Author Topic: Micronta 22-195 Multimeter Calibration  (Read 4312 times)

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Offline Fisher77

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Micronta 22-195 Multimeter Calibration
« on: October 17, 2016, 10:30:59 pm »
I am new to the forum. I have been hovering around for awhile, and decided it was time to join. Lots of great info here. I recently bought aquired a 22-195. I have checked the accuracy of all the ranges, but 1 has failed. I am in the US, and mains voltage is 120V AC. This bad boy is measuring it at 160V AC. I was wondering if anyone might know the procedures for calibrating this meter. I tried searching, but only came up with one post about this meter. Any help would be greatly appreciated. Thanks
 

Offline rcbuck

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Re: Micronta 22-195 Multimeter Calibration
« Reply #1 on: October 17, 2016, 11:44:10 pm »
If DC voltage measurements are correct, I would check C3. That is a .022uF, 1000V cap. I don't know where it is in the meter but looking at the schematic, it may be wired directly across the AC/DC switch terminals. When the switch is in the DC mode, C3 is shored out by the switch.

If the DC range is also reading high, check the value of R1. According to the schematic, it should be 10 Meg ohms. It is kind of strange as the schematic indicates it is 1/2 half of a resistor. The other half is labeled R29. So it may be a 3-pin resistor pack. R29 is also 10 Meg ohms and could also effect on the voltage readings.

There are no calibration details in the manual. There are 4 trim pots on the schematic. The only one that gives any indication of what it does is VR101, which is used to adjust the current for transistor gain measurements.
 
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Offline Fisher77

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Re: Micronta 22-195 Multimeter Calibration
« Reply #2 on: October 17, 2016, 11:54:37 pm »
Thanks rcbuck. I will check the cap and post back. The DC voltage is reading within specs. I have searched the net for anything on calibrating this meter, and I have found absolutely zero on it.
 

Offline ModemHead

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Re: Micronta 22-195 Multimeter Calibration
« Reply #3 on: October 18, 2016, 01:05:56 am »
VR3 is the AC calibration trimmer.  The AC converter circuit is just to the right of VR3 on the schematic, connected to pins AD0, AD1, AVX1, and AVX2.  All those components would be suspect.  C18 and C19 appear to be electrolytic, so there's a good start.
 
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Offline rcbuck

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Re: Micronta 22-195 Multimeter Calibration
« Reply #4 on: October 18, 2016, 02:57:38 am »
ModemHead, I couldn't figure out how they were rectifying the AC voltage. I suspected those components but was unsure. Thanks for pointing that out.

I agree with you, C18 and C19 is probably the best place to start. If one of those were open, that would explain the 160V reading on AC voltages (1.414 X 120).
 

Offline Fisher77

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Re: Micronta 22-195 Multimeter Calibration
« Reply #5 on: October 18, 2016, 03:33:18 am »
The caps are not electrolytic, or at least they don't look like it to me. They look like tantalum's to me :-//. Do I test these like regular capacitors, I have never seen one marked like this? Took some pics. They say 2.2/16 on them. All the resistors from the AD0, AD1, AVX1, and AVX2 pins are good.

Thanks for the clear schematic ModemHead, the one I was looking at sucked.
« Last Edit: October 18, 2016, 03:38:41 am by Fisher77 »
 

Offline ModemHead

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Re: Micronta 22-195 Multimeter Calibration
« Reply #6 on: October 18, 2016, 03:45:27 am »
Yep, they look like tantalum.  Maybe one is leaky (electrically speaking).  I'd try swapping them out, for lack of a better idea.

My google-fu is weak, I can't find a datasheet on this SMC4301F.

But just going by general principles, the makers of these DMM-on-a-chip devices usually provide an op-amp buffer ahead of the A/D, and provide access to the feedback loop externally.  The multimeter maker can then decide to add an RMS converter chip for a "true RMS" feature, or add some passives to make a precision rectifier circuit.  Although in this case with 2.2uF caps it looks more like a peak-detect type of thing, not really sure.
 
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Offline Fisher77

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Re: Micronta 22-195 Multimeter Calibration
« Reply #7 on: October 18, 2016, 04:26:01 am »
Will do. I dont have any 2.2uF tantalum's. I have some 10uF tantalum's. I do have some 2.2uF electrolytic's if they will work? If not, I can try this itty bitty electronics store we have in town. Other than that its hurry up and wait for the delivery.
 

