Looking for a vintage (inexpensive) benchtop True RMS multimeter - Page 1
Forum section: electronics / testgear

author: vferdman
author_rank: Contributor
timestamp: July 17, 2025, 05:13:48 pm
content: I've been looking for a benchtop multimeter for my vintage stereo repair needs. I got my EE wings in the 80's and therefore am very much a Fluke/Textronix/HP?Keithley type of guy. Suffice it to say that I still very much enjoy my Fluke 8060A, though I have others more modern ones. I am looking to have an LED display TRMS meter on the bench pretty much dedicated to measuring my output power on the amps. I test with sine wave, square wave, sawtooth, etc. Would love to have a TRMS reading just light up and meter be more or less semi-permanently wired into the test bed. I was looking at Keithley 179A and, although I never had one before, I do remember them in R&D labs I worked at in the early 80s. I also found a bunch of other vintage models, but not exactly sure if they are TRMS. Perhaps someone knows and can educate me.1. Fluke 8000A (doesn't state TRMS)2. Fluke 8600A (doesn't state TRMS)3. HP Hewlett Packard 3478A (doesn't state TRMS and has LCD display)4. Keithley 197A/197 (doesn't state TRMS)I do not need data logging or auto ranging (but will take it), willing to go with LCD (but rather have LED or similar light emitting display). Would love to pay about $50 or so since this is going to be mostly a one trick pony (reading those waveform voltages). But if there is one with good other features, I would not kick it out of my lab. So far it's a Keithley 179A that fits most of the requirements, but I do not know much about these old machines. There may be a better choice out there.Yes, I can get a cheapo Chinese meter for $20 and use that, but I really love good old US-made test gear as I have grown up on it and it has always been rock solid stuff. I also want it to run off the wall power, not batteries. I will power up the test bench with a power strip and I want that meter to come up and go down with the rest of it.

author: TimFox
author_rank: Super Contributor
timestamp: July 17, 2025, 05:31:58 pm
content: Quote from: vferdman on July 17, 2025, 05:13:48 pmKeithley 197A specs:https://download.tek.com/manual/197A_RevB_DocSpec.pdfACV and ACI are both TRMS.-hp- 3478Aa specs:https://accusrc.com/uploads/datasheets/agilent_hp_3478a.pdf?srsltid=AfmBOooK-LPYLwkQ3DTq8PVgsHkWQRfTivMj2nDUn5Wgu5-od6HQGiaeAgain, ACV and ACI are both TRMS.Fluke 8000A and 8600A: both are average-responding, rms calibrated, not TRMS

author: vferdman
author_rank: Contributor
timestamp: July 17, 2025, 05:39:35 pm
content: Thank you! This is great.

author: bingo600
author_rank: Super Contributor
timestamp: July 17, 2025, 05:39:53 pm
content: Fromhttps://www.rosenkranz-elektronik.com/userUpload/pdf/hp%203478%20a_68%20a%20datasheet.pdf

author: vferdman
author_rank: Contributor
timestamp: July 17, 2025, 05:52:27 pm
content: A little further down the rabbit hole is this little jem:Valhalla Scientific 2000 Auto-ranging Digital Watt-AmmeterThat may be the exact thing I am looking for. Anyone know about these?That may be way better for my needs, but I never used one or even seen one in person.

author: adam4521
author_rank: Regular Contributor
timestamp: July 17, 2025, 11:24:22 pm
content: Did you find a data sheet for that? Seems elusive. But power meters, or power quality analysers, might be a category to look out for.I have the HP3478A multimeter. I had it calibrated for a project about two years ago and they found it to be within specs, maybe 40 years after it was made. Straightforward to use, and plenty of documentation available. If you get one of these, you'll want to take a backup of the calibration constants which are just held in CMOS memory with a battery. There's a method/script for doing it on this forum but you need a GPIB cable and USB adaptor for your computer. It will add to the cost unless you have this stuff already.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 19, 2025, 12:28:53 am
content: Quote from: vferdman on July 17, 2025, 05:13:48 pmWell, apparently you don't love it enough to pay the price.Just saw a a few Fuke 8040A meters on Ebay. Working meters are around USD175, but there are also a bunch "as is" for around USD 35. It looks like a quite fun & smallish (portable) benchtop DMM.But from what I remember reading, the switches of that type of DMM tend to go bad over time, and they are difficult to clean.

