CQ Reviews:The Millen Model 90652

Solid-State DipperBY W ILF RE D M . SCHERER,* W2AEF

S EVERAL years ago CQ reviewed the M il­len Model 9065 I-A G rid-Dip Oscillator,' arevised version of the famous original unit.the Model 90651, both of which have be­come standards in the amateur and industrialfield. The M illen people have now come upwith a solid-state version, the Model 90652which overcomes the most objective short­comings hereto fore experienced with solid­state dippers. As a matter of fact, the Model90652 provides performance equal to or bet­te r than that realized with many vacuum-tubejobs; plus some additional features.

It has high sensitivi ty with deep and sharpd ips, Hi-Q with good coupling to test circuits,Q-multiplier setup for increased sensitivi tyand sharp responses with abso rption-type op­eration a t energized circuits (otherwise cus­tomarily conducted as straigh t low-Q diodedetection ) , headphone jack, self-containedbattery opera tion for convenient handlingand insta nt performance.

The instrument case has a 1/4-20 tappedsocke t hole for attachment of a wrist strapor other reta ining device to prevent droppingthe dipper from loca tions such as at antennatowers. A handy polypropylene carrying casealso is supplied.

Reta ined are original attributes such as fre­quency coverage of 1.7-300 me" overlappingin seven ranges, protected inductors, built-inmeter, rugged construction, absence of spuri­ous dips, one-hand operation, 2% frequencycalibration on a drum dial, anti-backlashdrive mechanism.

CircuitryThe circuitry for the M illen Solid-State

D ipper is shown at fig. J. The oscillator em­ploys a 3N I 28 single-gate MOSFET operating

*'1echnical Director, CQ.'''CQ Reviews The Millen Model 9065 1-A Grid­Dip Meter," CQ, May 1968, p. 62.:?Extended to 1.6 me in this model.

in a spli t-Colpitts circuit with the resonatingtank connected between the drain and gateof the MOSFET. As usual, the circuit is tunedby a split-stator variable capacitor with itsrotor grounded. The MOSFET source is at r.f.ground through a bypass capacitor.' The am­plitude of oscilla tion is controlled by a po­tentiometer, R3, that varies a d.c. potentialappl'ed to the source. This is the DET-osc.control which is used for Q-multiplier oper­ation as explained later.

Operating potential is ap plied to th eMOSFET d rai n which is at an r. f. potential ,because o ne side of the oscillato r tank is con­nected to it. The voltage-supply source musttherefore be r.f-i solated from th e drain. Ina vacuum-tube job, the isola tion (sim ila rlyrequired at the tube plate ) can be simply han­dled by a series resistor o f several th ousandohms ; however, it cannot be as sim ply ac­complished with a transistorized setup, be­cause the voltage drop would be too hi gh.This cou ld be avoided by use of an r.f. choke,but another problem would arise due to thevirtual impossibility of obtaining a satisfac-

3This is a specially fabrica ted capacitor thateliminates erratic operation and avoids spuriousdips. It has no leads and is built into the tuning­capacitor frame. It JS located directly at thesource socket terminal for the MOSFET.

The Millen Model 90652 So lid-Stole Dipper.

Septembe r, 19 7 1 . • CQ • 63

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Fig. l - Circuit diagram for the Millen Model 90652 Solid-State Dipper. Details are given in thete xt. J3 is for use with I.f. inductors which will be available on specia l order.

tory single choke for opera tion over the widefrequency ra nge of the un it.

In the Millen job the situa tion is takencare of by a network consisting of a small­value resistor, R s, in series with two series­connected different-value r.f. chokes, RFCI­R FCt , one of which has an iron core, eachswamped with a resistor. These primarilyhand le the isolation at the lower and middlefrequencies. Three ferrite beads, Xl-X" en­hance the required isolation at the higher fre­qu encies. T his setup permits a smooth andwide frequency range of oscillation withoutspurious "holes." As with the M illen vac­uum-tube units, spurious dips or suekoutsalso are avoided by bonding straps betweenpart of the case and selected points on thechassis.

MeteringThe r .f. potential across the oscillator

tuned circuit is indicated by a meter whosereading dips when the circ ui t is resonatedwith the test circuit to which it is coupled.The metering arrangement is activated by ad.c. potential obtained from a full-wave rec­tifi er across the oscillator tank. Two 1N 3604diodes, CR,-CRt are used. This setup pro­vides deeper and sharper dips than those ob­tanable with a hal f-wave rectifier at only oneside of the tank. The two diodes also providesome protection from strong r.f. fields thatmight otherwise damage the MOSFET.