Offline rcbuck

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Re: Micronta 22-195 Multimeter Calibration
« Reply #8 on: October 18, 2016, 06:06:23 am »
You could try the electrolytics as a test. They are probably physically larger but if they fit it could be a temporary fix. The tantalums will have lower ESR but I don't think it matters as the meter is only rated for 1000 Hz AC.

I have had mine for over 25 years and it has always be fairly accurate. Other than occasionally blowing the 500ma fuse due to doing something dumb, it has never given any problems.
 
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Offline Performa01

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Re: Micronta 22-195 Multimeter Calibration
« Reply #9 on: October 18, 2016, 11:05:23 am »
Will do. I dont have any 2.2uF tantalum's. I have some 10uF tantalum's. I do have some 2.2uF electrolytic's if they will work? If not, I can try this itty bitty electronics store we have in town. Other than that its hurry up and wait for the delivery.

Al-electrolytics would be better compared to tantalums in just about any respect – except for high frequency behavior, which includes ESR. For a circuit meant to operate at 1kHz maximum, the lower leakage and vastly better reliability of Al-electrolytics would certainly be of much more value.

The ultimate solution would be the use of polyester (mylar) cpacitors though, as they are still another order of magnitude better than any electrolytics, be it aluminium or tantalum.

http://www.ebay.com/itm/WIMA-MKS2-2-2-50-10-Polyester-Capacitors-/122090718455?hash=item1c6d2c84f7
 
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Offline Jeff_Birt

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Re: Micronta 22-195 Multimeter Calibration
« Reply #10 on: October 18, 2016, 01:03:10 pm »
Yep, they look like tantalum.  Maybe one is leaky (electrically speaking).  I'd try swapping them out, for lack of a better idea.

My google-fu is weak, I can't find a datasheet on this SMC4301F.

But just going by general principles, the makers of these DMM-on-a-chip devices usually provide an op-amp buffer ahead of the A/D, and provide access to the feedback loop externally.  The multimeter maker can then decide to add an RMS converter chip for a "true RMS" feature, or add some passives to make a precision rectifier circuit.  Although in this case with 2.2uF caps it looks more like a peak-detect type of thing, not really sure.

Radio Shack / Micronta seems to have used this chip on several meters. I recently picked up some 22-193 folding meters and they use the same chip. What I found on them is that the ohms function was decently accurate (measuring a 0.1% decade box) but the DCV was only within a few percent. This was consistent across all of the meters I could get working.

I did find a few posts on the web that suggested the SMC4301F was made by Seiko but I could not locate any other information. At lead RS put the schematics in the manuals :)
 
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Offline Fisher77

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Re: Micronta 22-195 Multimeter Calibration
« Reply #11 on: October 18, 2016, 02:34:23 pm »
Well looky at what I found this morning. There is no continuity between Resistors 24, 25, and C18. Also no continuity between said resistors and D20 either. Looks like either someone down the line overloaded it and burned up the traces, or battery corrosion leaked onto the board and rotted them out. By the look of it, I am putting my money on the latter.  Pic below. Now what is your opinions on the best course of action to fix this, wire?, leave it and not use the AC? What say you?
 

Offline retiredcaps

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Re: Micronta 22-195 Multimeter Calibration
« Reply #12 on: October 18, 2016, 02:53:01 pm »
Now what is your opinions on the best course of action to fix this, wire?,
In modemhead's blog, he documents how he fixes missing traces using 30AWG wire-wrap wire in Fluke meters.  Examples below.

http://mrmodemhead.com/blog/fluke-26-iii-corrosion-repair/

http://mrmodemhead.com/blog/fluke-77-iv-copper-corrosion/

http://mrmodemhead.com/blog/fluke-23-iii-dmm-repair/
 
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Offline ModemHead

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Re: Micronta 22-195 Multimeter Calibration
« Reply #13 on: October 18, 2016, 03:59:24 pm »
Good eyes.   :-+

Yeah, small wire, pre-tinned is helpful. Scrape and clean up the area to get bare copper available to solder to. Use flux, clean it off when finished. The secret to holding the wire in place while soldering is a small wad of blue-tack, poster putty or the like.
 
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Offline Fisher77

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Re: Micronta 22-195 Multimeter Calibration
« Reply #14 on: October 19, 2016, 03:39:19 am »
Thanks guy's. I will get it repaired, and post back with the results. Didn't get to touch it today, hope to get back to it tomorrow.
 

