Oscilloscope as a VTVM - Page 1
Forum section: electronics / testgear

author: dellsam34
author_rank: Regular Contributor
timestamp: May 04, 2021, 04:26:50 am
content: Hi all, I'm suppose to use a VTVM for some audio circuits calibration due to supposedly their high internal impedance but they are not cheap and most need repairs, I was wondering if I can use my Rigol DS1074Z-S as both, a function generator on CH1 & CH1 and as an Oscilloscope on CH3 & CH4, But first can an oscilloscope be a good replacement for a VTVM? if so can it be used for both tasks for testing the same circuit without problems?

author: bdunham7
author_rank: Super Contributor
timestamp: May 04, 2021, 04:48:57 am
content: A 100M 100X probe will work pretty well as long as the resulting voltage ranges and accuracy are good enough--and they probably will be at least comparable to old analog high-impedance VTVMs in the audio range. At low voltages you can also use a high-impedance bench DMM for DC measurements and the input impedance will be much higher than any VTVM.

author: wizard69
author_rank: Super Contributor
timestamp: May 04, 2021, 05:03:45 am
content: I will try with simple answer inline below.Quote from: dellsam34 on May 04, 2021, 04:26:50 amNo but there are a lot of qualifications here.QuotePossibly.Lets take the first answer. An oscilloscope can make reasonable measurements of DC voltages assuming it is properly calibrated. You can also do AC measurements on sin waves and get reasonable results with a little math. More complex wave forms are another issue. Now that is using a scope in a traditional manner if you are using the built in capability of modern scopes to measure voltages for you, how well the scope does is buried in the specs.This doesn't even get into the calibration SOP being dependent upon the behavior of a VTVM. In any event I find using scopes to do voltage measurements to be a bit frustrating at times. It might be better on more modern machines but the scopes I have access to are not all that useful for voltage measurements to any degree of accuracy. You mention audio circuits so I'm thinking that maybe high accuracy isn't needed. So you really need to determine if a scope is "good enough", which again probably depends upon the details.As for your second question that really depends upon the circuit. One thing to make sure of is that any connections do not create shorts. Also don't forget with audio circuits is is very easy to create a signal source, if you just need something to trace through the circuit. MP3 players, old audio equipment, PC audio cards and similar can all generate noise. They might not be calibration quality noise but you would need to know the specs of your scope to determine if the signal generator there is good enough. Since it is safe to assume you have a PC due to posting here, I'd look into software that can turn your audio hardware into a signal generator. Even if your scope is good enough signal generator wise it is always nice to have alternatives.

author: bob91343
author_rank: Super Contributor
timestamp: May 04, 2021, 05:35:18 am
content: Those are complicated answers. Let me simplify.An oscilloscope is a VTVM. While most VTVMs have an input impedance of maybe 11 Megohms, most oscilloscopes have an input impedance of 1 Megohm. However, with a 10:1 probe it becomes 10 Megohms, close enough.So the short answer is yes.

author: BrokenYugo
author_rank: Super Contributor
timestamp: May 04, 2021, 05:36:07 am
content: Anytime you see "VTVM" suggested in service data you can pretty safely read "11 megaohm impedance voltmeter with decent frequency response", or in other words, the test point in question is high enough impedance a old school analog VOM would load the circuit too much. A good modern multimeter (10 megs) is generally close enough, as would a scope with a 10x (10 megs) probe.

author: bdunham7
author_rank: Super Contributor
timestamp: May 04, 2021, 06:02:09 am
content: Quote from: BrokenYugo on May 04, 2021, 05:36:07 amThe problem with that is that capacitive circuit loading at even moderate frequencies may be much higher than a VTVM, especially the ones that have an input tube right in the probe. An HP410B is something like 25M + 1.5pF, while a typical 10X probe might be 10M + 15pF and an HP 34410A is 1M + 150pF. My cheapo 100X probe is 100M + 3.5pF. Even at just 50kHz, those would all load the circuit quite differently.

author: dellsam34
author_rank: Regular Contributor
timestamp: May 04, 2021, 08:35:44 am
content: Let me add more details to my original questions:What I meant by audio circuits calibration is that I will only be using audio test tones 20-20KHz extreme, sine wave 2V pp possibly with some noise or hiss.I will be feeding the line in of the pre-amp being worked on with a test tone using a function generator and monitor the line out of the pre-amp with a VTVM. My idea is to avoid getting extra hardware by just using my Oscilloscope for both tasks, Killing two birds with one stone if possible.I posted the model # of the oscilloscope in case someone is familiar with it.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 04, 2021, 01:48:40 pm
content: If the reason for using a VTVM because of its high input impedance then the scope doesn't have as high an impedance unless you use the 100x probe like others have said. However, if you need good bandwidth then a scope has good bandwidth compared to an AC voltmeter but may not be better than a VTVM.

