"A" Filament

Cleaning

You must be VERY careful when cleaning the electronics of a Mighty Mite, or you will have a non-functioning Mighty Mite. The heart and soul of the Mighty Mite is its ultra-sensitive Leakage circuitry. Leakage detection sensitivity is over 100-megohms. This is the one function that is not comparable in other tube testers, and is truly a highly valued feature.

Do NOT clean the guts of your Mighty Mite with any of these modern biodegradable cleaner/degreaser agents, such as Clean-Rite Purple Power®, Castrol Super Clean®, or anything similar. Regardless of how much you rinse the cleaning solution off, and regardless of whether months or years pass, these cleaners cause leakage problems that do not go away. Apparently they leave behind a microscopic residue that conducts current. But switch your Mighty Mite into Grid Leakage test after using these cleaners, and you will find your tube tester detecting Leakage even with no tube in any socket.

If this problem has happened to you, you may have a VERY tough time fixing the problem. I have had units sent to me with this problem for repair — the problem is rather obvious when you see a perfectly clean Mighty Mite detecting Leakage as soon as you turn it on. The only solution that I have found is to use a powerful Electrical Contact Cleaner/Degreaser and flood each socket (front and back) and each switch repeatedly until the problem is solved, lightly using a nylon brush to agitate the cleaner before it evaporates. Unfortunately, you must be VERY CAREFUL with this type of powerful Contact Cleaner, because it WILL eat into plastic. For example, it can ruin the meter faceplace, it can dissolve the pin straighteners found on some Mighty Mites, and it will even eat into the internal plastic layers found inside the "C" switch (there are clear-plastic spacers/shields between each switch layer inside switch "C".)

Therefore, I would suggest not using any cleaning solution on your Mighty Mite unless you need to remove stubborn grease or grime. You can blow the dust out of your tester with a garage Air Compressor. If grime must be removed, use a very mild soap, diluted, and rinse well and let dry for a very long time.

A properly working grid leakage circuit should not register almost any leakage (in any position A through K on Switch 'D') when no tube is being tested. (Note that Position H-K on Switch D will register a little higher, and this is normal.)

Grid Leakage should be at or near Zero with no tube being tested

Grid Leakage should be at or near Zero with no tube being tested

Remove the guts from the case. Inspect each wire leading to each tube socket terminal. Look for broken wires at each socket terminal, and inspect for any wires that may be shorting to other wires/terminals or shorting to the chassis. Inspect the AC line cord and top cap wire, and replace if necessary. Check that the meter needle is indexing at the far-left line on the meter with power off, and adjust face setscrew accordingly.

Check every resistor, and replace any that are out of tolerance. Most of the resistors are silver-band 10% tolerance, and a few (such as the 10-Meg resistor) are 5% tolerance. Replace as necessary. Pay special attention to the 10-meg resistors and also the load resistors under Switch "B'. Since most of the load resistors are silver-banded 10% tolerance, a natural variation in Emission test scores is normal and acceptable.

Always replace the 0.1 cap that is integral to the Shorts circuit. It is commonly defective. Likewise, replace the 20mf electrolytic cap. The remaining three caps are disc-style 0.01mf, and seldom defective. Replace only if defective. Notice that Sencore specifies that two of those 0.01mf caps must be 10% tolerance.

Check the internal 12AU7A tube with another tester, replace if weak, shorted, or leaky.

Make sure each rotary switch position has good continuity, and carefully clean as necessary.

Power on unit, and check each Filament voltage position for proper voltages. If any position does not register, you either a problem with the "A" Filament switch, or an open transformer winding.

Calibration

Calibration can only be attempted if all of the preparation has been thoroughly completed. Anything less is a total waste of your time. Calibration assumes that all circuit components are working properly, which means accurate resistor values, good continuity, and working capacitors.

Calibration of these tube-type Mighty Mites is straightforward. You need a variac and a voltmeter, preferrably a vintage USA digital Fluke for maximum accuracy. You will be adjusting the CAL and Zero Adjust Potentiometers, which are shown in this photo.

Plug the tester into the variac, Power-on the tester into CAL position, and set the Variac for exactly 115 volts. Allow tube tester adequate time to warm up, preferably at least five minutes. Monitor the line voltage with your Fluke.

Adjust the CAL control on the front panel until meter reads CAL on the right side of the meter.

Switch the function switch to Shorts-position, and adjust the internal "Zero Adjust Control" until meter reads Zero. (Note that Zero is not all the way on the left).

The CAL control and ZERO ADJUST CONTROL will interact with each other, so repeat the adjustments several times until both controls are calibrated as you switch back and forth.

You can now disconnect from the Variac. When using the tester at full line voltage, be sure that you READ THE INSTRUCTIONS and are using your tester properly. After inserting a tube for testing, you MUST switch to the CAL position first and adjust the CAL control. This properly adjusts the circuit for your line voltage, taking into account any load draw from the tube under test. You readjust this front panel CAL setting with each tube that you test.

Test Shorts and Leakage

Turn the function switch to SHORTS position, and rotate switch D to all positions, making sure that the shorts light does not illuminate. Insert a test socket, jumper two pins together, and make sure that these self-induced shorts are properly being detected.

Next, turn the function switch to GRID LEAKAGE position, and rotate switch D to all positions, making sure that the meter does not detect Grid Leakage with no tube under test. If it does, re-read the section above regarding cleaning. Insert a test socket, connect a high megohm resistor (such as 10-Meg) between two pins, and make sure that this self-induced Leakage is being detected in the appropriate switch position.

Leakage test