author: J-R
author_rank: Super Contributor
timestamp: July 19, 2025, 06:54:50 am
content: I think you've got it right, the Keithley 179A seems unbeatable for the price.Otherwise, I'm partial to the Fluke 8840A/8842A with the VFD display. I have a few of these and assuming the display is in good shape, they are great. Big, bright (depending on condition), and super-fast digits (slow/medium/fast rate options). Someone has made an LED replacement display kit as well.If you do go shopping for that, be sure it does have the AC option installed.

author: daisizhou
author_rank: Super Contributor
timestamp: July 19, 2025, 10:13:10 am
content: fluke 8920a very nice

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 19, 2025, 01:59:19 pm
content: I've used a 3478 for decades and am continually amazed at how accurate and trouble-free it's been. Yes, I need to change the battery.

author: vferdman
author_rank: Contributor
timestamp: July 19, 2025, 03:04:04 pm
content: I really like Fluke 8840A! As I said, I am not that familiar with benchtop models. 8840A sure appeals to me for display, being an old school Fluke and feature set. Looks like it can be had for under $200 or so. Thank you for pointing it out to me. I need to make sure it measures true RMS ac. I assume it does, but need to check. My main application at the moment for it will be measuring output power of audio amps using various waveforms. True RMS is a must for convenience here.It is a LOT of multimeter, though and I love it. Definitely will consider it.

author: Aldo22
author_rank: Super Contributor
timestamp: July 19, 2025, 03:11:25 pm
content: Quote from: vferdman on July 19, 2025, 03:04:04 pmWouldn't an oscilloscope be more useful?

author: vferdman
author_rank: Contributor
timestamp: July 19, 2025, 03:46:00 pm
content: I have an oscilloscope, but it's an analog one, a Tektronix 2225, and does not provide numerical readings. Just waveforms. So I have to count the divisions (not precise) and do the RMS calculation (not simple if waveform is not a sine wave). It's not all that accurate and cumbersome. A digital TRMS meter will give me TRMS voltage of my output waveform, which I square and divide by my load impedance to get a power number. A much simpler and more precise operation. Also, I want to have a meter connected to the output terminals all the time, a dedicated machine. All I would need to do is glance at it while I am looking at the scope for clipping to get a number. A digital scope would certainly solve that issue and would give me all I need in one device. But I am working with what I have and adding a bit at a time. It's much less expensive to add an old benchtop meter than a new digital scope. I do see a digital scope in my future, of course. Nice ones are a bit out of my price range, though. And I am a sucker for vintage test equipment. One day I would need to bite the bullet and go digital on the scope, which will reduce the footprint of my bench setup by a lot, so that is a very good reason, but as of now, I am staying with what I got.

author: KungFuJosh
author_rank: Super Contributor
timestamp: July 19, 2025, 05:02:41 pm
content: You can do everything you want with a Siglent SDS804X HD oscilloscope. They're around $400.

author: vferdman
author_rank: Contributor
timestamp: July 19, 2025, 05:10:56 pm
content: I understand. But have budget constrains.

author: vferdman
author_rank: Contributor
timestamp: July 19, 2025, 05:19:44 pm
content: Thank you for the recommendation. I also see a Rigol DHO804, which is even less expensive (under $400) and seem to have similar features. I also recognize Rigol brand a bit more, but I am not very well versed in digital scopes. Is there a reason I would choose Siglent over Rigol?

author: Grandchuck
author_rank: Super Contributor
timestamp: July 19, 2025, 05:25:32 pm
content: https://www.eevblog.com/forum/testgear/sds800x-hd-review-demonstration-thread/msg5293741/#msg5293741

author: KungFuJosh
author_rank: Super Contributor
timestamp: July 19, 2025, 05:26:00 pm
content: Quote from: vferdman on July 19, 2025, 05:19:44 pmThat's a whole can of worms. The Siglent is a better scope, end of story. The Rigol is still okay if your budget requires you to save a few dollars vs. getting a much better device. Rigol's support is atrocious. Siglent is good enough that they OEM for some of Teledyne LeCroy's scope models.

