Analog ohm meter (VOM) for  computer repair question. - Page 1
Forum section: electronics / beginners

author: Mp3
author_rank: Frequent Contributor
timestamp: August 20, 2021, 07:38:02 am
content: I almost bought an older analog multimeter the other day, when tracing continuity between thru holes on a PCB i was repairing, checking for trace damage after desoldering some components, I was becoming irritated waiting for my meter to auto range, even in manual mode they were all going too slow for me.So i busted out my cheapo analog multimeter which has X1 and X10 modes for resistance measurement (absolutely bargain basement model, was about $5 shipped on aliexpress)worked great except i could not get it to react whatsoever to a good connection which reads 7.6 megohms on my DMM's. It also has miserable hard wired leads and for what I paid i prefer to get a better ohm meter than spend cost and effort trying to get a bit more usability out of this cheap thing.Then i went to youtube to check out talk about some more common VOMs I could get on ebay for a decent price.A video by "The radio mechanic" on youtube made it seem like it would be a worthy tool to have... until he says not to use it on low voltage circuits like computers etc since it'll draw too much load and did not say anything else on the matter.Is there some reason i should not use a multimeter like this due to its different resistance to a DMM when working on these sorts of circuits?To be clear I would never use an analog VOM on something like this with power applied.I am just looking for a better option than my cheapo multimeter that I can use for this sort of continuity measurement purpose to find bad or damaged traces. The VOM i choose to replace this analog multimeter will hopefully let me also read higher resistance such as 7+ megohms.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 20, 2021, 08:18:09 am
content: The resistance readings and continuity tests are more like the weak points on analog meters. This is really where the digital ones shine. High resistance often with analog meters often requites a relatively high voltage (e.g. 9 V). The details can vary, but generally the voltage at the resistor to test is higher than with a digital meter. Low voltage helps with in circuit testingIt is more like finding the right digial meter. A good continuity test is relatively common, even with cheap ones and the beeper is one of the most used functions. Fast auto-ranging in Ohms mode is rare, but this is still usually fast than on can turn the knobb by hand. The option to use manual ohms ranges can be handy - not sure how common these days.

author: Mp3
author_rank: Frequent Contributor
timestamp: August 20, 2021, 08:29:53 am
content: Hmm, i might have to make a bench size adaptation of dt-830 then

author: theHWcave
author_rank: Regular Contributor
timestamp: August 20, 2021, 12:07:49 pm
content: Nothing against the DT-830, a classic bargain-bin meter but not one I would use for this job (I have two DT-830). What I use is the Brymen BM235 which is very fast in continuity (15ms) and turns the back-light on as well as the audible beep, which is useful in a noisy environment. Only problem is that if the resistance is more than 200 Ohm or so, it shows nothing in continuity mode. To see what the high resistance actually is you have to switch to resistance mode. Thankfully just one button flips between continuity and resistance mode, so that is no too much of an issue. A better meter allows you to null the lead resistance and they can show resistance with 0.1 ohm resolution. I have used this to narrow down a short to ground (failed capacitor) on a large PCB power rail, by probing various places and moving towards lower resistance...Whatever meter, most important are the probes. Invest in a good set, with very sharp and thin needles, gold plated if possible. With cheaper probes (including the otherwise nice Brymen standard probes) I often find they are too big for very fine traces and too dull to get reliably through the oxide layer on contacts.

