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VOM, ohms not zeroing

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	Post subject: VOM, ohms not zeroing Posted: Nov Mon 08, 2004 3:00 am
Member Joined: Jan Thu 01, 1970 1:00 am Posts: 80 Location: Salem, Oregon	Micronta 220204<BR>I got a used one, voltage and current ranges are accurate. It measures on ohms ranges, but zeroing problem.<P>On ohms, I can't zero the meter. The zeroing control works, but the closest I get to zero is about mid-scale. Same on all resistance ranges.<P>I've done this:<BR>* Opened the unit, it is clean, not corroded. <BR>* Sprayed tuner cleaner on zeroing pot, range switch positions.<P>* Panel Jacks appear clean.<P>* Changed test leads.<P>* Measured batteries, the 1.5 volt battery effecting the resistance range measures 1.68 VDC, a good one, swapped it anyway.<P>* polished the battery contacts.<P>I assume test results say there is some extra resistance in the circuit, but where?<P>Advice?<P><P>------------------<BR>
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Dean Huster	Post subject: VOM, ohms not zeroing Posted: Nov Mon 08, 2004 5:38 am
Joined: Jan Thu 01, 1970 1:00 am Posts: 2219 Location: Harviell MO USA 63945 (12 miles S of Poplar Bluff)	If a meter has ohms zeroing problems and it's a battery problem, it will not zero on the lowest range first because the "X1" range causes the heaviest current drain on the battery. Inexpensive VOMs have a single 1.5 volt battery and more expensive meters have a 1.5 volt battery for the lower ranges and a higher voltage battery (6v, 9v, 15v, 30v, depending on the model and age of the meter) for the higher ranges.<P>Usually, replacing the battery(ies) with new ones is the only good way of checking them since voltage measurements don't load the cells down enough to cause a significant voltage drop caused by high internal resistance.<P>You've sprayed down the switch and cleaned contacts -- you might check the tension on the test lead contacts to be sure they're tight. That leaves you with the most likely problem that someone was trying to measure the ohms in a live wall receptacle. Set the VOM on a voltage range and use another meter (preferably a DMM) to check the resistance of all the resistors used in the ohmmeter circuit on all the ranges. In addition, check the zero adjust control to make sure that it goes all the way from zero to its maximum resistance. Since you have the zeroing problem on all ranges, it could be a single resistor that is common to all ranges that's been overcooked. And resistors can go bad just with age. Ohmmeter ranges can have one or two less-stable carbon composition resistors in them while the other functions rely strictly on the more-stable metal film resistors.<P>If all parts check OK, you may have a contamination problem. If the meter uses a circuit board, you may have to get in there with some isopropyl alcohol and a toothbrush and scrub the thing down, front and back and see if that takes care of the problem. But usually, board contamination affects the higher ranges more than the lower ranges.<P>Dean<BR><P>------------------<BR>
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David Kulka	Post subject: VOM, ohms not zeroing Posted: Nov Mon 08, 2004 5:44 am
Joined: Jan Thu 01, 1970 1:00 am Posts: 1655 Location: Burbank, CA and Thailand	Do you know whether it ever worked correctly? SInce the voltage and current ranges work ok, I can only think that a a resistor in the ohms circuit is bad (or the wrong value), that the rotary switch is faulty, or that there's some broken wire (or miswire). <P>Do you have a second meter that you can check the resistors?<P>------------------<BR>
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Fred Scoles	Post subject: VOM, ohms not zeroing Posted: Nov Mon 08, 2004 6:24 am
Joined: Jan Thu 01, 1970 1:00 am Posts: 2078 Location: Oswego, NY, USA	The VOM's that used circuit boards to mount their resistors (such as Radio Shack-Tandy-Micronta, Precision, RCA;but the Triplett 630 series & Knight's did not use circuit boards) could rarely get a cracked or cold solder joint or a cracked copper trace that can give symptoms like this. My Precision 260VOM once read partial on DCV ranges, which was caused by a hairline crack through a solder joint on the board; found it using a 20X magnifier lens. Also, if someone zapped hi-volts thru the Ohms circuit it could also ruin the Ohms switch wafer contact in addition the resistor mentioned above. If more ranges than the Ohms were affected, then it could be caused by damaged meter surge protection diodes (installed across the meter terminals).<P>Fred<P>------------------<BR>
