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

rob aylestone

Well-known member
I take your point guys - but a meter that runs out of steam at 1K doesn't inspire me with accuracy, and I'm left wondering that the true RMS and RMS readings are actually pretty close in real terms - what are we looking at here? Electricians are concerned about noise on the AC mains waveform distorting the calculation of the RMS voltage - but in audio waveforms, we don't have anything other than a sine wave, noise free down to very low levels. The sparks drive this true RMS requirement. Is it really important in audio circuit measurements? Back in the days when we were aligning audio kit we did not have true rms to deal with at all.

I've got do disagree with my view of scopes too - I'd happily read the value on a decent digital meter (within it's spec) but I like the confidence of seeing the display on screen showing the waveform - after all, if there is a sizeable noise component, or harmonics, you'd see them and be able to measure them.

I guess I have to agree that even a 30 yr old meter of the Fluke's quality will produce meaningful readings given the right kind of input. I think maybe I've grown out of all purpose meters?
 

sweetbeats

Reel deep thoughts...
As I said, I was talking about a meter rated for reasonable accuracy across the audio band, not one that “runs out of steam at 1K”.
 

ecc83

Well-known member
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.
Good heavens Mr S! My diversion into the problems of measurement was just an example to show how fraught it can be...NO suggestion of a pop at you (or anyone else)

No, for the 'lone' tape enthusiast all that is needed is a reasonably accurate meter since most readings will be, as mentioned "relative". The manual* will give the output at say 1kHz (333Hz for cassette) for '0 VU' and then the response in dB ref that. Bias setting varies with manufacturer but again is usually a 'dB drop' ref some mid band level. There is no need for the OP's machine to be closely matched to anyone else's unless he is exchanging tapes and either needs 'precision' or is using Dolby NR.

I would however strongly suggest a test tape to at least set the replay level for a known flux and so enable the correct recording levels to be set.

*Good idea to download some service manuals and bone up on the alignment procedures. I had stacks of Sony manuals and they give comprehensive level diagrams with voltages in volts (rms!) and dBs...dBu although some makes might use dBV (the guitar amp industry is almost exclusively dBV ) Good to get very familiar with both dB types.

Note, response measurements are generally made at -10VU for 15ips recordings and -20 dB VU (or Dolby B level for cassette) Should have said. As always, check the book.
Dave.
 
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sweetbeats

Reel deep thoughts...
Hey Dave I wasn’t offended and I didn’t sense any offense meant at all. I figured you were presenting a spectrum of issues, but wanted to make sure.

I agree on the test tape too. I think if somebody is going to get into maintaining a tape machine, it’s going to require a proper test tape in the kit. I see people wanting to avoid the test tape, like they don’t really need it, or thinking they can make their own or whatever, and the cold reality is you’ve gotta pony up and get the tape, MRL or whatever. Calibration of the playback electronics is always the second phase of the electronics calibration, and I consider there are 4 phases, meters, playback electronics, record electronics, bias. Bias can and should be done on its own too, but it’s also part of the electronics calibration because it influences and can be influenced by the record EQ adjustment. And that’s the thing...tape machine electronics calibration is not just about the levels. It is also checking and adjusting the playback and record frequency response...and how we can check if the heads are garbage. Need the tape. I know you know all this, but for future readers I’m dumping the info here.
 

ecc83

Well-known member
Hey Dave I wasn’t offended and I didn’t sense any offense meant at all. I figured you were presenting a spectrum of issues, but wanted to make sure.

I agree on the test tape too. I think if somebody is going to get into maintaining a tape machine, it’s going to require a proper test tape in the kit. I see people wanting to avoid the test tape, like they don’t really need it, or thinking they can make their own or whatever, and the cold reality is you’ve gotta pony up and get the tape, MRL or whatever. Calibration of the playback electronics is always the second phase of the electronics calibration, and I consider there are 4 phases, meters, playback electronics, record electronics, bias. Bias can and should be done on its own too, but it’s also part of the electronics calibration because it influences and can be influenced by the record EQ adjustment. And that’s the thing...tape machine electronics calibration is not just about the levels. It is also checking and adjusting the playback and record frequency response...and how we can check if the heads are garbage. Need the tape. I know you know all this, but for future readers I’m dumping the info here.
Ha! Don't assume I know ANTYTHING friend! Been a very long time since I serviced tape machines and at my advanced age and with rattling around with various pills there will be big holes in my recollections!

I would advise anyone who is serious in servicing tape gear to look for some old analogue test kit such as an audio generator and a mV meter. There was quite a lot around a few years ago but it can only get scarcer and more expensive. A decent DMM can be used to calibrate it.
A distortion analyser is very useful but not vital or one can use 'Right Mark' software freebie, even quite a basic audio interface will be way below the distortion and noise of any tape system.