author: Vtile
author_rank: Super Contributor
timestamp: August 20, 2021, 04:08:34 pm
content: Analog meters do have their places, not too many these days, but I still use 100% analog AVO8 from 1960's (note: highly expensive as new, compare to ie. Simpson VOM) outside work when I do something with mains powered it is my voltage monitor (working alone), also at times I do use it for mains current. The 2.5kV range is also handly for quick HV low current measurements. Again many will disagree because of no CAT rating (or DOG).My go to setup is pair of analog FET-VOM and 5.5 digit Keithley DMM. FET-VOM is always used when no exact value is not needed, ie continuity. There is a few "usable" FET-VOMs, but one needs to read their homework closely. In example my another FET-VOM (Sanwa) is decent, but not worth the price, because of the extremely slow needle movement, which eats out only advantage of FET-VOM compared to most DMMs.Reason many do say do not use analog VOM to modern low voltage electronics is that analog meters gets their operating power directly from circuit they are measuring. The construction of analog meter is extr3mely simple there ia d'arsonval meter movement (needle) and network of voltage dividers operated by manual range switch. All multimeters and voltage meters do have so called input impedance (in voltage range) and with analog meter (especially cheap and even most more expensive ones still in production) this input impedance can be as low as a few kilo-ohms with low voltage ranges, while even dirt cheap DMMs usually have 1 Megaohm input impedance for all volt-ranges. With Ohms law you can calculate the current the meter is drawing from measured circuit applies all multimeters. Also the ohms range in analog meter is inefficient compared to DMM, for same reason as why there is low input impedance in volt-range there is high-current and high voltage applied to resistance measurent, the d'arsonval movement needs more power (current to be precise) than transistor amplifier inside DMM. This higher resistance measuremt voltage and/or current used in analog meter is higher than what delicate uProcessors, transistors etc. might handle, not to mention you might get wrong readings from in circuit resistance measurement if the measuring voltage is over few hundret milliohms as silicon PN junctions start to conduct.Analog FET-VOM is totally different breed than pure Analog-VOM or DDM. They use analog movement, but do have analog (FET)transistor based amplified input circuitry as do DMMs, the difference is that after input circuitry FET-VOM do not do digitalisation of measured value (as do DMM), which was slow or extremely expensive and a bit later the flickering readings still needed to be slow. Only when bar-graph displays did come up, DMMs started to get close. Unfortunately they aren't still on par with good analogs (except few most expensive advanced bench dmm's) what comes to visual simulation/emulation of analog needle movement. FET-VOMs have long been (almost) obsolete meter breed. FET-VOM looks exactly like analog-VOM externally, but internally are much more close to DMMs.7.5 megaohms is rather much for any analog-VOM (except vintage meggers, which might look a bit like analog-vom). One option for you ia to do logic tracer or equivalent accessory to your analog vom....And yes there is usually no one do it all meter. They are compromise usually, also for me it seems that development of handheld DMMs are somewhat stopped for economical reasons (better to sell two trade specific meters than one combimation including both) . I do have $800 handheld DMM for day-job, would I use analog instead ...heck no ... Have I faced situation where I would have had desire to get my not so portable FET-VOM from home .. indeed ... Would I use state of the art digital-analog FET-amplified handheld meter ... yes please. Would I bough new $800 dmm for my hobby ... no I rather use my $35/piece old flagship meters.PS. Lets not go to tube VDVMs they are obsolete( and belongs to museum or tube enthuastics), even compared to FET-VOMs predating both FET-VOMs and DMMs, without advantages.

author: richmit
author_rank: Contributor
timestamp: August 20, 2021, 05:28:47 pm
content: Quote from: Vtile on August 20, 2021, 04:08:34 pmI have the Sanwa EM7000, and use it regularly. But you are right about the slow needle.Do you have a recommendation for a modern FET-VOM?

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2021, 05:29:47 pm
content: One thing to check about non-DMMs: Classic VTVMs, which require line power and have a moving-needle meter, have a 1 megohm resistor in the probe, close to the tip, which usually resulted in a total of 11 megohm input resistance on DCV. This is useful when measuring DC voltage in the middle of AC or RF amplifiers, since the probe resistor isolates the cable and meter capacitance from the circuit under test.I believe that the original FET VMs, which emulated the VTVMs, have the same probe resistor, but I don't know about later units.I have not seen a DVM with such a probe resistor.The downside to this resistor is that direct connection to the meter will be uncalibrated.

author: Renate
author_rank: Super Contributor
timestamp: August 20, 2021, 06:35:48 pm
content: I think that auto range is the work of the devil.If you don't know even within a magnitude what you're looking for...

