cheap little Sperry VOM - Page 1
Forum section: electronics / testgear

author: cosmicray
author_rank: Frequent Contributor
timestamp: March 03, 2015, 09:18:05 pm
content: The name says Sperry, but I doubt it's related to the famous Sperry brand.The little VOM has a minor problem. It reads 6V when it should be showing 9.4V. There are so few parts here, the problem should be easily tracked down. Removed the fuse and battery, and checked the resistors one by one. They all looked correct, other than the one marked R2. The color coding on that one has me confused. If I read it left to right (black-blue-grey-red-brown) it would expect to be 0-6-8 x 100 @ 1%. Actual measured resistance is ~825. If I read it the other way (brown-red-grey-blue-black), I end up with 1-2-8 x 10^6 @ undefined %. That seems like an impossible value, so left to right must be correct. I have lifted the board gently, but see nothing other than the usual flux around some of the solder joints.When checking the single resistance scale, the zero wheel almost, but not quite, gets it to full scale zero. I have tried rotating that 1K adjustment trimmer, but see no changes to either issue.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 03, 2015, 09:25:56 pm
content: The first thing I'd do is change that crusty, rusty trim pot. Looks like it's marked "1K." Looks like it's set approximately half way, so I'd set the replacement at 500 ohms, then check and adjust for voltage, then check resistance. Not that I'd expect great results, but cheapies have their place.

author: Electro Fan
author_rank: Super Contributor
timestamp: March 03, 2015, 10:45:22 pm
content: Sometimes a project make senses or is fun just for the sake of the project but if you wanted a nice one of these - probably at less expense (if you factor in time and any parts cost), here is another route:http://www.amazon.com/Sperry-Instruments-HSP5-Analog-Multitester/dp/B006LQKCZA/ref=sr_1_1?ie=UTF8&qid=1425422512&sr=8-1&keywords=sperry+meter

author: Seekonk
author_rank: Super Contributor
timestamp: March 03, 2015, 11:35:59 pm
content: There is only a need to adjust one thing and that is the meter movement. My bet is that crappy little pot.

author: Howardlong
author_rank: Super Contributor
timestamp: March 03, 2015, 11:39:34 pm
content: Except for the colour, it looks exactly like the meter I started with 35+ years ago. Learned an awful lot with it, too.

author: xrunner
author_rank: Super Contributor
timestamp: March 03, 2015, 11:56:59 pm
content: Quote from: Seekonk on March 03, 2015, 11:35:59 pmYea.Are all the voltage scales reading off? You didn't say, but they probably are, hence the pot.

author: cosmicray
author_rank: Frequent Contributor
timestamp: March 04, 2015, 01:21:43 am
content: Quote from: xrunner on March 03, 2015, 11:56:59 pmI only tried the 10V scale (with a 9.4V source). It read low, at 6V. As the zero-ohms will not get quite to full scale, that makes me wonder if the meter movement is the issue (i.e. zero ohms might ought to be closer to mid scale on that dial). Using a DMM, I measured 125 mV across the meter wires, when the pointer is as close to full as I can get it. No idea what I should be seeing for full scale deflection.The internal adjustment pot does look like crud. I checked it with a DMM, and it appears to be wiping 0-1K correctly. When I initially opened the meter, it was set more towards the CW position of adjustment.

author: Macbeth
author_rank: Super Contributor
timestamp: March 04, 2015, 05:31:58 am
content: The 6k8 resistor - you aren't measuring it in circuit are you? If so then you need to find all the parallel resistances too then apply Kirchoffs law. Or just desolder one leg and re-measure.As for the meter - throw it in the bin. My guess is the coil is damaged - by reading a high voltage on a low range, dirt ingress, or perhaps someone actually tried measuring the mains with this death trap? (I notice the newer Amazon version of this meter is "CAT II rated" at 300V not 500V AC)Even if it was working at its best - a movement with 2k/volt is useless for electronics work. 20k/volt is the standard for analogue meters.

author: SeanB
author_rank: Super Contributor
timestamp: March 04, 2015, 07:58:27 pm
content: Have you tried a new fuse? As it is in series with the input, and I have seen a few go high resistance it is a quick check. Otherwise replace that 1k sensitivity pot, as it affects all ranges, and calibration is basically getting the best compromise on all ranges, or getting one DC range spot on with the others having some error. The ohms pot is also one that can change resistance and go high. What is the meter DC resistance, using another meter with a low measurement current. It will be around 100-500R.