Dave.
 

38869420

Member
Haha its true, i'm well and truly lost and this is all way over my head, but it's a good read and I appreciate the discussion.

I'm after True RMS because Sweetbeats recommended I make sure it has it. The triggering reason for a new multimeter is to get something that'll help in all my electronics stuff, but all started because I want to measure voltages on the test points in the 388 for biasing voltage. So I measure it for it's current bias (Maxell 35 90) and then measure it after I rebias it for RTM LPR35. Calbrating the machine is still a while away for me as I learn what i'm doing and collect all the neccessary bits (test tape, scope, tension springs etc) I know a scopes important, but a decent multi should be the first step i'd assume.

And thanks Sweetbeats, the 85 can be found for decent prices, so looking at that & the 87 in second hand listings. Building my own Voltmeter is on the list for things I wanna try and do too.
 

sweetbeats

Reel deep thoughts...
Haha its true, i'm well and truly lost and this is all way over my head, but it's a good read and I appreciate the discussion.

I'm after True RMS because Sweetbeats recommended I make sure it has it. The triggering reason for a new multimeter is to get something that'll help in all my electronics stuff, but all started because I want to measure voltages on the test points in the 388 for biasing voltage. So I measure it for it's current bias (Maxell 35 90) and then measure it after I rebias it for RTM LPR35. Calbrating the machine is still a while away for me as I learn what i'm doing and collect all the neccessary bits (test tape, scope, tension springs etc) I know a scopes important, but a decent multi should be the first step i'd assume.

And thanks Sweetbeats, the 85 can be found for decent prices, so looking at that & the 87 in second hand listings. Building my own Voltmeter is on the list for things I wanna try and do too.
So I appreciate where you are at and the fact it is confusing. Not too many years ago I was exactly where you are now.

Clearly the “True RMS” factor is open for debate, but years ago when I was where you are that’s what was recommended to me, so that’s what I did, and why I’m passing the recommendation along. Is it a red herring? Maybe. But at minimum I think an RMS measuring AC meter is essential, and funny thing it seems any meter that has decent accuracy across the audio band happens to be RMS measuring. At least what I have and any time I check on this sort of thing for people in your situation.

NOW...I’m sorry to do this but here is a wrench in the works...I kind of lost track what you were wanting to measure first and foremost with your pending new-to-you DMM...look on page 44 of the 388 manual. That’s the page with the specs. Look toward the bottom of the right-hand column on that page and you’ll find the spec for the bias frequency...100kHz. Yes. Somebody correct me if I’m wrong, but if you want to set the bias as outlined in the factory procedure, and remember this assumes you are using Ampex/Quantegy 457, you set the voltage amplitude to 150mV AC measuring across the test points but you need a meter that is accurate to 100kHz! This is where a scope comes in handy. So just forget finding a DMM for setting your bias. And forget about 150mV because that only applies to Ampex/Quantegy 457 on the 388. Years ago I talked in depth with one of the two analog service gurus at Teac in Montebello, CA, and he told me LPR35 bias requirements were vastly different than 457. And I verified this for myself using the LF modulation method of setting bias, which involves recording a subsonic LF tone, and adjusting the bias to minimize the resulting distortion. I can’t recall the frequency...6Hz? 10Hz? Google it. This is also called the bias rocks method because the distortion from the subsonic tone sounds like boulders tumbling. This is all a real PITA on the 388 since it is a 2-head machine and you can’t do it on the fly, AND the LF modulation method becomes a challenge at the 7.5ips tape speed and the super narrow track format. But I was able to do it, and find a balance point between minimum distortion (minimum boulder noise) and noise (which sounded like a pulsing pssss-pssss-pssss-pssss). I can’t recall what then the measurement was at the test points...110mV using my Fluke 85? Again, NOT ACCURATE to 100kHz but it is a relative measurement at that point because I set bias using an auditory process, and then set each track to that amplitude using the test points. It sounded good. You can do the same with your 35-90. Or you can use the overbias method, but you’ll have to use your ears because there’s no instruction for how far overbias to go, and, again, it’s a PITA on the 2-head machine. Or get a scope...even a 10MHz 2 channel scope will measure the test points relatively accurately.
 

sweetbeats

Reel deep thoughts...
Oh and here’s something else to chew on...I read accounts of people liking 388’s “lo-fi” sound. The 388 is not a “lo-fi” machine. Okay, maybe compared to a wider format higher speed tape machine or an at least decent digital system the 388 would be considered HF limited, but lots of people, myself included, can’t hear past 16kHz anyway...but my point is when properly setup the 388 is a nice sounding system, plenty of HF bandwidth there AFAIC...I mean, have you heard a properly setup “high speed” cassette 4-track? I think that sounds nice too, and the 388 is similar track width but twice the tape speed. Anyway...how many people out there with 388’s are setting the bias using a meter like the Fluke 85 that’s rated to 20kHz to measure 100kHz bias frequency? Worse yet how many are using a non-audio spec DMM? I believe either of those options will register low at 100kHz, so the operator would then be dialing in more bias to compensate for the low reading...over-bias...which equates to diminished HF performance...NOW it sounds bitchin’, all lo-fi and whatnot. Tongue in cheek, but I bet that is not too uncommon. Proper test gear is important. So is understanding the specs and following the instructions in the manual.
 

