Thursday, March 17, 2011

Heathkit IT-22b Capaci-Tester



Heathkit Capaci-Tester IT-22b
One of the pieces of test equipment I brought home from the last hamfest was the Heathkit IT-22b Capaci-Tester.
It is a fairly simple piece of equipment designed to test capacitors for shorts or open circuits.
A critical piece of information I learnt while researching this tester was that it is NOT designed to test electrolytic capacitors despite having what looks like DC red & black connectors on the front.
Instead of a meter is has a 'magic eye' tube which uses a vacuum fluorescent display and electronically deflected shadow wedge to indicate the state of the component under test.
For an excellent display of 'magic eye' tubes take a look at www.magiceyetubes.com
For more information on the Heathkit IT-22b take a look at the information provided by Joe Sousa at www.radiomuseum.org and included below.



Fellow Radiophiles,
I present here a functional analysis for each position of the rotary switch of the IT-22 capaci-tester. I don't have access to a manual, I am working with the schematic and a picture of the front panel.
Another description of the test capabilities is at http://www.heathkit-museum.com/test/hvmct-1.shtml
According to this link, this tester does not check leakage or electrolytic caps. Note that the test signals used by the tester are all AC type signals at 19MHz for opens and 60Hz for shorts. No DC bias is provided for electrolytics. The primary usefulness of this tester is the ability to test non-electrolytic paper, mica, film or ceramic capacitors for shorts or opens, without removing them from the Radio or TV.
It would be good to see a set of operating instructions for this IT-22 set.
Before I start on the general functionality, note that there is no DC power supply. The eye target anode and triode plate only work on the positive halves of the 540VAC power transformer secondary. The 55VAC winding provides a bias that is negative when the tube is active on the positive halves of the 540VAC winding. One drawback of this approach is that the eye pattern may not be as sharp as with a DC supply.


Heathkit IT-22 front panel
I will refer to the switch drawing in the schematic, and rotate the switch clock-wise, starting from the OFF position. The front panel shows 5 switch positions with OFF at the center:

  • Short Test
  • Short Check
  • AC OFF
  • Open Check
  • Open Test








Position Short Test 
The control grid is connected to the Red test lead and to the negative bias source provided by R1 and R2, via taps 4+5+10. This is a DC test because the Hartley oscillator is disabled, and the test is conducted during the positive half cycles of the 540VAC power transformer winding. A shorted input capacitor opens the eye. A good capacitor under 1uF will close the eye. The absence of a capacitor at the leads also closes the eye. The reactance of 1uF at 60Hz is 2.6k, which will start to load the R1R2 significantly. Above 1uF, the eye should start to open up due to the low capacitive reactance. The CT-1 instructions at the link above say that this test works for capacitances up to 20uF. Larger capacitances look like a short.
Position Short Check 
The control grid is grounded by taps 5+9. The triode plate and deflection anode go to a low voltage and open the eye. The Red test lead is open. The LC circuitry at the cathode is prevented from spurious oscillation by the R4=3.3K damping resistor. This test is meant to warm up the unit from the off position and the capacitor should not be tested until the eye is completely open in this position.
Position AC OFF 
The slaved power switch in series with the power transformer is open. A 3.3k resistor shunts away any charge present at the capacitor to be tested.
Position Open Check 
Switch taps 9+10+11 are grounded, thus shorting out the cap under test without any current limiting resistor, before any tests are conducted. The control grid is tied to C5 via switch taps 5+2, thus completing the feedback loop of a Hartley oscillator via L1B. The -8V DC grid leak bias that develops at the grid due to the Hartley oscillation, cuts off the triode. The resulting high voltage at the triode plate and deflecting anode close the eye completely. The oscillation frequency for the similar CT-1 model is19MHz. This test is meant to warm up the unit from the off position and the capacitor should not be tested until the eye is completely closed in this position.
Position Open Test 
The Hartley oscillator loop is still closed by taps 5+2. The oscillator coil secondary L1A is now tied to the active RED test input  via C6=470pF. R4=3.3k is still in parallel with the capacitor input. A non-shorted capacitor greater than 50pF at the input will lower the oscillating frequency to a point that oscillator amplitude is much lower, or extinguished. The -8V grid leak bias is thus reduced and the eye closes. The reduction in oscillation amplitude with at lower frequencies is due to the Q of the inductor being dominated by series resistance of the L1 inductor, in particular the L1a winding that is loaded by the cap under test. A shorted cap will also close the eye, so the short test should be done first.
Corrections and comments are invited.
Regards,
-Joe


Heathkit IT-22 Capaci-Tester circuit diagram






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