Offline Fisher77

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Re: Micronta 22-195 Multimeter Calibration
« Reply #15 on: October 19, 2016, 05:37:08 pm »
Ok, got the repair done. Time to put this thing back together, and test it. Don't look to shabby to me for the first time I ever tried this. I didn't have 30AWG. I used 22AWG.
« Last Edit: October 19, 2016, 05:41:00 pm by Fisher77 »
 

Offline Fisher77

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Re: Micronta 22-195 Multimeter Calibration
« Reply #16 on: October 19, 2016, 06:12:56 pm »
And success :-+. Thanks for all the help ModemHead, rcbuck, and others that posted. Only other thing I would like to do to this meter is see if there is a way to add a back light to the display. Any ideas?
 

Offline ModemHead

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Re: Micronta 22-195 Multimeter Calibration
« Reply #17 on: October 19, 2016, 10:41:03 pm »
Excellent, your repair looks great.  Especially since it works.  :-+

I used one of these for years until I "adopted" some vintage Flukes.  I still have the Micronta somewhere in a box.  I don't remember anything about the construction of the LCD.  It may be difficult to get a backlight working though because the LCD is probably "reflective" and not "transflective", meaning the back side of the LCD is pretty opaque...
 
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Offline Fisher77

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Re: Micronta 22-195 Multimeter Calibration
« Reply #18 on: October 20, 2016, 04:20:09 am »
Thanks ModemHead. I looked at the LCD a little, but it is behind a board. I think you may be right about it being reflective. I will try to get some pictures of it as soon as I can.
 

Offline Reedy9621

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Re: Micronta 22-195 Multimeter Calibration
« Reply #19 on: June 09, 2019, 04:02:39 am »
Hello, I'm a newbie to posting on any sort of blog so I hope I'm doing this right. Anyways, I wanted to share some photos regarding a display backlight I just installed in my Micronta 22-195 Multimeter. It was not easy, but I was determined to finish it. I’ve used that meter for over thirty years and always wished it had a backlight. The plastic box that holds the LCD and the elastomeric connector wasn’t designed to hold anything else. Especially not six layers consisting of a reflector sheet, two diffusers, a prism sheet, plastic lens, LED’s and two polarizer sheets. It took a little carving and sanding but I got it to fit. To get the light to come through I peeled off the silver reflective tape on the back of the LCD but left the polarizer. Of course I destroyed two calculators and an alarm clock to figure all this out before I dared trying it on the meter display. It takes 16 volts to light it. I’m temporarily using a boost converter running off the meter’s batteries (6V) and switched by the ON/OFF button. It draws about 55mA, but that boost converter is overkill. I’ve been trying to build a smaller one, but I’m finding it more difficult than I thought to come up with something that works and is stable. But that’s just due to my level of knowledge.  Hell, for all I know that boost converter might have an adverse effect on the meter, but I haven’t noticed anything different. It seems to be as stable as it’s always been. I eventually want to put the light on a timer, like two minutes. I took a 555 and made a touch switch timer so I can just reach up and touch a corner to turn the light on, but haven’t implemented it yet.
 Anyways, I’ll stop here. I wonder if anyone else has done this mod?
« Last Edit: June 09, 2019, 04:17:49 am by Reedy9621 »
 
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Offline Fisher77

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Re: Micronta 22-195 Multimeter Calibration
« Reply #20 on: June 21, 2019, 01:39:07 pm »
I am going to have to start looking for old phones, and an old laptop now. What state are you from Reedy? I am curios cause you mentioned
fly fishing for bass. I bass fish all the time here in Texas. Never have fly fished for them. I've looked into it, but the wind blows constantly here.
Never have been sure how that would affect the flies, casting, etc. I am in the same boat as you. Hard time balancing work, bass fishing, and electronics.
 

Offline Reedy9621

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Re: Micronta 22-195 Multimeter Calibration
« Reply #21 on: July 20, 2019, 05:05:14 am »
I live in Oregon in the middle of the Willamette Valley. Yeah the wind doesn't help concidering the size of some bass flies. It's almost like trying to cast a handkerchief.
Where about in Texas do you fish?  I'd like to check it out on Youtube. My favorite area is the Cascade Lakes in central Oregon.



« Last Edit: July 28, 2019, 07:43:03 pm by Reedy9621 »
 


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