Nuts & Volts

hand-wired for the display, U6, RN1,Q3, and Q4. The other was the mainboard 2-3/4” x 3-7/8” (RadioShack#276-1688). The display board getsfolded back and mounted on the samethreaded standoffs as the main boardwith proper spacers. I used a plastichousing that’s common to BUD andSERPAC — available through Mouser.Once the timing resistor switching(R8-R10) is done, using U2, there is nomore critical wiring to do. Just keepU1’s Inputs (p2, p3) short and in theclear as much as possible. All the datasheets for the display, chips, and plastichousings are downloadable throughMouser. Mouser has best price on S1.The best price on P1-P5 and U5(74C926) was from Unicorn Electronics(www.unicornelectronics.com).The 2 kHz clock oscillator (U4A)can be adjusted to the correctfrequency with R15. This does notneed to be exact: plus or minus 20 Hzis adequate. Use two resistors here.One will be as large as you can gowithout going over the target frequency,the other will be a small value tofine-tune it. I used a 51 kΩ in serieswith a 6.2 kΩ and came within 2 Hz ofthe 2 kHz target. All resistors are 5%carbon film. R9 (10 MΩ) should be a1% metal film, not for accuracy, butfor its stability. Carbon resistors withThe Standard for checkingCapacitors in-circuitGood enough to be thechoice of Panasonic,Pioneer, NBC, ABC, Ford,JVC, NASA and thousandsof independent servicetechnicians.Inexpensive enough to pay for itself in justone day’s repairs. At $209, it’s affordable.And with a 60 day trial period, satisfactionguaranteed or money-back policy, the onlything you can lose is all the time you’recurrently spending on trying to repair allthose dogs you’ve given up on.CapAnalyzer 88Athis high a value can have wildand unpredictable temperature coefficients.I used a 5% carbon film inthis unit but will replace it the nexttime I have an order going out. Thesemay be hard to find, but NewarkElectronics has them.Once the unit is completed, calibrationis achieved by adjusting P1-P4and P5. The nice feature about thesecalibration controls is that theycompensate for all circuit componenttolerances from their design centersincluding clock frequency error. Startby making a rough adjustment on thehigh end of each range. Then dropback to range one (0-999 pF) andadjust P5 (zero) to just eliminate anyparasitic display to “000.” Now readjustP1 to whatever standard you areusing. Then adjust ranges two throughfour to a standard on their high end.You should now see “000” on allranges with no capacitance in Cx. Ifranges two through four show anyparasitic reading, C1 will have to betweaked somewhere between 2,000-5,000 pF. When making these tests,use the small test receptacles (A-29071-ND that are wired in parallelwith Cx’s pin jacks) if possible. Thesewill accept lead diameters of 0.20-.40inches, which will accommodate 90%of tested capacitors. When necessary,Locate shorted or leakycomponents or conditionsto the exact spot in-circuitStill cutting up the pcb,and unsoldering everypart trying to guess atwhere the short is?$209Your DVM shows the same shorted reading allalong the pcb trace. LeakSeeker 82B has theresolution to find the defective component.Touch pads along the trace, and LeakSeekerbeeps highest in pitch at the defect’s pad. Nowyou can locate a shorted part only a quarter ofan inch away from a good part. Short can befrom 0 to 150 ohmsLeakSeeker 82BAvailable at your distributor, or call 561-487-6103Electronic Design Specialistswww.eds-inc.comuse short leads out of the pin jacksand subtract any residual readingsthat these add (2-10 pF) beforeconnecting the test capacitor.For calibration, use the beststandards you can scrounge up that arenear the high end of each range. I amfortunate enough to own a 1% capacitordecade substitution box, but youcan purchase a couple of 1% capacitorsfrom Digi-Key that will calibrate thetwo most critical ranges (one and two).These are 1,000 pF, p/n P3824-ND($0.63), and 100 nF, p/n P3872-ND($1.15).Calibrate, Test, UseAs opportunity presents, you canrecalibrate with better standards onranges three and four. This unit’saccuracy is only limited by the accuracyof the standards you calibrate itwith. In my case, that was 1%, which isquite adequate for test bench use.Although the display is quitestable, there will be instances wherethe Cx value is so close to the nextwhole digit (i.e., 99%), that it cancause LSB flicker. If that happens,simply move your free hand near thecapacitor in Cx (2-3 inches) whilereading the display. That will addthat last fraction of a picofarad andstabilize the LSB to the next wholedigit that it is already so close to.The average current draw on thisunit is about 35 mA, which is a prettyhefty load for a nine-volt battery. I ranaccelerated life tests, assuming 1,000tests per year at five seconds per test,and it appears the battery would lastalmost as long as its shelf life. For thefront panel, I tried something new. Idrew this up from one of my schematicCAD programs, along with text. Ithen printed this out on glossy photopaper and pasted it to the case withspray adhesive. Looks nice, but Idon’t know how durable it will be.Time will tell, I guess. I built this unitfor less than $30 and am very satisfiedwith it. The first tests I performed wereto quantify and label all those airvariable capacitors and trimmers. Itwas a breeze and a joy. NV50 May 2006