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 Post subject: Micronta 22-220 Analog Multimeter Calibration
PostPosted: Feb Mon 26, 2018 5:48 am 
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Location: Fountain Valley, CA 92708
I have a Micronta 22-220 that is in excellent condx cosmetically but I note that it is out of spec for the DCV scales (+/-3% for 1-2-10-30 versus a DMMCheckPlus). The resistance measurements are dead on. The DCV readings seem uniformly low. Typcally:

1 Volt Scale
0.5 VDC shows 0.47
1.0 VDC shows 0.93

3 Volt Scale
0.5 VDC shows shade under 0.5
1.0 VDC shows shade under 1.0
2.5 VDC shows shade under 2.4

10 Volt Scale
0.5 VDC shows approximately correct
1.0 VDC shows shade under 1.0
2.5 VDC shows shade under 2.4
6.0 VDC shows 5.6
10.0 VDC shows 9.3

There are no apparent adjustments except two VRs on a daughter board which do not seem to affect the DCV readings. Nice meter; would want to keep it because to the 10 MOhm per volt, etc.

Questions:

(1) Does anyone happen to have a manual for this (the 22-220)?
(2) Does anyone happen to know if the DCV readings can be calibrated?
(3) Am I doomed.

My thanks in advance.

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 Post subject: Re: Micronta 22-220 Analog Multimeter Calibration
PostPosted: Feb Mon 26, 2018 10:48 am 
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Location: Lincoln City, OR
Greetings to Bill and the Forum:

Bear in mind that the +/- 3% is 3% of full scale reading. Of the voltages posted in your table, only about half of them are out of spec... and the ones that are out, aren't out by much. If the meter were mine, I'd be perfectly happy with it. Bear in mind that Leigh says that most voltages in antique radios are + / - 20%.

If one were using the meter as a standard to calibrate other meters or in very high precision work, the discrepancies would be bothersome.... otherwise, no big deal.

If the discrepancies were uniform over the entire deflection range of the meter, you might be able to correct it by tweaking the multiplier resistors. However, in your case, the discrepancies seem to be non-linear.... in which case, the problem is in the meter movement itself. Not practically correctable.... and as I said, were it mine, I'd be fine with it.

Just my $.02 worth; your mileage may differ.

Regards,

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 Post subject: Re: Micronta 22-220 Analog Multimeter Calibration
PostPosted: Feb Mon 26, 2018 1:58 pm 
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Location: Fountain Valley, CA 92708
Jim,

Thanks for the observations and insights. I never considered meter movement and the age of the meter. While it has pretty much been a "hangar queen", I cannot speak for it's life before I acquired it in 2013. Still in all, it is surprising that there are no calibration pots on the unit.

I am sure there were some design considerations involved in that state of affairs.

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 Post subject: Re: Micronta 22-220 Analog Multimeter Calibration
PostPosted: Feb Mon 26, 2018 3:50 pm 
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Greetings to Bill and the Forum:

An analog VOM is a pretty simple device, actually. If the meter movement itself is accurate (adjusted at the factory by tailoring the magnetic field strength and spring tension) and the multiplier resistors all meet spec (usually 1%) then the meter will meet its accuracy specifications. If the meter movement looses magnetism over the course of many years, the meter will become less sensitive.... but the only cure for that is to send the movement to a meter shop and have it re-magnetized and calibrated to spec. Multiplier resistors don't usually age like carbon composition resistors do... so there's no need for cal pots.

The situation is different for a VTVM. Here you are dealing with tube aging, perhaps non-precision components in the bridge circuit or the AC rectifier circuit and so cal pots are de rigueur.

If you want real accuracy, you need a differential voltmeter. I have let a couple slip through my fingers at swap meets over the years; now I'm sorry I didn't try harder. Oh, well....

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 Post subject: Re: Micronta 22-220 Analog Multimeter Calibration
PostPosted: Feb Mon 26, 2018 5:17 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: St. Louis, MO, USA
I keep a supply of precision resistors and capacitors around and periodically check my equipment, espically the really old ones.

