545 kHz Initially I coutd not trim the frequency down far enough. Removing the 39-pF capacitor across the trimmer solved ihis problem The amplitude, offset, and distortion adjustments all went off without a hitch. {The calibration procedure in the manual is written quite clearly and offers two methods of calibration—for those with and without other test equipment.)

There was a smafl bit of residual distortion superimposed on the output of the X100 range, i suspect it was multiplexing noise from the dispiayfttriver circuit, Bypassing the main voltage regulator ground to the power cord ground through a .5-mF, 600-volt capacitor eliminated the distortion.

Finally, the LED frequency display would not latch in the XJOQ range. The manual men-lions this problem and suggests delaying the clock pulses with a 1 GO-Ohm resistor. I had no tuck. Through some experimentation, though, I found that a I5k-0hm resistor in the delay fine solved the problem and the display ran pertectiy.

Sok how did my unit compare with the published specs'* ll came quite close. Output was 2.00 volts p-p and was flat from 10 Hz to 25 kHz, where it began to very slowly dip down a bit. Response was down 0.91 dB at 95 kHz and 1,5 dB at 155 kHz, Range extended from 10 Hz to 155 kHz.

Photo A. Front panel of the Dick Smith Electronics Function Generator. 24 73 Amateur Radio • May, 1987

Photo B. Instde view of the completed and modified kit Note the fuse holder at the upper left, the trimpot at the toner left, and the by* pass capacitor at the bottom left behind the pots A new frequency-adjusting pot is below the visible pot.

The digital display was accurate to ± one digit In typical use. this resulted in a maximum indicated display error of 50 Hz at 155 kHz, From a cold start, frequency drift was -3 Hz in the first two hours. After warmup, total drifi in eight hours was 1 Hz. Power consumption was 6 Watts at 120 volts. Needless to say. J was impressed with the unit's performance.

Modifications

After using the K-3520 for a few weeks, f became aware of a few shortcomings and set out to modify the kit. I found that the clean, open component layout, clear schematics, and detailed circuit description in the manual open up this kit to all sorts of modifications. An experimenter s dream!

Due to the circuit design, the frequency-adjustment range ¡s very nonlinear DSE tries to get around this by providing a coarse-and fine-adjustment pot. This helps a little, but it still gets touchy in spots. I found that replacing the T-Meg linear taper "coarse adjust" pot with a SOOk-Ohm, reverse-audio taper pot made the unit much easier to adjust. If a reverse audio taper pot is not available in your area, you could use a standard audio taper wired for reverse operation. It still isn t linear, but it's much better than before.

The only drawback is that this mod raises the bottom frequency limit from 10 to 20 Hz Since I use my unit for audio work, this was not a problem, t also replaced the 4,7k-Ohm resistor in series with the pot with a miniature PC-mount lOk-Ohm linear taper pot. This modification allows setting of the unit's high-frequency limit anywhere between 75 and 400 kHz" Keep in mind, however, that this is a compromise The higher the top limit, the more nonlinear the frequency-adjustment range becomes.

With these two mods, you can tailor the response limits to allow greatest adjustment ease for any given application, f strongly advise caution when you're experimenting above the unit s rated 170-kHz limit. Although the output stage in my kit survived numerous visits beyond 500 kHz, the output siages in a supposedly "wideband amplifier under test did ncif Above all. assemble the kit according to instructions first, Do not make any changes or modifications until you are sure everything is working as it should!

Conclusions it has been weft over three months since I assembled my unit, and IE am both impressed and pleased with its performance, tt ts a well-designed, easy to use, and attractive piece of equipment There are many smail touches m its design that show the quality of this kit All in all. this is a fine piece of gear, Congratulations to Dick Smith Electronics for coming up with a winner. The K-3520 function generator kit offers amazing performance for an even more amazing price! ■

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