Buying a Multimeter as I learn how to maintain this stuff...

38869420

Member
Heyo,

TLDR; I need a multimeter that will relatively accurately measure mV down to about 50mV.

I'm not an electronics guy, but i'm slowly learning the important things I need to do to keep my Tascam 388s, 426, 112, Space Echo, old synths & samplers etc running well. So the main reason I want to upgrade my multimeter is i'd like to check the millivolts when I bias my Tascam 388. I want to traditionally bias it (record test tone, listen to test tone, adjust, repeat) instead of doing it by voltage like the manual states. BUT i'd then like to take note of the tapes bias (LPR35, Maxell 35 90 mainly) so I can readjust if need be without having to do the re record & listen method, and to share with my friends & you guys for future reference.

Basically, the 388 has 2 test points on their channel cards that you measure the voltage of 120mV to bias it to the standard tape of the time (457 I think?)

From the manual;
1-5-4. Recording Bias Adjustment
1) Connect AC Voltmeter between TP 2 and TPG ón the REC/PLAY AMP PCB.
2) Adjust trim pots on the BIAS PCB for a 120 mV reading on the AC Voltmeter.

Asides from that, it'd be nice to have a decent multimeter for everything else. So i've been looking at the UNI-T UT139C which seems to be a pretty capable multimeter. Reading the manual for it's voltage measurements it says it goes down to 60mV up to 600mV with an accuracy of 1.0%. The A goes to 20mV & the B to 40mV but seems their upper limit is lowered too.

Not sure which would be most suitable for this job, plus future work as i'm still very much learning. Anyone got any insight, or could recommend a better multi that wont break the bank too bad?

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rob aylestone

Well-known member
Most of those test procedures are so much suited to a scope. Meters are great with dc levels, but lots of your measurements for alignment are based on AC, so azimuth, for example isnt that visible on a meter. It’s been a long time since I even had a reel to reel, but if I wanted to keep one going, I’d be looking at a cheaper meter firstly. The ones you’re looking at are the kind of thing an electrician would use and might be a bit ott for general electronics. Being able to measure circuit impedance for installations I wouldn’t find that useful. For dc and 50/60hz measurements the cheap ones suffice, and then spend a little on an audio frequency scope, as it can measure ac or dc accurately, but most importantly show you distortion, clipping and waveform displays. It can show you the presence of harmonics and allow these kind of issues to be fixed, where a meter cant.
 

38869420

Member
Most of those test procedures are so much suited to a scope. Meters are great with dc levels, but lots of your measurements for alignment are based on AC, so azimuth, for example isnt that visible on a meter. It’s been a long time since I even had a reel to reel, but if I wanted to keep one going, I’d be looking at a cheaper meter firstly. The ones you’re looking at are the kind of thing an electrician would use and might be a bit ott for general electronics. Being able to measure circuit impedance for installations I wouldn’t find that useful. For dc and 50/60hz measurements the cheap ones suffice, and then spend a little on an audio frequency scope, as it can measure ac or dc accurately, but most importantly show you distortion, clipping and waveform displays. It can show you the presence of harmonics and allow these kind of issues to be fixed, where a meter cant.
Ok great, thanks! Any tips on the type of scope to get? It’s like walking into a field of bees and asking to find the one that makes leatherwood honey lol
 

sweetbeats

Reel deep thoughts...
#1 criteria for selection when buying a used scope:

It works.

You don’t need a 100MHz scope...you don’t need one with lots of external trigger or time delay bells and whistles. A basic two channel 20MHz scope is fine. If you want it to be reliable then you are buying new or newer or professionally refurbished from a tech retailer, and expect to pay...$250-$300 for a decent refurbished/tested one. New or newer? Much more. If you go cheapsies, then expect to deal with the same thing we deal with with any older analog electronic device...it needs work. I’m partial to Tektronix.