author: daisizhou
author_rank: Super Contributor
timestamp: July 19, 2025, 09:13:05 pm
content: What frequency range do you use? TRMS is related to frequency.If it is within 100khz, any of the above mentioned ones can be used.If it is higher than 100khz, then only FLUKE 8920a is the best choice, of course 8921 or 8922 are also good。In general, I prefer 8920a8920a is the only choice within 20Mhz (pay attention to measurement accuracy)

author: vferdman
author_rank: Contributor
timestamp: July 19, 2025, 11:57:47 pm
content: My frequency range is audio gear. 20kHz tops. I work on vintage hifi gear.

author: daisizhou
author_rank: Super Contributor
timestamp: July 20, 2025, 12:15:41 am
content: Quote from: vferdman on July 19, 2025, 11:57:47 pmAs you said, the FLUKE 8060a is excellent for audio.The same is true for the Fluke 8920A in the high frequency field.At least I think that with 8920a, measuring TRMS voltage value below 20Mhz has an unparalleled advantage

author: bdunham7
author_rank: Super Contributor
timestamp: July 20, 2025, 01:24:00 am
content: Quote from: vferdman on July 19, 2025, 03:04:04 pmYes, the 8840A is TRMS (if it has the AC option--that's what you need to check) and the AC TRMS converter works throughout the audio range.If you want cheap, the HP 3478A is your biggest bang-for-buck for your needs.For your specific needs, the Fluke 45 has TRMS with adequate bandwidth, uses RS232 communication which is easier to set up than GPIB and also has a unique audio power mode that can display amplifier power output directly assuming a 2/4/8/16 ohm load. IDK what they go for but I'm assuming under $200.

author: MarkusAJ
author_rank: Contributor
timestamp: July 20, 2025, 01:55:12 am
content: Quote from: vferdman on July 17, 2025, 05:13:48 pmI can see your point, I'm still having and keeping in mint condition my Tektronix 475A, big sentimental valueBut, for audio work I suggest checking the REW, free software. If you use good ADC/DAC like the MOTU, or the Focusrite Scarlett you will be able to measure everything in 144dB dynamic range.Even if you would decide to use 16 bit ADC/DAC (an average $25 value on ebay) you will get 96dB dynamic range, better results than any of the listed meters, and you will get almost everything else required in audio tuning/measuring.Check this link:https://www.roomeqwizard.com/

author: GigaJoe
author_rank: Frequent Contributor
timestamp: July 20, 2025, 02:21:02 am
content: Fluke 8000A (doesn't have TRMS)2. Fluke 8600A separate board , coming as an option , custom fluke TRMS board ,quite good up few hundred kiloherz3. 3478A chips has HP labeling , but can be same AD6364. Keithley 195 , 197A/197 HP 34041a - all have AD6375 fluke 8840 \ 42 AD637 as optional board, some people do it yourselfFLUKE 8800A - separate board \ option, cusom fancy converter up to 1Mhzwith LCD display, need a correct workplace lightning , some light need to reflected from a display , otherwise difficult to work ...look at Keithley 179A , LED large display, set on 200V range, you have 10mV resolution ... enought .. and quite accurate for audio AC range ... on negative side ... lowest range for resistance 2K , for fraction of ohm a bit difficult to measurein the end i may pointed to something differenthttps://www.ebay.com/itm/356847829832has dual display good display, , you can see AC and DC in the same tame, TRMS 100K , ( same AD636 ) it measure 2 parameters in the same time , reasonable accuracy, slow 2 medium 5 fast 20 measure \sec , MIN\MAX , REL. dbm on diff load. diodes\buzz , reasonable price ..... probably need TLC , caps replacement

