Test Cards# 5+6 for L3-3 tube tester transformer
Description
The price and order number is for TWO cards, #5 and #6. It is very important to test each winding of the mains transformer for every new acquired tester. Transformer problems by itself are rare, but with testers on the market they are not rare at all. Reason being simply, nobody will keep a tester with a problem which he can't fix.
Transformer damage
There are ways to damage a tube tester transformer. With some testers, the transformers are tiny, and it may seem strange, but when you do something wrong, you see it often in time. It gives a loud hum, or pilot light burns dim. With L3-3 however, the transformer is very powerful. This is needed, because it can do testing in a programmable circuit inside the tester. Like when testing a powerful rectifier that way, like for instance the "83" or 5U4G, not only the 50 Watt energy for the tube must be supplied, but also this 100 Watt DC output, the will pump into the dummy load must be supplied. (That big resistor bank, in the back of the tester).
Though the transformer is very large, this doesn't mean it cannot damage. I can tell you, it does not damage when you set it to 115V and then plug it into 230V. That will saturate the core, and the mains fuse blows immediately. But there is no protection when some defect shorts one of the AC anode supply windings for a rectifier test. That gives enough heat to burn one of the windings, and melt some wire isolation inside the transformer, but not enough to blow the primary fuse. Other transformer problems can result from excessive current draw from a low voltage heater winding. A damaged winding may give no problems at first sight. Just the transformer get a bit warmer than normal, but no smell or noise. The problem is, that winding has a few percent lower voltage. For instance 8% lower. When the winding is loaded however, voltage drops 20% even at not very high current. So a tube test can still be done, but at too low heater voltage, of course results will be wrong. Yet, you may not find out quickly, and throw "bad" tubes, which were not bad at all.
Cable tree damage
The cable tree holds many wires, These are (glass?) fiber isolated, and around that is some plastic. The fiber is very good. The plastic stays like new over time, which is also very good, but the price for that seems, it has a low melting temperature. If you short the transformer via the cable three, that particular wire deep inside will melt the isolation of itself, and also of other wires. It can cause any problem whatsoever. This causes perhaps a load on one or more transformer windings, which will not necessarily damage the transformer. The tester may even seem to work, as long as you do not use that particular function the wire is for, or perhaps it gives only a problem when particular plugs are used. Such problems may take a long time before you find out.
Test routine
It is good practice to check all transformer voltages before a calibration is done, and also when you buy a new tester. With L3-3 however, the transformer has many terminals and it cannot be reached from all sides easily. Also you need to take the tester apart, and all together this becomes a terrible exercise, nobody likes it.
I made test cards for it
I found it is possible to connect many of the voltages to the plug board, and you can just plug a Volt Meter into the holes then. Some other voltages are already connected to the plug board by default, and it just needs overlay cards, to make sure you plug in the right holes.
The test results
It is important to write the results in a table, because not only absolute numbers say something about the transformer, but also how the numbers relate to each other. With this table, calculate the error in percent for each voltage. From there, you can see if there is a problem with just one winding, or with whole transformer. If all voltages are too high or too low all together, with quite the same percentage, this means probably the transformer is good, and it needs just calibration. However, when a voltages has a remarkable different deviation than the others, this winding is bad, or there is something loading the winding which should not be. Perhaps some circuit error, or a cable tree burn. So in case, only one winding is bad, de solder the wire fron the transformer, and see if the voltage becomes normal again.
What to do if all voltages are too high or too low, with the same percentage?
This would need recalibration of the 'triangle' calibration mark. There is no official procedure for this, because this is an undocumented factory calibration. Yet I found it often to be not very good any more. Please remember, because there is no official procedure for this, it needs some judgement before you change something. So in case you come to the conclusion, the mains calibration mark is offset, and you want to correct it, FIRST simply make an artificial, new calibration mark just with a pen, or with some tape, on the meter glass. Then ignore the old calibration mark, and use your own. Now with this settingn go through the cards 5,6,7 and see if things have improved. So you should be closer to the required voltages. Simply use this new calibration mark. Then when you come to the conclusion you want to change it, you can adjust R85 such that the needle moves from your new (tape or pen) mark to the original mark on the panel meter scale.
How NOT to do this
Sometimes I see people use the internal 6.3 Volt heater voltage of the electronics as a reference for the mains calibration. This is written nowhere and it is totally wrong. Reason is, the internal 6.3 Volt heater voltage is only 6.3 Volt indeed with a fully loaded transformer, but you are hardly ever doing so. Moreover, because test instructions say so, you have to set the mains calibration (Triangle) mark BEFORE you begin. So it can already be seen this must be done with an unloaded tester, and in this case internal heater voltage for the internal electronic goes up some 5...7%. So this internal heater voltage, please regard it irrelevant for a thing called CALIBRATION. It is not what this voltage should be used for.
Procedure for mains calibration (Triangle) mark:
- Use card #5 and Connect an AC Voltmeter to the 7 Volt heater winding, as shown on the card.
- Rotate the mains adjustment knob and choose the setting which makes the needle move get closest to the triangle mark. You will not always be able to put it exactly on the mark. So it better is to use variac, because then you can put it exactly on the mark.
- Also be careful, the mains voltage may suddenly change 5% in the middle of your work. So keep a multi meter connected to the mains voltage. If that happens, you need to start over.
- Now the measured voltage should be as indicated on the card, within it's minimum and maximum limits.
- Verify this also with the other heater voltages and with the high voltages. Though heater voltage has first priority.
This will give you an overview if only one voltage is out of tolerance, or the whole tester.
Recommendation
At first, you can leave the new calibration marking on the glass for a while, and see if the tester works better now. Also verify this with cards #5 and #6 of course. When this seems what you need, the only question is now, how make the needle point at the original calibration mark. This is done by change the value of R85. Be careful, there is some 300Volt on R85 to ground.