Regarding your multimeter question, here’s the thing. You can’t just get any meter with a decent accuracy rating. You are measuring audio band signals, 20-20,000Hz, so you need something that is accurate across the audio band, and you’ll know this by looking at the specs. If it’s designed/rated for audio bandwidth measure to you’ll see accuracy specs in the AC voltage range for multiple ranges of the audio band. Maybe 100Hz, 1kHz and 10kHz ranges or something. You also want a meter that measures AC voltage in RMS...a “True RMS” meter. If you get something that is neither of these you’re measurements will not be accurate and it’ll be money wasted. I like Fluke meters.
 

38869420

Member
#1 criteria for selection when buying a used scope:

It works.

You don’t need a 100MHz scope...you don’t need one with lots of external trigger or time delay bells and whistles. A basic two channel 20MHz scope is fine. If you want it to be reliable then you are buying new or newer or professionally refurbished from a tech retailer, and expect to pay...$250-$300 for a decent refurbished/tested one. New or newer? Much more. If you go cheapsies, then expect to deal with the same thing we deal with with any older analog electronic device...it needs work. I’m partial to Tektronix.

Regarding your multimeter question, here’s the thing. You can’t just get any meter with a decent accuracy rating. You are measuring audio band signals, 20-20,000Hz, so you need something that is accurate across the audio band, and you’ll know this by looking at the specs. If it’s designed/rated for audio bandwidth measure to you’ll see accuracy specs in the AC voltage range for multiple ranges of the audio band. Maybe 100Hz, 1kHz and 10kHz ranges or something. You also want a meter that measures AC voltage in RMS...a “True RMS” meter. If you get something that is neither of these you’re measurements will not be accurate and it’ll be money wasted. I like Fluke meters.
Thanks
 

38869420

Member
The multimeter I linked to is 45Hz~1kHz which seems it won’t cut it, but it is true rms. I don’t have 1k to drop on a Fluke at the moment so just trying to understand what specs I need that’ll serve me well for my tape machines, synths and samplers mainly.
 

ecc83

Well-known member
The multimeter I linked to is 45Hz~1kHz which seems it won’t cut it, but it is true rms. I don’t have 1k to drop on a Fluke at the moment so just trying to understand what specs I need that’ll serve me well for my tape machines, synths and samplers mainly.
That meter is only "true rms" for power frequencies since it won't read the harmonics in a 500Hz square wave say. You have to pay a lot of money for a 20-20kHz test meter (I have a Fluke 83) and you would in fact be better off to look for a S/H AC mV meter such as those of the 'Levell' brand.

Or, make one! https://www.eleccircuit.com/6-ranges-ac-millivoltmeter-circuits/
The above shows the circuit of a mV meter that would be suitable for tape recorder alignment. Not up to such electronics construction? All I can say is "learn!" Tape machines need constant care and alignment if they are to deliver good results and you are unlikely to find an expert locally and unlikely to be able to afford them if you could!

If you did build that meter I would add a couple of reverse biased diodes to the input since a brush with erase bias would probably blow the op amp. A times ten probe is also easy to make or you might find a cheap scope probe on 'da bay'.

Dave.
 

sweetbeats

Reel deep thoughts...
The multimeter I linked to is 45Hz~1kHz which seems it won’t cut it, but it is true rms. I don’t have 1k to drop on a Fluke at the moment so just trying to understand what specs I need that’ll serve me well for my tape machines, synths and samplers mainly.
I wouldn’t spend $1K on a meter either. Several options for working used Fluke 83 or 85 meters on eBay right now. I have an 85.

People don’t want to hear it, but if you want to maintain your own tape machine, calibrate it etc, even for a 1/4” machine the price of admission for the gear to maintain it is $600-700 on a good day (meter, scope, signal generator, degausser, test tape, spring gauges, etc.)
 

rob aylestone

Well-known member
The big thing I learned, a very long time ago, was that the colleagues who had the very expensive, and for the 70s, clever test gear, often got just a bad results as I did, because we didn't have the experience to interpret test results. Our teachers would smugly watch us produce result with clear evidence of faults, which we'd measure and diagnose ....... wrongly. Maybe we didn't spot the large capacitor or inductor that was removing the HF, because we'd picked a test point after it, not before. The comment about square waves is spot on - but even applies to any waveform other than a sine wave. Your measurements make no sense unless you realise what they're actually telling you. Some alignment procedures especially of older kit have precision requirements. If a test point is supposed to have 10.4V on it, is 10V close enough? Perhaps not if they have specified it to a tenth of a Volt. I managed quite well with a moving meter - an Avo 7, that was ancient even back in the 70s. Old but solid and accurate. The thing was working out where what I was trying to measure with it was likely to be wrong. Then I'd use the scope for measurements. I think my old scope was a dual trace 20MHz Tektronix and that was fine. I did buy a Ferrograph test set, but later sold it because I had the other things it had built in.

