Heathkit IT-11 Capacitor Checker Refurbishment

Heathkit IT-11 Capacitor Checker after cleanup and refurbishment

Restoring a Heathkit IT-11 Capacitor Checker for use at my local Makerspace.

One of our Makerspace mem­bers donat­ed an old Heathkit capac­i­tor check­er that had belonged to his father for use at the mak­er­space. It was gen­er­al­ly in good shape, but had bit of dust and grime buildup from many years of use.
The Heathkit IT-11 was built from 1961 to 1968 when it was replaced with the IT-28 which had a beige/tan front pan­el. This unit had sev­er­al capac­i­tors dat­ed ear­ly 1963, so this unit is near­ly 60 years old.
Capabilities include capac­i­tance (4 ranges), resis­tance (3 ranges), capac­i­tor leak­age (3 to 600 VDC 16 steps), and using exter­nal stan­dards, induc­tance, trans­former turns ratio, addi­tion­al capac­i­tance and resis­tance ranges. There are three leak­age set­tings for dif­fer­ent types of capac­i­tors includ­ing Electrolytic 2 mA, Min ‘lyt­ic 15 uA, and Paper Mica Etc. of 2 uA.
The check­er uses a “Magic Eye” elec­tron-ray indi­ca­tor tube for visu­al­iz­ing when the bridge cir­cuit is bal­anced, and a read­ing can then read from the bal­ance dial indicator.

Heathkit IT-11 Capacitor Checker before major clean­ing. Very crusty bal­ance dial indicator

The first part of the refur­mish­ment was clean­ing the case, front pan­el, line cord, and knobs. After dis­as­sem­bling the unit I used a mild cit­rus based clean­er and tooth­brush on the case, knobs, and line cord with a final water rinse.

Heathkit IT-11 Capacitor Checker clean­ing the case

The front pan­el was cleaned using the same mild cit­rus based clean­er on a cloth tow­el to pre­vent any clean­er from get­ting into elec­tron­ics or knob bush­ings, with a water damp­ened tow­el wipe to remove any remain­ing clean­er.
Next up was clean­ing all the switch con­tacts and poten­tiome­ters with DeoxIt D5 con­tact clean­er. The vac­u­um tube pins and sock­ets were also cleaned with the DeoxIt D5 con­tact clean­er and then the glass envelopes were cleaned with iso­propyl alcohol.

The 59 year old elec­trolyt­ic and paper capac­i­tors were at the end of their use­ful life so I replaced them with some Vishay, Nichicon, and Illinois Capacitor parts.
I used my ESI 250DE Impedance Bridge to select the replace­ment 200 pF, 0.02 uF, and 2 uF film capac­i­tors used in the bridge cir­cuit for best accuracy.

Heathkit IT-11 Capacitor Checker tube side after capac­i­tor replacement

The orig­i­nal 600 volt DC fil­ter con­sist­ed of two 40 uF 350 VDC capac­i­tors in series, effec­tive­ly mak­ing a 20 uF 700 VDC capac­i­tor. Unfortunately Heathkit did­n’t prop­er­ly design this fil­ter cir­cuit in my opin­ion.
In a per­fect the­o­ret­i­cal world plac­ing capac­i­tors alone in series would be fine, but in prac­tice the two elec­trolyt­ic capac­i­tor capac­i­tance val­ues will not match, even when they are brand new.
If one capac­i­tors capac­i­tance changes in the series string, so will the volt­age across that capac­i­tor. Another issue is unequal capac­i­tor leak­age cur­rents, which will also cause volt­age across the capac­i­tors in the string to be unequal. Heat, age, humid­i­ty, elec­tri­cal stress can also affect the capac­i­tance val­ue. Before you know it the volt­age across the capac­i­tor has exceed­ed it’s rat­ing, caus­ing pre­ma­ture aging or fail­ure.
The solu­tion is to uti­lize bal­anc­ing resis­tors in par­al­lel with each capac­i­tor in the series string.
There are sev­er­al dif­fer­ent for­mu­las for cal­cu­lat­ing the val­ue of the bal­anc­ing resis­tors, but my favorite is by VA1DER, when using high qual­i­ty matched capacitors:

  • N is the num­ber of capac­i­tors in series
  • Vrate is the rat­ed max­i­mum volt­age for any one capacitor
  • Vbus​ is the bus volt­age – the expect­ed volt­age across the whole series of capacitors
  • IΔleak​ (in μA) = 0.0015 x C x Vbus
  • C is the rat­ed or aver­age mea­sured capac­i­tance of one capacitor
  • Rbalance​ in megohms

I usu­al­ly half the resis­tance, to give myself a safe­ty fac­tor of two, as long as the bal­ance resis­tor’s pow­er dis­si­pa­tion is a rea­son­able amount.

