L3-3 tube tester. Test Button reversal

The concept of the L3-3 is, to show the heater voltage if no test button is pressed. So to see any test result, you always need to press the "Test" button. If you work with the L3-3 for longer than 30 minutes, you get a blister on your thumb, because the button is so heavy. Besides you have one hand less, because you always have your thumb on that "test" button. This was for me an important shortcoming. I found out, with some small modification on the switch, you can solve this problem.

To remove the switch from the top, you need a special tool, or as I did it, I grinded an old screw driver to fit exactly one of the two slots. Then, with the screwdriver in only one of the two slots, you can rotate the screw, and remove that funny screw easily.



Mark the blades well.

When you take apart the switch, you must be careful it doesn't fall apart, or you will have a hard time getting the switch back together as it was. I would say it is very wise to write a fat stripe on the side, with a marker pen. Also make a few clear pictures. You could also mark the blades with one dot, two dots, three dots etc. It may be a good idea to tape the first SIX blades together a little bit too, since these stay as they are. Anyway make sure no "accident" happens,

Now, the work to do is very little.

1) You have to remove the last two blades, these are the bottom ones (7+8) in this picture, but you leave the soldered wires on. The blades stay in the same position, so they are not rotated or anything. It is just you swap them, and leave the wires on. After this, you mound back together the switch, and this part is done. Remove the tape.

2) Now the blade with the bended end is at the last position. First thing you do now, is preliminary change the shape of the last blade, until it functions normal. After this you are note done yet. Now you have to check that bladed 1...6 open and close nicely as they did before. Un tighten the screws a tiny bit, and try to see what rotating them does. Do NOT re-bend the blades 1...6, since this is already nicely factory adjusted. It is just their position by rotating them, that you need to re adjust, so the silver contacts nicely touch each other in the middle.

3) Now comes the adjustment of blades 7+8. This is the "final" meter switch. If you press the button, first thing that must happen, THIS switch opens. (Whereas originally it was such that LAST thing that happens, this contact closes). This is important to do right. So you need to adjust the shape of the blades 7+8 accordingly. Basically this is done, by making the distance "D" in the last picture quite small.

4) Final control:

When you press the switch, FIRST 7+8 open, and after that the blades 1..6 do their function.

5) Now you solder off, counted from the top to the bottom: wire 1 and 3, and solder them back on, reversed. MARK them before you start! If you make a mistake, it will drive you crazy. Then the same is done with wires 4 and 6, and your 're done.

6) Tighten the screws finally, and re-check if all contacts are nicely in the middle, and open and close nicely as they should. Mount back in the switch and you're finished

AFTER. How to "TEST" button was reversed.

Detail of modified switch

Capacitor must be placed.
A good value is 100....300uF

After the meter is reversed, it advised to mount a capacitor if 100uF across the meter. This is better, because after the modification, if you rotate the measurements switch, the needle might move relatively fast from one position to another. Perhaps this is not even a problem, since it's the same with other testes, but let's just protect the meter best way we can. Actually I used my L3-3 a while without protection cap, and it ok like that. Yet, errors can happen when you are experimenting with self made cards. Or plug the wrong, by FORGET a plug. For instance this may cause the plate voltage be set to 300V, while you have chosen a 15V meter scale. The coil itself can take this 20x overload, the electrical energy is still small, and the coil will not overheat from this. It is just the mechanical energy which is large, and the needle will hit the scale end violently, which deforms the needle or the coil even. So this should be avoided. Not just after the test switch in modified.

The meter is 150uA, and we do not want any capacitor leakage causing difficulties. This can not be guaranteed with a plain electrolytic capacitor, since the voltage over it will be just a few millivolts, and the cap will not regenerate chemically (as it will in a normal circuit). You may use a 100uF foil cap but they are quite bulky. Do not use the modern "energy storage" foil caps, such as used in switched power supplies, since these are leaky by default. Same for paper in oil. So to take the best, I choose for a special kind of capacitor, the so called "SEALED WET TANTALIUM". These are expensive, but can be found cheap on Ebay as NOS . These caps have lowest leakage possible, long life time, no storage problems, and tight tolerance. So all problems of aluminum electrolytic caps, these don't have. Most are Russian, some USA Sprague also are offered. When you get a NOS cap, they are perhaps 40 years old, and they're still perfect. These are real diamonds, because with the older ones there were no limitations for chemicals used, and their original use was for stability. I can not prove it, but I think specially the Russian types are the strongest of all. You do need to re-format them before use or testing. For this, connect the cap with a 1Meg Ohms resistor to a voltage source which equals it's working voltage. After 30 minutes it's finished. Now you can first time use the cap, like test it with a meter, or put it in a circuit. Sealed Tantalum caps should be close to their rated capacitance, because they're finest quality products. You may want to test this, but probably their is no need anyway if you don't have a tester. On the picture here, you see where the cap can be mounted, just put in in parallel with the meter protection diodes. Some L3-3 have these diodes close to the banana jacks on the stabilizer board. Just look for them, you can't miss it. So on that case, the caps are mounted there.

Now the meter will move gently.

Important: Check with a sensitive voltmeter than you have polarized the cap correctly.