author: vferdman
author_rank: Contributor
timestamp: July 20, 2025, 01:42:10 pm
content: I am really starting to think about one of the 12 bit digital scopes under $400 that are out now. I would like to have one and now these things are super affordable. I would be able to have my power measurements done with one very easily as most of these are now 4 channel devices. I could have two channels dedicated to output power. It's changing my thinking a bit...Also, this is not an urgent need. More like a "nice to have". I already have enough handheld meters to get what I need done. I am just thinking of improving my work environment and flow. A digital scope would go a long way in giving me more room and capabilities for sure. Just need to come up with the budget.As a stop-gap, I think Keitley 179A is a really good choice. They are fairly inexpensive and for just mostly measuring TRMS AC voltage under 20kHz should be good. Also has that retro look that we all love.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 20, 2025, 04:06:26 pm
content: Scopes with a 12-bit ADC count up to 4095 (Or +/- 2047), and that gives them the resolution of a 3 1/2 digit DMM. But they won't have the accuracy. For a scope with an 8 bit ADC drift due to warming up and thermocouple effects is hardly an issue, and if you want to use a 12-bit scope to see details in signals, this (slow) drift is also not an issue. Even "High res" modes with 14 to 16 bit resolutions are useful for details. However, If you want to start using your 12-bit oscilloscope as an DMM, (with an absolute accuracy) then both drift and accuracy do become a problem.I just had a look at the Siglent SDS800xHD datasheet. It specifies 1.5% accuracy for <5mV and 0.5% for >5mV.DMM's only do slow signals. Creating a microcontroller project with a "scanner card" form is not so difficult. You can dedicate a few channels to a programmable reference voltage to periodically calibrate the scope. A nice extension to such a project is to extract measurement data such as RMS values from the scope over SCPI and display them on a bigger (7-segment or TFT) display.On a sidenote. you can buy a "Power Analysis" software extension for this scope (and it's quite expensive, 4 channel scope is EUR 500, while the software extension alone is nearly EUR 300). The advertisements suggest this power analysis is for analyzing power losses in high speed systems, such as the different phases of an SMPS circuit. I never looked into details, as I can't afford the software as a hobby. Previously, "Power analysis" extensions were only available for the higher end scopes (> EUR 1500).I also had a look at the datasheet of the Siglent SDS5000X series. These have 14 bit ADC's and prices start at EUR 3000 for a two channel scope. DC gain accuracy is specified as only 3% I guess because this is an older generation scope, and DC accuracy was deemed less important for a scope back when it was designed. I have not checked the "western" brands, as those are out of my price range of interest. But check it carefully if DC accuracy is important for you.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: July 20, 2025, 09:04:52 pm
content: when i did stereo amps repair or some digging ... i use dual screen gwinstek DMM , similar to what i suggested in a previous post , there is no way back to single screen , when you see AC and DC voltage in the same time ,197 extremely rudimentary to modern time , even RELative mode doesn't exist ... ( and i bet you don't drive manual ... right)

author: edavid
author_rank: Super Contributor
timestamp: July 20, 2025, 09:46:05 pm
content: Quote from: vferdman on July 17, 2025, 05:13:48 pmJust out of curiosity, what is the purpose of measuring output power with a non-sine waveform?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 21, 2025, 12:03:59 am
content: Yes, TRMS isn't that useful to most people. If you're measuring sine waves you don't need it. If you're not, the answer probably isn't meaningful anyway, unless you really want the heating power of the waveform. In 50 years of doing this stuff I've absolutely needed a TRMS meter maybe twice.

author: TimFox
author_rank: Super Contributor
timestamp: July 21, 2025, 02:46:42 am
content: The main use I have had for TRMS is to (carefully) measure noise.

author: Martian Tech
author_rank: Regular Contributor
timestamp: July 21, 2025, 04:34:54 am
content: How about a Keithley 199?