Heathkit IT-11 Capacitor Checker with bal­anc­ing resis­tors in par­al­lel across capacitors

I fol­lowed the Test and Calibration pro­ce­dures from the Heathkit IT-11 Assembly Manual. The hard­est part of the cal­i­bra­tion was com­ing up with the prop­er resis­tances to gen­er­ate the sim­u­lat­ed leak­age cur­rents for each of the three ranges. There are two dif­fer­ent pro­ce­dures for cal­i­brat­ing leak­age cur­rents. The first involves using spe­cif­ic resis­tance val­ues and volt­ages, and the sec­ond uses a meter that can mea­sure mil­liamps and microamps. I end­ed up using the sec­ond, as it is more accu­rate. I did­n’t have a 2 megaohm vari­able resis­tor, so end­ed up using a 1.5 megaohm fixed resis­tor and padding it to achieve 2 uA.

Heathkit IT-11 Capacitor Checker anoth­er view of the tube side components

The Heathkit IT-11 capac­i­tor check­er is still a great tool for mea­sur­ing capac­i­tor leak­age, but most new mul­ti­me­ters with capac­i­tance modes and ded­i­cat­ed LCR meters eas­i­ly out­per­form this check­er by a wide mar­gin.
It is still use­ful tool as long as the per­son using it under­stands it’s lim­i­ta­tions and is will­ing to use it safe­ly, due to the high volt­age poten­tial on it’s “Test” out­put jacks.

Heathkit IT-11 Capacitor Checker after restoration

7 Replies to “Heathkit IT-11 Capacitor Checker Refurbishment”

  1. I’m still a lit­tle fuzzy on the math. The schemat­ic that I have for the IT-11 shows 600V across those two capac­i­tors. When I do the math, I get 2.36 Megaohms, or 23.6K Ohms. I don’t see how that trans­lates to two 330K resis­tors. Can some­one help me out? Am I mak­ing a basic cal­cu­la­tion error or what?

    1. The cor­rect cal­cu­la­tion for the capac­i­tors I used in the refur­bish­ment is 2.36 megaohms.
      The 330K resis­tors in the pic­ture were used for test­ing, and lat­er replaced with Vishay HVR3700002324FR500
      which are 2.32 Mohm, 0.5W, high volt­age, safe­ty met­al film resistors.

  2. It’s a great review. Thanks for shar­ing. I’ve have the same tester and it’s use­ful to check and renew capac­i­tors. It’s some­time use­ful to check them with the real volt­age that they’re sup­posed to with­stand. New testers are more pre­cise but sure­ly don’t have high­er volt­age tests. Useful to any­one who restore old­er valve equipment.

  3. Hi, I just restored one of these units. Can you tell me what is meant by “MIN’LITIC”? Are the tan­ta­lum capacitors?
    Thank you

    1. Hi Mirach,
      The Type label “MIN.‘LYTIC” is short for Miniature Electrolytic.
      Min Lytic capac­i­tors are clas­si­fied as High capac­i­tance Low work­ing volt­age and small phys­i­cal size.
      At low­er work­ing volt­age rat­ings most elec­trolyt­ic capac­i­tors have high­er ESR val­ues for the same capac­i­tance val­ues as units rat­ed at high­er work­ing voltages.

      Greg(Barbouri)

  4. Thank you. Unfortunately my IT11 can­not be con­fig­ured for 230V, it only has two black wires in the pri­ma­ry. strange because in the wiring dia­gram the input trans­former has 4 wires. Perhaps this is a first ver­sion. I’ll have to use a exter­nal transformer..

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