Dennis


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 Post subject: Re: Micronta 22-220 Analog Multimeter Calibration
PostPosted: Mar Sun 04, 2018 4:35 pm 
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Joined: Mar Sat 03, 2007 12:53 am
Posts: 688
VR1 on the daughter-board should control the DC calibration. When you adjust the pot you should allow some time for the reading to settle.


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 Post subject: Re: Micronta 22-220 Analog Multimeter Calibration
PostPosted: Mar Sun 04, 2018 7:19 pm 
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Location: Fountain Valley, CA 92708
Quote:
VR1 on the daughter-board should control the DC calibration. When you adjust the pot you should allow some time for the reading to settle.
Alan,

Thank you for that! It has cleared up the question immensely! For the record, I made some adjustments to VR1 and gor the following results (to compare with my initial message above).

1 Volt Scale
0.5 VDC shows 0.50
1.0 VDC shows 1.00

3 Volt Scale
0.5 VDC shows 0.5
1.0 VDC shows shade over 1.0
2.5 VDC shows shade over 2.5

10 Volt Scale
0.5 VDC shows approximately 0.5
1.0 VDC shows 1.0
2.5 VDC shows approximately 2.5
6.0 VDC shows 6.0
10.0 VDC shows shade under 10.0*
*PentaRef lists the 10V standard as 9.999

As you can see, the measurements are well within spec and I am a happy camper because these are good units and, inasmuch as they are DUAL FET, they are on the order of 10MΩ per volt. For the record, I included picture in the event future inquiries are made.

I still am looking for a manual for this unit if anyone has one. Also, Alan, would you happen to know what the other VR is for?

Thanks to all.


Attachments:
Micronta 22-220 #1.jpg
Micronta 22-220 #1.jpg [ 75.49 KiB | Viewed 381 times ]
Micronta 22-220 #2.jpg
Micronta 22-220 #2.jpg [ 97.16 KiB | Viewed 381 times ]
Micronta 22-220 #3.jpg
Micronta 22-220 #3.jpg [ 59.91 KiB | Viewed 381 times ]

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Bill, k6whp
"Cheer up, things could get worse.
So I cheered up and things got worse."
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 Post subject: Re: Micronta 22-220 Analog Multimeter Calibration
PostPosted: Mar Sun 04, 2018 7:51 pm 
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Joined: Oct Tue 29, 2013 9:42 pm
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Location: Fountain Valley, CA 92708
..belay the request for a manual or schematic as I found an .pdf scan I made. I attached the schematic below but, with the size restriction, t is practically worthless.

However, if anyone needs the manual from me an email at k6whp [at] arrl [dot] net and I will oblige. (It's 6.2 MB.)


Attachments:
Micronta 22-220 Schematic.jpg
Micronta 22-220 Schematic.jpg [ 107.37 KiB | Viewed 375 times ]

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Bill, k6whp
"Cheer up, things could get worse.
So I cheered up and things got worse."
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 Post subject: Re: Micronta 22-220 Analog Multimeter Calibration
PostPosted: Mar Sun 04, 2018 8:45 pm 
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Joined: Oct Tue 29, 2013 9:42 pm
Posts: 404
Location: Fountain Valley, CA 92708
..and, finally, disregard the request for what the other VR does. It's the ACV adjustment. See attached (revised) picture. I also located the ACTUAL manual so if anyone needs a detailed schematic, I will do the best I can for you.

This is what comes of having too much time on one's hands! I plan to spend the rest of the day polishing ans sharpening my DMM leads.

:roll:


Attachments:
Micronta 22-220 Multimeter Voltage Adjustments.jpg
Micronta 22-220 Multimeter Voltage Adjustments.jpg [ 84.85 KiB | Viewed 367 times ]

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Bill, k6whp
"Cheer up, things could get worse.
So I cheered up and things got worse."
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