author: Vtile
author_rank: Super Contributor
timestamp: August 20, 2021, 07:43:39 pm
content: Quote from: richmit on August 20, 2021, 05:28:47 pmUnfortunately there is not many of them produced anymore, you can calculate them by one hand fingers. What I know the best is the Gossen-Metrawatts Metraport 3A, Simpson might still also produce something (FET-VOM)... Then there is Sanwa (maybe I one day reverse-engineer the circuit around the meter and see if it can be modified to be faster) I drooled after Metraport, but could't justify the cost, even more so since I do have late Goertz model (precessor of gossen-metrawatt) with slightly (by an a hair) better electrical specifications. I also do not have seen or heard how it behaves, is it slow or some other quirks... I would say that if money is not an issue then MetraPort 3A.If you do find good condition Conway or Marconi FET-VOM (sold in US with those names) with BNC and one banana plug only connector, then that one is really sweet unit for everything under mains energy levels. It is actually late 1970, maybe early 1980 unit from Warsaw Poland, original name as Meratronic - meratester. This is my forementioned pair for bench DMM. The specs are pretty unbeliavable for battery operated unit, sweeps table with all handheld DMMs I have seen so far. Also one should note that the AC range from 0Hz up to 1 Ghz is only with original probe and only 7.5V peak IIRC. Only real downside is that the knob is fragile soviet plastic... Shows nicely ESD and other ghosts hovering around ... or charge carried by wind. 😂I haven't gone through the market in past three years, but I do suspect the available models are not increasing.

author: kripton2035
author_rank: Super Contributor
timestamp: August 20, 2021, 07:49:08 pm
content: to check for continuity you should really build one of this :http://kripton2035.free.fr/Projects/shorty-display.htmlsorry it's not for high ohms. it's fast, very fast. works from 0.1mΩ to around 2Ω

author: james_s
author_rank: Super Contributor
timestamp: August 20, 2021, 07:50:38 pm
content: I don't find the auto ranging on my Fluke 87 to be a problem, it is fast and accurate. It also has a nice fast responding analog bargraph which gives me most of the advantages of an analog VOM for varying signals. I still enjoy using a good quality analog VOM, the classic Simpson 260 is a lovely instrument and hard to beat for what it is, but I struggle to think of a case where I'd recommend someone buy one for anything more than nostalgia. You have to be careful using a VOM on modern gear, the voltage and current used in the continuity and resistance ranges is high enough to damage modern semiconductors.

author: Vtile
author_rank: Super Contributor
timestamp: August 20, 2021, 08:27:28 pm
content: Quote from: TimFox on August 20, 2021, 05:29:47 pmYes, I'm not personally for tubes, but VTVMs are really nice devices, unfortunately they are absolytely antiques by now after good FETs (and more so chopper FET arrangements) invented with advantages of solid state devices were adapted to old VTVM concept. Tubes in general are fascinating, more quicksilver inside the better (referring to Daves TV broadcast station episode from past)Quote from: james_s on August 20, 2021, 07:50:38 pmI have seen review of Simpson FET-VOM here at EevBlog forum. You are right analog passive meter (the premium models eg. New Simpson) worth the price for majority (only for niche use). Even FET-VOMs are questionable, as new price, but old good units are usually cheap since DMM and for drifts, tuning, poor connections, relative levels they still beats most DMM. Fluke 87 is nice, but not so... Few collagues have one, not impressed, good for industrial troubleshooting and is robust not much more.. why I can not plot graph with it, why I can not program it freely for Go-NoGo and accessories, why there is no IR-test mode, why there is no possibily to create custom screens with PC, why there is no (...and definitely not with phone app life span of 2 years compared to meters 20 years)..... list continues to infinity. The baragraph lacks resolution compared needles, the screens are shadowing or lack pixels. With OLED models the battery life is more of an joke than anything.Thomson* light mirror galvanometer** adapted to this era for meter would be interestig.One should not underestimate the training value of analog meter.. They force user to think electricity!!!*Better known as Lord Kelvin.** D'arsonval is one type of galvanometer. One might wonder where term galvanically isolated comes.

author: Renate
author_rank: Super Contributor
timestamp: August 20, 2021, 08:52:00 pm
content: @kripton2035I know that the 82 ohm doesn't run 100% (does it?), but I'm concerned for it.At 100% it's doing more than 1/4 Watt.(Nice little project!)