author: necessaryevil
author_rank: Regular Contributor
timestamp: March 04, 2015, 08:04:26 pm
content: Well, there is a simple step-by-step solution which works for all kaput cheap-ass multimeters:Take the multimeterThrow the multimeter in the trashcanBuy a Gossen, a Fluke, an Agilent or any multimeter which got a thumbs up from Dave

author: xrunner
author_rank: Super Contributor
timestamp: March 04, 2015, 08:07:59 pm
content: Quote from: cosmicray on March 04, 2015, 01:21:43 amI can help you rationalize buying a nice new one. I'm really good at that.

author: Seekonk
author_rank: Super Contributor
timestamp: March 04, 2015, 09:04:32 pm
content: You are not in the spirit of things. Add an additional battery and op amp to boost the drive to the meter.

author: cosmicray
author_rank: Frequent Contributor
timestamp: March 04, 2015, 09:31:44 pm
content: Quote from: Seekonk on March 04, 2015, 09:04:32 pmThe op amp would have to drive external field coils, to balance the inaccuracy in the meter movement.

author: cosmicray
author_rank: Frequent Contributor
timestamp: March 04, 2015, 09:34:21 pm
content: Quote from: xrunner on March 04, 2015, 08:07:59 pmYes, I got the message/memo/email. Quite clearly. If anything, this was a debugging exercise, to learn likely failure points. The meter may have been so cheaply made, that the entire thing is a failure point.

author: cosmicray
author_rank: Frequent Contributor
timestamp: March 04, 2015, 09:56:07 pm
content: Quote from: SeanB on March 04, 2015, 07:58:27 pmNo, I did not think to check the fuse for resistance gone high. That is easily done. The fuse reads (at first contact) about 9 ohms, but then settles down to around 3 ohms. So to prove/disprove this theory, I put a jumper clip across the fuse terminals in place of the fuse. Always a handy thing to do when a penny does not fit this fuse type holder. I still see the same effect on the ohms scale, where one end of the adjustment will not quite make the pointer reach zero. So the fuse, however questionable, is not the issue.Currently disconnecting the PCB, will report back soon.

author: cosmicray
author_rank: Frequent Contributor
timestamp: March 04, 2015, 10:21:36 pm
content: The pot is not so good. Measuring A-B shows ~1065, a bit high but not that far out. A-W and W-B are *both* showing ~700 ohms. Not at all a good reading. Apparently the wiper is adding ~200 ohms of it's own resistance. All of this measured with the pot out of circuit. I have a 1K cermet, that I got from rat-shack. It should answer the functional question, even it I cannot close the case with it.edit: What I'm going to do is tack solder a 680+330 1/8 w to simulate the pot. If that works, then I'm going drill some very small holes to align the cermet 1k pot, then run a few small jumpers to connect to the correct lands. The cermet appears to be ~1mm taller than the fuse holder clips.

author: cosmicray
author_rank: Frequent Contributor
timestamp: March 05, 2015, 01:54:57 am
content: The cermet 1K is connected with a few short pieces of wire. I can now select various combinations of the cermet and the zero ohms wheel, and get a full scale zero ohm reading. The extra 200 ohms on wiper was what was keeping me from getting that before. I checked a few mid scale resistor values (3.3K and 4.7K) and they were approximately where the indicator should have been.The voltage is still problematic. Turning the cermet all the way to one end, gets the 9.4V reading to now show slightly over 7V. But the pot lands are actually wired with A shorted to W, so it is a rheostat. The far end of the turns, effectively is setting the rheostat to a zero ohm short. So something in the voltage divider is dropping more volts than it should, otherwise the pot/rheostat would be used to calibrate.The mystery resistor (brown-orange-grey-blue-black) reads 1382 ohms with one end lifted. The blue multiplier may have been in error. All the other passives read (within tolerance) what would be expected. That leaves the two diodes as possible candidates, but one is used for AC rectification (I think).

author: cosmicray
author_rank: Frequent Contributor
timestamp: March 09, 2015, 11:46:53 pm
content: So, I think I understand what the problem is. But I need to confirm one thing ... if the meter is rated 2K ohms/volt, and it'ss on the 10V DC scale, it should be presenting a 20K impedance load ?