The d.c. potential is applied to a 2N5429J F ET d.c. am plifier in the source circuit ofwhich is a I rna meter. The meter circuit it­self is the zero-suppressed type; that is, read-

ings are indicated for only the upper portionof the current range. This, in effect, providesan expanded scale in the useful working areaand thus further contributes to dee p dip in,dications.

The suppressed zero, or the de lay in read­ings below a certain current, is obtai ned witha bias applied to the J FET source. This bias isobta ined from a stiff voltage divider and iscontrolled by the emitter-collector junctionof a 2N 1131 bipo la r transistor which formsone leg of the divider in series with the sourcereturn of the JFET. The current through the2N 113 1 transistor, and thus the bias fo r theJ FET, is adjusted by varying the base bias onthe 2N 1131. T his is hand led by the METER­

SET control, R i,A jack in the source return is furnished for

headphone usc as may be desired in someapplications.

Q-Multiplier SetupOne function usually provided with a dip­

per is that of diode detection for utilizing theinstrument as an absorption-type de tector orfreq uency meter at energized circuits. TheModel 90652 goes th is one better by allowingthe oscillator to fu nction as a Q-multiplierwhich markedly increases the sensiti vity andprovides sharper peak readings as the unit 'stuned to the frequency of energized circuits.This is realized by selling the DET.-OSC. con­trol to the point just below where actual os­cillation commences which is where regen.eration is produced.

ConstructionThe type of construction, size and styling

64 • CQ • September, 1971

The Millen Model 90652 Solid-Stote Dipper withcarrying case.

with a sturdy copper-plated case are the sameas that of the Millen vacuum-tube models;however, the weight is only 2Vz Ibs (withbatt ery ) compared with 4\4 Ibs for the othermodels.

T he meter has a taut-band suspension andthus is less susceptible to mechanical shockor sticky operation.

T he tuning capacitor is dri ven by about a1.75: I anti-backlash gear train operated bya knurled thumbwheel attached to the drum­dial shaft in an arrangement that conven­iently permits one-hand handling and opera­tion of the instrument.

T he inductors for the different ranges havemolded form-fi tted plastic covers. They aremarked as to frequency and are color- andletter-coded to correspond to the relatedranges at the drum-d ial window where thefreq uency ra nge for each inductor is also in­dicated opposi te the corresponding scale onthe dial.

Hi-Q and efficient operation on the highestra nge ( 120-300 me) is enhanced with a sil­ver-plated ind uctor. High-Q with better-than­usual coupli ng to the test circuit on thelowest ra nge ( 1.6-3.5 me) also is maintainedby a specially-wound inductor with a pow­dered-i ron core.

The instrument is powered by a self-con­tained 9-volt transistor-radio alkaline bat­tery. The estimated battery life is up to 6months when the dipper is operated 2 hoursper day. The power is turned on with theMETER-SET control which is operated by ablack kn urled thumbwheel that shows a yel­low band around its edge whenever the bat­tery is engaged.

66 • CQ • September, 1971

Operation and PerformanceExcept for some of the control adjust­

ments, operation of the 90652 in general issimilar to that employed with other instru­ments of like nat ure. For conventional dipperuse (on de-energized circuits) the DET.-OSC.

control is advanced fully clockwise to the osc .position . T he METER-SET control is advancedto apply the battery power and is turned to thepoint where an initial meter reading of one­hal f scale or more is obtai ned. Because of thesuppressed-zero, the meter will not producea reading unt il the control is advanced quitea way. T he instrument is now set for g.d.o.­type operation.

During such operation, good coupling pluspositive dips of good depth were obtained onall ranges with our unit. No spurious suckoutsor dips were experienced at any time. Withthe METER-SET adjusted for a full-scale read­ing at the maximum-obtainable current pointo n any range, an on-scale reading was indi­cated at any freq uency within the associatedrange. No readjustment of the M ET ER-SET

was necessarily required, although at oneend of two ranges the meter read ing droppedtoward the lower end of the scale in whichcase, fur ther advancing the control to raisethe reading in the particular area might bedesi rable for more convenient observations.Of no consequence, as far as operation goes,is that because of the type of metering setup,a meter reading may still be had when theMETER-SET control is well advanced and noind uctor is installed.

Frequency calibration of the instrument,checked with a freq uency coun ter, was withinthe manufactu rer's specification of 2% toler­ance. T his relates to that when the oscillatoris decoupled from a test circuit, inasmuch asthe oscillator frequency can be "pulled" by atest circuit, depending on the degree of cou- :piing between the two. For maximum fre ­quency accuracy, the least coupli ng shouldbe used that still enables a dip indication tobe had. This is a requisite with any other sim­ilar instrument used in like applications.

