Marconi MU-101 Calibration Problem

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Marconi MU-101 Calibration Problem

Post by TPK »

I have a Marconi MU-101/Hickok 533 tube tester that I am in the process of doing the final English Calibration. I am using the Tube standard calibration ( Variac/Isolation transformer/amp meter, through a 10K 10 Watt resistor) which is supposed to give 5ma and translate to 1000 Umhos on the meter.

I keep getting 7.5 ma on my amp meter and while rotating the bottom half of the English control meter on the tester reads around the 500 mark instead of 1000.

I am using the 6L6 tube settings as well,, J R 5347-2. Bias 23 English 88. I would like to know what the correct English meter reading should be when calibration is done this way.

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Re: Marconi MU-101 Calibration Problem

Post by Jac »

Hello Paul,

I am not familiar with the calibration method with the variac. I think this was rather used with the 539 tester.

I am not sure if you can use it with the smaller testers, but I suppose the only effect you get from this is a fixed reading on the mmho scale. Like as you you say, when it transfers to 1000 mmho, you need to read 1000mMho.

I can tell you a little bit what to do, but I am not so familiar with the Hickoks, and when I have to work on one, I need to dig deeper into it too before I can begin. Also these work very strange, to my opinion only to get it patented.

The English test is a load test for the tube, and I do not know how it's calibrated, but it's not a mMho test. The potmeters should be balanced. Also what you measure with the English test, is kind of undefined. So I take a tube of which I know it is kind of weak, and I check if this is reproduced with the English test. Normally when the rest of the tester is good, the English test is also ok.

As you get only half the value, as you write, perhaps that has to do with the tester working only on a half wave. So on the other half there is no current. Same confusement there is always with the AVOs.

There is a calibration procedure for each Hickok, but unfortunately I don't have the factory manual for the 533. I know it excists, but those who have it are hiding it for the rest of us. That's a pity or a shame, call it what it is...

The difficulty is, the Hickok i needs to indicate mMho values like you are used to from normal datasheets, but they test the tubes at totally different voltage and current for that. So they play with the bias voltage such that it looks right. Like this you get 6000mMho for the 6L6 like you are used. Don't know for sure, but I always thought this is what they are doing.

Very brief, this is how I work:

FIRST CHECK THE METER. Impedance, full scale and linearity must be perfect, no stick-slip.

1) Set the mains such that you have 93V transformer primary. (Check for the 93V in the internet, but I am pretty sure this is right). Set the calibration mark. If this gives problems, there you have a problem inside somewhere.

2) Check ALL voltages on the transformer. Sometimes one can be bad, it's a common error, and the whole tester is compromised by it.

3) Check the output voltage of the Bias circuit, at all positions of the pot meter. Small linearity errors can be corrected by bending the arm.

4) I take normally a 6L6 characterized at 250V plate and screen, set the bias of the Hickok at the roll chart value, and not adjust the slider of the calibration pot meter until the mMho is indicated correct for this particular tube. So when the tube is 6300 it needs to say 6300.

Sorry for not having a better answer.

User manuals are in the internet, but if you can get hold of the official factory manual, please let me know!

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