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tubesrgood	Post subject: VOM, ohms not zeroing Posted: Nov Mon 08, 2004 6:30 am
Joined: Jan Thu 01, 1970 1:00 am Posts: 1770 Location: Warwick,RI	Well,it may be coincidence,but I have had three Micronta VOM's all of which had the same ohms zero problem.<BR>It took a little work to solve the problems but I was eventually successful.<BR>I seem to have had a *combination* of problems including lousy solder joints,dirty switches that took more than one shot of cleaner to do the job and poor battery holder contacts.<BR>I don't think these VOM's are really well constructed in terms of soldering at the factory,etc and I think the owner has to work on these to get everything up to snuff.<BR>Malcolm<P>------------------<BR>
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Fred Scoles	Post subject: VOM, ohms not zeroing Posted: Nov Mon 08, 2004 9:22 pm
Joined: Jan Thu 01, 1970 1:00 am Posts: 2078 Location: Oswego, NY, USA	In my reply yesterday, I should have said "Simpson 260" not "Precision", sorry about listing the wrong brand. <P>I have a Micronta #22-049 VOM (1970's ?) that is notoriously inaccurate; and a few of the Shack DMM's I've used all worked, but were inaccurate compared to other brands; this supports what others mentioned above. <BR>Fred <P>------------------<BR>
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David Kulka	Post subject: VOM, ohms not zeroing Posted: Nov Tue 09, 2004 5:14 am
Joined: Jan Thu 01, 1970 1:00 am Posts: 1655 Location: Burbank, CA and Thailand	Actually, Malcolm and Fred's comments make the most sense - "what you are seeing is normal". Gulp.<P>Not to sound negative, but maybe the best move would be to deposit said meter into round waste receptacle and shop for an inexpensive but better one? I'd think you could get a pretty decent used VOM for $20 or $25 that would perhaps outlive all of us, give accurate readings, and might perhaps worth more years from now, should you decide to sell it.<P>Just a thought.<P>------------------<BR>
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m123	Post subject: VOM, ohms not zeroing Posted: Nov Tue 09, 2004 9:09 pm
Joined: Jan Thu 01, 1970 1:00 am Posts: 80 Location: Salem, Oregon	m123 wrote: <font>quote:</font><HR>Originally posted by Dean Huster:<BR><B>If a meter has ohms zeroing problems and it's a battery problem, it will not zero on the lowest range first because the "X1" range causes the heaviest current drain on the battery. Inexpensive VOMs have a single 1.5 volt battery and more expensive meters have a 1.5 volt battery for the lower ranges and a higher voltage battery (6v, 9v, 15v, 30v, depending on the model and age of the meter) for the higher ranges.<P>Usually, replacing the battery(ies) with new ones is the only good way of checking them since voltage measurements don't load the cells down enough to cause a significant voltage drop caused by high internal resistance.<P>You've sprayed down the switch and cleaned contacts -- you might check the tension on the test lead contacts to be sure they're tight. That leaves you with the most likely problem that someone was trying to measure the ohms in a live wall receptacle. Set the VOM on a voltage range and use another meter (preferably a DMM) to check the resistance of all the resistors used in the ohmmeter circuit on all the ranges. In addition, check the zero adjust control to make sure that it goes all the way from zero to its maximum resistance. Since you have the zeroing problem on all ranges, it could be a single resistor that is common to all ranges that's been overcooked. And resistors can go bad just with age. Ohmmeter ranges can have one or two less-stable carbon composition resistors in them while the other functions rely strictly on the more-stable metal film resistors.<P>If all parts check OK, you may have a contamination problem. If the meter uses a circuit board, you may have to get in there with some isopropyl alcohol and a toothbrush and scrub the thing down, front and back and see if that takes care of the problem. But usually, board contamination affects the higher ranges more than the lower ranges.<P>Dean<P><BR></B><HR> <P>Dean:<BR>Thanks for good advice. BTW, we used to communicate. If you recall, I live in Oregon, mobile home.<P>Mike<P>------------------<BR>
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m123	Post subject: VOM, ohms not zeroing Posted: Nov Tue 09, 2004 9:14 pm
Joined: Jan Thu 01, 1970 1:00 am Posts: 80 Location: Salem, Oregon	Thanks to all for good data. <P>I also had several of these Micronta 204 models, one new, it agreed quite well with several digital VOM's until a miswired transformer test blew it. They do seem fragile. <P>------------------<BR>
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