author: bdunham7
author_rank: Super Contributor
timestamp: May 04, 2021, 01:58:17 pm
content: Quote from: dellsam34 on May 04, 2021, 08:35:44 amThat's a bit less demanding than I originally imagined. Is this a solid-state pre-amp or does it have vacuum tubes? Is it one specific model and if so, can you tell us what it is? It seems very odd to me that a VTVM would be specified for this purpose unless it just happens to be from a certain era when a VTVM would have been the most common available measuring device with a decent frequency response. The output impedance of a hi-fi pre-amp should be pretty low, typically 100 ohms or so. Any modern DSO, including yours, should be adequate with any probe or even no probe. Many DMMs will work as well but you have to read the specifications to know. Just use the DSO and the RMS measurement and you should be fine.

author: TimFox
author_rank: Super Contributor
timestamp: May 04, 2021, 01:58:43 pm
content: In vacuum-tube era electronics, "VTVM" can have two meanings:1. A DC voltmeter, typically with 11 megohm input resistance (1 megohm in probe), that also included a separate input to a rectifier to give peak-to-peak values on AC voltage (lower input impedance, no resistor in probe). Some (such as the -hp- 410 series) included an AC probe with a diode in the probe head to measure AC up to high RF frequencies.2. An AC voltmeter, covering audio through a few MHz frequency range, such as the -hp- 400 series, where the panel meter is inside a feedback loop for good accuracy over the frequency range. Input impedance of the 400 series is 10 megohm in parallel with 15 or 25 pF (depending on range). seehttp://www.mcmlv.org/Archive/TestEquipment/HP400.pdf

author: bob91343
author_rank: Super Contributor
timestamp: May 04, 2021, 05:18:38 pm
content: I can see no problem at all using an oscilloscope for the indicated purpose. You don't even need the probe.

author: dellsam34
author_rank: Regular Contributor
timestamp: May 05, 2021, 01:41:05 am
content: Yes it's all solid state pre-amps for several tape decks (8-track, R2R, cassette), and yes the service manual is from the era where the VTVM was the only tool to measure AC voltage.

author: mansaxel
author_rank: Super Contributor
timestamp: May 05, 2021, 07:04:11 am
content: Quote from: dellsam34 on May 05, 2021, 01:41:05 amFluke 8060 if you can get it (because it was designed with audio work in mind). Or any other DMMwith a decent frequency response. Accuracy/resolution providing that frequency response is OK and instrument is to spec will not be a problem, a 3 1/2 digit DMM is plenty.Your oscilloscope, yes, why not? Do keep peak / RMS differences in mind. That goes for all measurements of AC, of course. A lot of the "Use this specific meter" instructions in service manuals was aimed at eliminating such pitfalls, and is what kept some models in use long after they were replaced in other fields.As has been mentioned above, the -hp- 400 series are eminently suited to this work, as is the 410 series, and as long as you don't have to go way high in frequency (but bias adjustments will definitely work), the 427a will do stellar service too; it's rated +/- 2% to 1MHz.Another alternative would be the transistorised AC Volt meters made by a few different makers, mostly Asian, during the 70s. They were explicitly aimed at AF adjustment work and as a solid-state successor to the VTVM in AF AC measurements. There even were some brands with dual pointers, for stereo, one supposes.Most of the scope alternatives I've described above are pretty cheap, with the exception of good specimen (with probe) -hp- 410c and the Fluke 8060, although I've recently scored a perfect 8060 for 16€ plus shipping.The wiggly pointer is a great interface for adjustment. Not to be dismissed because it's old and imprecise.And, you can never have too many meters!/Måns, Flukes: 8020A, 8022A, 8060x2, 10, 123. -hp-: 400, 410b, 427a, 974a. Gossen: MetraHit 16, 25. H&B: Elavi 3. Avo: Model 8, Solartron: Big Nixie DMM. Marconi: TF1041b (less magic smoke ATM).

author: dellsam34
author_rank: Regular Contributor
timestamp: May 05, 2021, 08:52:38 am
content: So from the responses above if using the oscilloscope for both supplying the pre-amp with a tone in CH1 and CH2 and measuring its output from the pre-amp with CH3 and CH4 is no problem, It's a matter of switching between channels only, No ground loops to worry about, no false readings?I know some suggested some VTVM's and DMM's but that's not what I'm looking for.