38869420

Member
Yeah, this is why I was looking for a low mV accurate DMM, good to know a scope is needed to measure this so will put my focus on that. Basically want to find the mV rating for the LPR35 so I can then adjust it like that instead of doing the 2 head Bias dance every time. Measure & note the mV for the current bias, then do the 'bias rocks' method to find the mV for the LPR35, like you did and note that. I'm lucky to have 2 now functioning 388s, so i'm thinking I might have one set up for 35 90 (what i've recorded on up till now) and one for LPR35 (what i'll use in the future as 35 90 is getting scarce to get NOS for a decent price, and i'd rather use new tape). So basically i'd like to measure the test points for the 35 90 bias on my main machine, replicate that in the second machine and then set up the main machine for LPR35.

Now this is a very basic question, but something I need to clarify as I look at these things. So does a 10MHz scope mean it's bandwidth? Would that cover a range of synth, sampler, tape machine stuff in future or would I be better getting a 20MHz or 100MHZ 2 channel machine? Also, would this do the job? https://www.amazon.com.au/Hantek-DSO5102P-Digital-Oscilloscope-2Channels/dp/B07MJB7YV5

And my 388 sounds great, not lo fi at all with a nice, open top end, and I also find that a weird description of the unit as it replicates what goes into it really well and does amazing things in the high mids. Its the perfect point of being different enough to digital to notice but high quality enough to not offer a diminished return. Like, I can HEAR a 388 and I love that.
 

rob aylestone

Well-known member
If you are an electrician and need accurate mains voltages for your disconnect calculations, them today’s big problem of noise on the line messes it up because the notion that our AC waveform is a pure sine wave is long gone. They need a way to produce an accurate value. Our needs of small voltages with HF content really don’t align with this, and being honest, one of the reasons I like a swinging meter is because you usually are tweaking for maximum, so all you need is to see if it’s going up or down.
 

ecc83

Well-known member
If you are an electrician and need accurate mains voltages for your disconnect calculations, them today’s big problem of noise on the line messes it up because the notion that our AC waveform is a pure sine wave is long gone. They need a way to produce an accurate value. Our needs of small voltages with HF content really don’t align with this, and being honest, one of the reasons I like a swinging meter is because you usually are tweaking for maximum, so all you need is to see if it’s going up or down.
Err? Quite. I cannot see the need for 'true rms' meters when you are only measuring sines? AC meters are average reading but scaled to read rms for sine and the signal is hardly likely o be so distorted as to cause significant error. This: https://www.brltest.com/pdf/pdf_meters/796.pdf is what I used for gitamp speccing and testing. It would make a superb meter for TR alignment IMHO, easily reads past 100kHz. The twin pointer, dual channel version is doubly useful.

I also had at home a Wayne Kerr Radford noise meter. BEEEutiful thing that read down to 10uV iirc and WAS true rms plus various weighting filters. I got it for nowt and had little use for it so passed it on to a chap on the SoS forum. Would be the Dog's Undercarried for tape work!

N.B. If OP goes for a mV meter such as above try to find one with a scope output for future usage.

Dave
 

38869420

Member
I think it would but it's quite expensive - I rather like Rigol - my analyser is one of theirs. I'm actually thinking about buying one of these.
cps
Take into account these are Australian prices, so,usually abour 20-40% more than US. $450ish for a new scope is ok, gonna ask a friend who hoards tonnes of stuff if hes sitting on an millivoltmeter too.
 

sweetbeats

Reel deep thoughts...
Yeah, see, in all honesty for what I use a scope for I’d rather have an older Tektronix analog scope that’s been refurbished or something. I think you pay less, and likely have something more reliable than something with all the features you won’t ever use.
 

38869420

Member
Yeah, see, in all honesty for what I use a scope for I’d rather have an older Tektronix analog scope that’s been refurbished or something. I think you pay less, and likely have something more reliable than something with all the features you won’t ever use.
Yeah I get that but it seems rare to find them in Australia. They’re all over the US eBay for cheap but shipping is $150 to $800 on them lol. Got some eBay searches saved though, so it’s a matter of time.
 
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