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 20, 2021, 09:46:14 pm
content: I have several Multimeters, both analog and digital.You almost have to use both types to see which type you like.Most serious repair guys have both types.The real limitation of any meter is the input impedance. AsVtilepointed out.There are analog meters with very high input impedance but these are FET input types, which essentially are the equivalent of the old Vacuum Tube Volt meter, much better than regular analog meters. One of my meters is a Fourdee PSM 37 Military type (high impedance FET). Look it up. Maybe you can find one cheap, I think I paid $10. I'm in the US so this unit might not be common in other places. It has a 100K ohm range so you can read 10,000K easily on the meter. It also has an interesting function where it measures resistance with a low voltage. with this setting the effects of regular PN junctions in diodes and transistors in the circuit you are examining do not register. This may be somewhat helpful.When in doubt I use a Calibrated HP 3456 volt ohm device. I don't think you really want to go to that expense,I will look for a Meratronic, sounds interesting..In my experience the analog meters that are sold used are not to be considered to be anywhere near calibrated/ I don't know if this matters to you, do you want to know if something is 5.0 volts or 5.1 volts?? Besides it is hard to see more than two digits on the meter anyway. Someone just told me that for quality purposes, they had to use a Simpson analog multimeter at work and that the company would not get the things recalibrated since buying a new one was cheaper than a proper certified calibration.I do not think the digital meters wander as much as the analog ones over time, maybe someone here knows?

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2021, 10:18:37 pm
content: High-impedance analog voltmeters.I own two: the senior unit is a true VTVM, an -hp- 410A. As I discussed above, it has a DC probe that contains a 10 megohm resistor to make a total resistance of 110 megohms. The tube complement is interesting: a 6SN7 operated at low plate voltage to minimize grid current, and two 6AG7 pentodes to complete the differential amplifier. The AC probe uses an Eimac 2-01C vacuum diode that is good up to UHF.The junior unit, Hickok 1605M, is solid-state, with a vacuum diode in the AC probe, and is basically a clone of the later 410C. Apparently, the original units used a photochopper, but in mine that was replaced by a high-end Analog Devices op amp. It also has 110 megohm input resistance (with 10 megohms in the probe), and the AC probe (with EA53 vacuum diode) is specified from 20 Hz to 700 MHz. Except for the AC probe diode, it is solid-state, but requires line voltage.Both of these units have a DC probe, an ohms probe, an AC probe (with diode) and a ground lead.The popular VTVMs, such as the RCA VoltOhmysts, used 1 megohm in the probe for 11 megohms input resistance.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 22, 2021, 09:50:31 pm
content: If the OP is just measuring supply volts on computer chips, almost any meter will be just fine.I think this is the most likely use for a voltmeter in a computerUnfortunately there is a lot of pos and neg volts supplies in a computer and then you would have to exchange the pos and neg probesHence a cheap digital meter is probably the choice IMHO.

author: james_s
author_rank: Super Contributor
timestamp: August 23, 2021, 07:01:29 am
content: That is something I remember being struck by as a great feature when DMMs were first becoming affordable, they do auto-polarity. It's really convenient to be able to read positive and negative voltages seamlessly, you can't do that with an analog VOM, at least not any that I've used.

author: TimFox
author_rank: Super Contributor
timestamp: August 23, 2021, 02:46:37 pm
content: Analog VTVM-type meters always have a polarity switch (manual), since one terminal is grounded.

author: Vtile
author_rank: Super Contributor
timestamp: August 23, 2021, 09:08:27 pm
content: Quote from: james_s on August 23, 2021, 07:01:29 amIt is called at least "center-zero" with analog movements.. Common feature with at least active (amplified) analog meters. With like 5 seconds of thinking analog meter movements used in VOMs could be "center zeroed" with the adjusting screw of the needle (if rotation angle is big enough).Google "center-zero analog" or "center zero galvanometer"

author: james_s
author_rank: Super Contributor
timestamp: August 24, 2021, 04:44:36 am
content: Oh I've seen center-zero analog panel meters but not analog multimeters, certainly most of them don't center zero. You might have enough range in the zero adjustment to center the needle but then the scale is going to require manual math to get the value. I'll stick with a DMM.

author: TimFox
author_rank: Super Contributor
timestamp: August 24, 2021, 01:06:59 pm
content: The (mechanical) zero-adjust screw should not have enough range to send the needle to center scale from left-scale.The Keithley (moving-needle meter) electrometers had an (electronic offset) zero-center capability, along with similar electronic meters, but the passive analog VOMs did not.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 25, 2021, 04:41:08 pm
content: The D'Arsonval meter is really the most accurate at midpoint and the spring set on most meters is set for this, if you change the setting on the mechanical adjustment of the meter too much, all bets or off on accuracy