Detection UseAs described earlier, abso rption-type de­

tection at energized circuits is obtained byoperating the dipper as a Q-Multiplier. Thisfunction is set up by rotati ng the DET.-OSC.

control toward the DET. position, unt il themeter reading suddenly drops or fails to con­tinue to fall. This will be at the border of os-

[Continued on page 96]

CO Reviews Millen Dipper [from page 66]cillation and the point of regeneration wherethe detecting sensitivity and Q are high. Inour tests, the sensitivity was five- to ten-timesgreater th an that found with the customaryd iode-type detection of other instruments.

Furthermore , responses were sha r plypeaked without the broaden ing effect other­wise due to the diode-loading effect experi­enced with the usual g.d .o. or dipper devices.With Q-Multiplier operation a very slightchange in frequenc y calibration was noted;however, this was less than that experiencedwith a switch between the oscillate and diodemode with conventional instruments.

Diode-type detection also may be had withthe 90652 simply by further reducing the set­t.ng of the DET .-OSC. contro l to below the re­gcneration point.

Headphone operation for detecting beatnotes with other signals, while using the oscil­late mode, also was conside rably more sensi­tive than wi th convent ional-type un its,

As ment ioned earlie r , the osci lla to rMOSF ET s protected against overload-dam­age to some degree; nevertheless, care shouldbe taken to avoid operation close to verystrong r.f. fields. Should any damage result,the 3N 128 MOSF ET is a low-cost type that isrelatively easy to obtain. In addition, it plugsinto a teflon socket, making replacement asimple matter. The effect on calibration willbe negligible,

The solid-state dippers operate at lowerpower levels than do vacuum-tube jobs andthus do not pu t out adequate power to oper­ate the customary r.I, bridges used in theamateur field . They also are weaker as a sig­nal source for other applications such as withrece iving equipment. but in some cases th 'smay be an advantage, since stronger signalstend to overload a receiver and generate con­fusing birdies.

The Millen Model 90652 Solid-State Dip­per is priced at $ 110 with battery and carry­ing case, A lso included are instructions forusing the device in various applications.' Itis a product of James Millen ManufacturingCo., Inc., 150 Exchange Street, Malden,Mass. 02148 .

-W2AEF

"For those not familiar with the applications andmethods o f use for dippers, a three-pan articleon the subject may be fo und in the May, Juneand July t968 issues of CQ under the title of"Using the Grid-Dip Meter,"

96 • CO • September, 1971

C.W, Results [from page 54]

to put up a dipole and make Malta availablto the boys on 28 me,

You can thank Kjell, SM~CCE for making 4U I ITU available in the contest. He dilthe operating in both the c.w. and phone sections of the contest. (I believe there wasome token operation by others during th ,phone week-end.) I

What do you do when it's so cold in t h~

shack that the Vibroplex is sending out a diwhen you want a dah? Dick, F2QQ juslQRT, but not before he had run up a winninlscore on 40. (Me, I have no such problewith the old pump handle I use.)

Here's a potential member for the QCWANick UB50E has been hamming since 1927so the boys over there date back quitespell too.

Bob, VS6AE was an ex-VS6 less thanweek after his contest operation on 21 meHe is now back in West Australia so maybanother one in Zone 29 in November.

That winn ing all-band score by ZS3Adidn't come easy for Jurgen, DJ3KR the station operator, even with the use of a rombithat eventually conked out anyway. The station is located at an ionospheric researecenter and the high powered pulse transmiters used for sounding purposes raised havowhen they were in operation, 15 minutes auof every hour.

Where was W3MSK in the contest? Oh!they were in there all right, but now signinW3AU. Ed has joined the confusing two let­ter brigade.

We again want to thank the many European contest managers who processed thelogs of their respective clubs. EspeciallMilos Prostecky OKIMP, Werner StiehmDJ8SW, Wojciech Klosok SP9PT, J. MatzonHA5FA and Kl aus Voight DM2ATL.

The same old crew with two new additionsover here. Andy Malashuk WIGYE, BobEntwistle W I MOO, Freddie C aposellaW2 1WC, Bern ie Welch W81MZ and two newmembers, Ral ph Nichols W1CNU and GeneWalsh K2KUR. 1 had better not leave outJoan of the CQ staff, or she will not save allthose pretty stamps for me when she opensthose overseas entries.

That's it for this one, now you can startyour letters of complaints, or maybe a fewkind words for your hard working Com­mittee?

73 for now, Frank, WIWY