Electronics wise, you need to be able to generate waveforms, measure them and view them. Meters combined with scopes probably do everything engineers need for the usual alignment processes. Ebay is great - but remember that if they say 10V and it's really 9.5V, the meter is totally useless. If you buy expensive test kit, they usually offer you annual alignment - for a very good reason.
 

ecc83

Well-known member
Avo model 7? ! Geez! I started in electronics pre transistors but we had Avo model 8s...Heh!

The Avo was hardly "precision" either. Accuracy was + or - 2.5% of FSD iirc? But it was the best we could afford. I have found a $50 Tenna digital meter that claims a 1% accuracy for AC voltage. The most sensitive AC range is 60mV but a FET opamp can be used to boost that by 20dB very simply.

Actually, an Avo would make a good basis for that DIY mV meter I linked to. Avos had several voltage scales and a decibel scale as well, just run it on the 100uA current range. At a pinch you can use a DAW to generate sine tones. Obviously they will only be as 'flat' as the quality of the soundcard/AI allows.

Dave.
 

rob aylestone

Well-known member
I think you just accepted the +/- for what it was, but repeatability and reliability were the thing. My teacher always said later in life when chatting that what was the point of measuring to an accuracy that was totally unimportant? 6 decimal places on some meters now? 14. how many Volts?
 
Heyo,

TLDR; I need a multimeter that will relatively accurately measure mV down to about 50mV.

I'm not an electronics guy, but i'm slowly learning the important things I need to do to keep my Tascam 388s, 426, 112, Space Echo, old synths & samplers etc running well. So the main reason I want to upgrade my multimeter is i'd like to check the millivolts when I bias my Tascam 388. I want to traditionally bias it (record test tone, listen to test tone, adjust, repeat) instead of doing it by voltage like the manual states. BUT i'd then like to take note of the tapes bias (LPR35, Maxell 35 90 mainly) so I can readjust if need be without having to do the re record & listen method, and to share with my friends & you guys for future reference.

Basically, the 388 has 2 test points on their channel cards that you measure the voltage of 120mV to bias it to the standard tape of the time (457 I think?)

From the manual;
1-5-4. Recording Bias Adjustment
1) Connect AC Voltmeter between TP 2 and TPG ón the REC/PLAY AMP PCB.
2) Adjust trim pots on the BIAS PCB for a 120 mV reading on the AC Voltmeter.

Asides from that, it'd be nice to have a decent multimeter for everything else. So i've been looking at the UNI-T UT139C which seems to be a pretty capable multimeter. Reading the manual for it's voltage measurements it says it goes down to 60mV up to 600mV with an accuracy of 1.0%. The A goes to 20mV & the B to 40mV but seems their upper limit is lowered too.

Not sure which would be most suitable for this job, plus future work as i'm still very much learning. Anyone got any insight, or could recommend a better multi that wont break the bank too bad?

