Craftsman 82026 Multimeter Part Replacement and info Varistor

[nico]

Hi,
Im new here and Im looking for some info and a replacement for a Varistor for a Craftsman 82026 Multimeter(Im pretty sure this was a very common model) I was using it when something poped inside. I believe I was switching from ac to dc volts possibly. Whatever it was a good friend said that what I did souldn't have caused a problem. I opened it up and I see that a Varistor blow up. It seems to function except for continuity and I believe dc volts. The part number is 820KDD7 or 820KD07. I can't tell if the second to last character is a D or a 0
It really looks like a 0 though. There is a sAs labled above that. It's labeled MOV on the board. It was a nice meter that had a Frequency counter and if all I need is one part Im ready to replace it, but can't find this part. ANyone know if the replacement part is different. I'm pretty sure the usermanual doesn't have a schematic or parts list.
Any and all help is greatlly appreciated.
Thanks in advance,
Nico

 

[Sceadwian]

Simply removing it should allow the meter to function, if it doesn't replacing it won't help and something else in the meter was damaged during the power surge. I'm going to hazard a guess that any MOV that's rated a bit above the max voltage your meter is supposed to read would be acceptable for replacement. Little curious as to what you did though.

 

[nico]

removed Varisotor Craftsman 82026 Multimeter Part Replacement and info Varistor

Thanks for the quick response Sceadwian!
I was just able to get to removing the Varistor last night(tad busy), I removed the varistor and it does now work. Although I have a minor problem.
The meter doesn't want to zero out so to speak. When you set the dial whether its dc volts, ac volts, ohms or Frequency. It usually will start out at like .06-.01 and drops down to either .01 or finally to 0.00, but if you move the meter or the probes(doesn't matter if the probes are connected) it climbs back up again then drops. When I tested volts on a batttery(comparing to another meter) they where real close, and didn't matter if I moved the meter around at that point. Any ideas why this happens? It's kinda annoying and not sure if it will be real accurate. Since I have no way to compare the Frequency setting. I think it did this a little bit before the problem occurred but now it seems its a little worse. I know there are 3 tiny pots to adjust inside would that help? Any ideas on the adjustment process?

What I did to cause this(if I remember correctlly) I thought I was testing dc volts and had it on ac volts and when I turn it to the other setting bam! I thoguht from what I have been told this shouldn't happen. I know I was switching the setting from one to the other when it happened. It was a bummer when it happened because my other meter got fried at the same time as this one. So I had no meter to test anything. I think the other one just blew and was on the wrong amp setting(dumb on my part) It was a cheap meter, but I was out of luck at that point. The thing was it was a Arcade video game(cocktail version) and the monitor was shot. We had power but the the Higvoltage unit must have blow up at some point and there was a large mess on the baord from a blown up cap, so all was not lost since I couldn't do anymore with a meter most likelly.

Also how does removing the varistor make it work. I kinda thought it would be a open circuit and not work. I'm very glad it does though
Before(when it didn't work) The continuity would just beep, and nothing on any of the other settings.
I'm assuming a short in the Varistor caused this. Thanks again for your help and also in advance on my other questions. I greatly appreciate it
Nick

 

[Sceadwian]

MOV Varistors are effectively open circuits until a high voltage is placed across them, then they conduct, which causes them to act like a dead short. This is fine for low surge spike values, obviously you sent something a little more serious to yours and it blew, after the arc was done whatever arrangement of atoms remained was still conductive enough to effectively be a short circuit.

Now keep in mind I'm guessing here, but if you passed through the section that measures DC current you probably shorted the probes to whatever it was connected to, when you moved past this the current stopped, I'm guessing again that the source had some moderate to serious inductance involving moderately high voltages, and the inductive kickback sent a large surge to the meter and the MOV went on a permanent vacation, apparently the surge was enough to fry the MOV but not enough to kill the meter after the MOV failed. All things considered it did it's job.

As far as the zero problem you're having goes I'm not altogether sure, but perhaps it didn't completly save the meter. Also I'm not sure where the MOV was in the circuit and although they're effectively open circuits they still do provide at least the slightest of returns for a signal which might explain the odd readings you're getting as the MOV is no longer there at all. Until you get a new MOV you could try replacing it with a 1meg resistor to see what that does.

All of this advice is at your (and what remains of your meter) own risk.

 

[nico]

Thanks, will try a 1 meg resistor,how many watts should it be though? I'm not sure where I can get a close MOV with the same specs since I didn't have any luck finding the original specs.
Thanks again for the help and explanation.
I greatly appreciate it.
Nick

 

[Sceadwian]

According to my Ohm's law calculator, if your meter is rated for 1000 volts, if shorted across the leads a 1meg resistor will draw 1ma of current or about 1 watt. So maybe a 2 watt resistor? You could try a 10meg resistor that would only be .1 watts so a 1/4 watt resistor would be fine at 10megs. I think the 10meg 1/4 watt would be easier to find than a 2 watt 1 meg.

 

[kchriste]

I know there are 3 tiny pots to adjust inside would that help? Any ideas on the adjustment process?

There is no point in trying to adjust the meter without a proper calibration standard. You'll only make things worse if you adjust those POTs.

I don't see what adding a 1MΩ or 10MΩ in place of the MOV would do except lower the input impedance of the meter and really screw up the resistance function in the upper ranges.
What is the max DC voltage that the meter is rated for? The max AC?
It is highly likely that the MOV only partially protected the meter, and that other components have been damaged. Hard to fix without another meter.

EDIT: All I could find on the 820KD07 was this datasheet:
https://pdf1.alldatasheet.com/datasheet-pdf/view/188070/MCC/820KD07NX.html
It is possible that this is incorrect, because it shows the MOV as being rated at 50Vrms which is too low if it is across the VΩ and common leads of the meter. Which meter terminals is it across?

 

[mvs sarma]

Nice that you D.M.M started working
the M.O.V under concern is 820KD07. the data-sheet can be had from here
look for 70mm devices on page3
https://www.electro-tech-online.com/custompdfs/2010/02/mov_standard_series_5_40mm.pdf

this helps select a suitable device from local availability. all the best.

 

[nico]

Sorry for the delay on getting back to this post. I haven't been able to get on here in a while.

Sceadwian- Thanks again, but I think since someone mentioned a part# I will try and locate a replacement first and see what happens.


kchriste- thanks, that is a good point. I better not adjust the the pots. You connects to the Common and the Amp/OHM/Battery. Is that still to low then?

mvs sarma- Thanks for the link. I hope I have the right replacement part
Thanks again to everyone and sorry for the delay,
Nick