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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 29th Jun 2020, 1:00 pm   #21
Matthew kane
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Join Date: May 2019
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Default Re: Metrix 310B tube tester Meter replacement

Nope too early of a call. Both pivot jewel screws are disintegrated inside.
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Old 30th Jun 2020, 12:51 am   #22
Radio1950
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Location: Brisbane, Queensland, Australia.
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Default Re: Metrix 310B tube tester Meter replacement

That's a brave attempt at a new screw!
Aldous Huxley would be proud.

Great photo of the movement.


Provided that the rest of that original M310 meter is OK, and that there are no iron filings in the magnetic pole area, I think the meter is salvageable.

If you run out of ideas, and you are ready to bin it, consider posting the old meter up to me and let me try with my junk collection?
I don't wish to intrude on your skills, but I may have something.
No guarantees.


By the way, don't touch the rest of the M310 Tester just yet.
The mathematics of the theory of operation has grabbed me and I cannot let it go.
I think I have it all solved.
Those original M310 shunt values are actually correct.

Had to be, as Metrix of France was/is a metrology company, making high quality instrumentation gear.
They probably had primary standards in their lab, and real expertise.

In a few days I hope to reveal all!
.

Last edited by Radio1950; 30th Jun 2020 at 1:12 am.
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Old 30th Jun 2020, 11:37 am   #23
Dekatron
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Default Re: Metrix 310B tube tester Meter replacement

Since the tester is working with AC-voltage the tube under test is half-wave rectifying that voltage so only half the current flows in the circuit. This is the same as with any of the AVO VCMs.

The meter branch is specified with a total resistance of 50 Ohm, and with 1 mA flowing it means that 50 mV will drop over the meter and shunts which are in parallell.

With 50mV across the meter and shunts you can see that for the 100 mA range to work out you get 50 mV / (50 Ohm in parallell with 1.01 Ohm) = 50.5 mA, which is just a small portion from 50 mA which is the half-wave rectified current that you would expect.

The same holds true for the other ranges with, including the 100 mA range as above:

100 mA range: 50 mV / (50 Ohm in parallell with 1.01 Ohm) = 50.5 mA
30 mA range: 50 mV / (50 Ohm in parallell with 3.57 Ohm) = 15 mA
10 mA range: 50 mV / (50 Ohm in parallell with 12.5 Ohm) = 5 mA
3 mA range: 50 mV / (50 Ohm in parallell with 100 Ohm) = 1.5 mA

Another way of seeing this is to just divide the 50 mV across the meter/shunts with the shunt resistance and then adding the 1mA flowing through the meter - so for the 30 mA range you would get 50 mV / 3.57 Ohm + 1mA = 14 mA + 1 mA = 15 mA.

To get 50 mA exactly on the 100 mA range you would need a shunt with some 1.02 Ohm, 50 mV / (50 Ohm in parallel with 1.02 Ohm ) = 50 mA, but Metrix might have used 1.01 Ohm as it might compensate for the small heating of the resistor and meter branch.

Half-Wave rectified AC and its relation to Mean DC and RMS DC can bee seen in the included PDF.
Attached Files
File Type: pdf Bull-DMMglossary-E.pdf (76.7 KB, 5 views)
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Old Yesterday, 10:52 am   #24
Matthew kane
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Default Re: Metrix 310B tube tester Meter replacement

OK I tried the dpm as per instructions here. The transformer has been replaced before and no filaments winding from 16-117v. Current reading is abnormally lower than what it should get with tubes I have tested, high current pentodes. Think I might use this metrix as a base for a revamped tester to supply up to 500v for anode and screen supplies. Remove existing - 50v max neg grid bias and add up to 150v with dpm readout. Going to get quote from my trafo supplier to see how much a new custom toroid will cost. Also add a new pot and dpm method to calibrate mains.
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Old Yesterday, 10:36 pm   #25
Radio1950
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Default Re: Metrix 310B tube tester Meter replacement

Hi again

This is proving to being all quite curious.

If you have used a 100R resistor for the DPM, try now removing that resistor, have the high impedance 200 mV DPM directly across the 1.01R shunt, and use the M310 on "100 mA" range with a test valve.

I still have to do some investigation, and will get back with some more info next weekend, after I go home from present travels.

Dekatron's post is very good.
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