View attachment 110002
I wonder whether a meter for electricians would have a sufficiently high impedance so as not to load the circuit which you are esting. Of course, if it is an "electronic voltmeter," which is a modern outgrowth of the old VTVM (vacuum-tube voltmeter), it would probably have a high enough impedance to do the job. Another consideration you might want to hold is whether you would want an analog or digital voltmeter. One important characteristic which separates an analog meter from a digital meter is that an analog voltmeter will reflect very small changes in the signal level you are testing, and those changes happen immediately. By contrast, a digital multimeter has a "run-up time" of at least a fraction of a second before the new reading is displayed, during which time the reading could change. As a totally blind person, I have become keenly aware of this difference. I have somewhere around my house an analog meter which is a standard triplet voltmeter which we adapted for reading without sight by connecting an oscillator circuit across the meter movement. The pitch of this oscillator changed with variations on the meter's deflection scale. I could hear even the AC ripple on a line in many cases, so it worked beautifully for me if I were turning a trimpot on a piece of equipment, giving me an instant change in pitch as I turned the adjustment. If I needed to know the exact reading, the little box we built had a braille scale on the face; you would flip a switch from "read" to "compare" and turn the knob until you heard the same pitch you got on the reading. The pointer then showed the reading by its position on the scale. By contrast, my talking digital multimeter has a "run-up" time of a second or so before I hear a reading after I hae changed the signal going to the meter. This meter would make it rather difficult to make a fine adjustment on a circuit because I would have to wait about a second or so to hear the new reading. Sadly, my analog meter got misplaced somewhere upstairs in our house when we did some remodeling; I continue to hunt for it as I have time. The truth is that I want both types of meters. The digital meter is neat for measuring a static signal level; but it approaches worthlessness when trying to measure a dynamic signal level because the reading I hear is probably a second behind the actual condition on the circuit. In all fairness, though, I should say that perhaps they have some digital meters that would respond more quickly than the ones I hae. I know that a certain amount of time is spent to generate the speech I get from my meter, so that might account for some of the delay; but that doesn't account for all of it.

On some of those tests, you might someday want to obtain an oscilloscope for looking at bias, distortion, and the like.

I might also remind you about one characteristic of some trimpots on circuitboards. Since they are said to be adjustments made by a technician, they are not expected to be turned very often. If you planned to adjust one very often to tweak in the bias exactly, I can envision the possibility of wearing out the wafer over which the wiper runs so that the pot wouldn't be reliable. If I were going to adjust the bias, I might consider finding a good copromise setting which would do quite well for all the tapes I planned to record, perhaps making the adjustment to slightly favor the type of tape you used most often.

That's my take on meters as I understand your purpose in getting one.
 

ecc83

Well-known member
I think you just accepted the +/- for what it was, but repeatability and reliability were the thing. My teacher always said later in life when chatting that what was the point of measuring to an accuracy that was totally unimportant? 6 decimal places on some meters now? 14. how many Volts?
Tongue was in cheek a bit Rob! Much of the AC signal measurement in TR lineup is 'comparative, i.e with reference to 1kHz say. Bias is often set (from the manual) as a current through a sense resistor in the rec' head winding or from a 'drop' at HF. Sony used to tell us to set for equal levels at 1kHz and 10kHz iirc? To set for an absolute flux level would mean the purchase of a test tape. How much are they these days?

You could go nuts on test gear! Ideally you want a distortion analyser. Scope? Yes, great but unless it has digital voltage readout they are not accurate things to measure voltages (and you have to convert from peak to peak to rms don't forget)

The bottom line is that tape machines need gear to set them up and to get the best results from 'modern' tape formulations. Money that might be better spent on a simulation plug in for your DAW?
 

rob aylestone

Well-known member
I bought a spectrum analyser with tracking generator. I’ve used it far more than I thought. It’s got all kinds of uses I didn’t really think about. Learning to drive it is a different issue!
 

38869420

Member
Ok, so the scope research is doing my head in, so i'm looking for a Fluke 85 second hand. Is the 85 III decent for my needs? Seems to be ok deals on it but are there True RMS models or are they all True RMS? I'm reading conflicting specs out there.
 

rob aylestone

Well-known member
You seem very attached to measuring True RMS? I don't see what for? These meters are now 30 years old, so anything you buy might be pretty inaccurate anyway, and for audio purposes we don't measure 50/60Hz very much? If you want to check that your 1K tone is really .775V and not .7V this meter will not be the right test instrument to do it. It is a general purpose multimeter designed primarily for electricians, not electronics. That's why we'd have a scope too - because a scope lets you look at the waveform and measure it. It shows you what you are measuring. What do you want the meter to actually do for you?
 

ecc83

Well-known member
You seem very attached to measuring True RMS? I don't see what for? These meters are now 30 years old, so anything you buy might be pretty inaccurate anyway, and for audio purposes we don't measure 50/60Hz very much? If you want to check that your 1K tone is really .775V and not .7V this meter will not be the right test instrument to do it. It is a general purpose multimeter designed primarily for electricians, not electronics. That's why we'd have a scope too - because a scope lets you look at the waveform and measure it. It shows you what you are measuring. What do you want the meter to actually do for you?
Erm..? I have to disagree with you slightly Rob. As I said before, a scope is not an accurate device for measuring voltages. The Fluke 85 WOULD be far more accurate. The 'point' of true rms is that of accurately reading a non-sinusoidal waveform but that hardly matters for tape lineup signals because they should not be distorted. Many machines are set for 3%THD at 0 VU and you will not detect 3% THD on a scope. Tape performance is a balancing act between noise, distortion, frequency response and, in some cases, low print. (why I am glad to be rid of them!)

Yes, get a decent, $50 ish meter but they are a bit of a pain when measuring dB referenced voltages. Better to buy the biggest and best VU meter movement you can and build a mV meter around it. The DMM can be used to calibrate it.

I am sure the OP is rather concerned by my suggestions of a 'DIY electronics' approach to this matter but the fact is, if you don't have at least basic level of how the electronics in TRs works, you are liable to cluck up their alignment. Building some basic test gear will give a grounding.

Dave.
 

sweetbeats

Reel deep thoughts...
I feel bad for the OP. We are all probably confusing the sh1t out of him because of our differing opinions, but that’s how it can go sometimes.

Rob, I don’t understand your statement about true RMS. Whenever you see an amplitude spec for audio band measurements it is representing an AC RMS measurement. Audio band amplitude is measured RMS, not peak-to-peak. So if you go get a meter that doesn’t measure AC as RMS your measurements will be off. Yes the differential can be calculated, but the rub is that likely that meter that only reads peak-to-peak is also not rated for accurate measurements across the audio band, so it’s double jeopardy.

To the OP my Fluke 85 has worked well. It measures audio band measurements well enough as far as accuracy. I did have to purchase the LCD screen repair you find on eBay...I think it was, like, $20, but that was and is all it’s needed.
 

ecc83

Well-known member
I feel bad for the OP. We are all probably confusing the sh1t out of him because of our differing opinions, but that’s how it can go sometimes.

Rob, I don’t understand your statement about true RMS. Whenever you see an amplitude spec for audio band measurements it is representing an AC RMS measurement. Audio band amplitude is measured RMS, not peak-to-peak. So if you go get a meter that doesn’t measure AC as RMS your measurements will be off. Yes the differential can be calculated, but the rub is that likely that meter that only reads peak-to-peak is also not rated for accurate measurements across the audio band, so it’s double jeopardy.

To the OP my Fluke 85 has worked well. It measures audio band measurements well enough as far as accuracy. I did have to purchase the LCD screen repair you find on eBay...I think it was, like, $20, but that was and is all it’s needed.
I agree, we are almost certainly confusing the guy but this is not a simple matter.

The "RMS" factor is really a bit of a blind alley? For the best part of a century we measured AC waveforms with an "AVERAGE" sensing moving coil meter (yes, even if said MC was driven by a valve, later transistors!). If you really wanted to know the RMS value of a signal, for power calculation say, you needed a Hot Wire Ammeter!

RMS measurement is also 'time dependent' and different meters have different integration times. I once hooked up an AP test set, a Levell mV meter and my Fluke 83 all to a pink noise source, each gave a slightly different reading!

Dave.
 

sweetbeats

Reel deep thoughts...
Okay, point well-taken, BUT...would you agree integration time is a lesser issue compared to the difference between RMS and peak-to-peak measurement?? I mean, some of this is relative to the operator and their studio and equipment, and once the test equipment is on board, even if it might vary by some small amount from somebody else’s studio and test equipment, it’ll be consistent calibration to calibration for the OP, and close enough to targets specified in the service manual. At least that’s my thinking.
 
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