author: Gertjan
author_rank: Frequent Contributor
timestamp: July 21, 2025, 06:55:37 am
content: How about the Keithley 199?IMG_1852 Keithley 199-2000pix.jpgTrue RMS (AD637), large LED displays you can read across the room, very suitable for audio: dB (absolute and relative), bandwidth (measured) 500kHz -1dB.Keitkley 199 manual with specificationsI also have the Fluke 8840A. Although it is the Fluke equivalent of the K199, my K199's are used all the time, while the 8840A sits on the shelves.- The 8840A has only a 2A current range, so small currents are difficult. K199 has a 30mA range (DC and TRMS AC)- The K199 is easy to calibrate yourself (e.g. touch up a single range). The 8840A is not. (But both are very long-time stable)- The very nice dB functions of the K199. The 8840A has no dB function whatsoever...- The 8840A is much deeper than K199, so needs a deeper shelve.I wrote aK199 driver for TestController, so you can use it for things like logging and measuring frequency responses.(I also wrote aTestController driver for the 8840A)In the end, I think you will need a scope AND a bench DMM. I could not live without either of them.Regards, Gertjan.edit:Martian Tech was faster with the same suggestion. Great minds think alike.

author: daisizhou
author_rank: Super Contributor
timestamp: July 21, 2025, 07:44:33 am
content: Quote from: Gertjan on July 21, 2025, 06:55:37 amWhat is the highest frequency that Keithley 199 can measure?(Less than 5% accuracy)

author: Aldo22
author_rank: Super Contributor
timestamp: July 21, 2025, 08:14:55 am
content: Quote from: daisizhou on July 21, 2025, 07:44:33 amI think it's0.15%+300 to 100kHz, which is pretty good.But that's for sine wave, I assume.If you really want to measure square wave TRMS up to 20kHz, it could still be tight to stay below 5%.

author: the Chris
author_rank: Regular Contributor
timestamp: July 21, 2025, 08:19:23 am
content: Even though it is an exotic piece of equipment, the Keithley 194 is worth a consideration. It will offer 3.5 digits with up to 1MS/s or 4.5 digits with 100kS/s while sampling rate and time can be set individually. It will not only output TRMS of any waveform with bandwidth appropriate for the set sampling rate but also average, standard deviation, integral, peak-to-peak and more. They were built as 1-Ch and 2-Ch models and sometimes can be had cheaply.I have one. If there is a measurment I can run for you, just drop me a line.Christian

author: Gertjan
author_rank: Frequent Contributor
timestamp: July 21, 2025, 09:21:53 am
content: Quote from: daisizhou on July 21, 2025, 07:44:33 amMy measurement: 500kHz -1dB:Keithley 199 Freq-curve-1000pix.pngThis frequency curve is also a nice demonstration of the use of the K199 dB mode, combined with TestController sweep function.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 21, 2025, 09:45:23 am
content: Quote from: Doctorandus_P on July 20, 2025, 04:06:26 pmThe RMS calculation uses quite some oversampling. So the limitation from the ADC resolution is not that relevant. With some 1 M fold oversampling one in theory gets an extra 10 bits when it comes to noise. If done right the RMS value calculated from the scope can be quite good. 0.5% accuracy is not that bad for AC, especially with more than just mains frequency.It is still a different reading with a larger BW, that can add noise as a background.

author: tooki
author_rank: Super Contributor
timestamp: July 21, 2025, 09:48:13 am
content: Quote from: Conrad Hoffman on July 21, 2025, 12:03:59 amWell, outside of the realm of audio amps, it's certainly important in some areas. For example, measuring the effective voltage being delivered to a motor or solenoid using PWM.

author: bdunham7
author_rank: Super Contributor
timestamp: July 21, 2025, 04:10:47 pm
content: Quote from: tooki on July 21, 2025, 09:48:13 amAs with audio amps, the real world loads you list are reactive. TRMS doesn't give you the whole story either way. Whether it is truly useful or not in a given situation requires some thought and I'd agree with others that it probably isn't as useful as its universal adoption would lead one to believe.

author: vferdman
author_rank: Contributor
timestamp: July 21, 2025, 04:23:04 pm
content: I use resistors for power testing, not speakers. This is not a complete test, just a sanity check that an amp can push x watts of power into an 8 ohm resistor before going into saturation. I am not doing rocket science here, guys. I need a very simple, bright and big display that is semi-permamntently hooked up to my load so I do not have to use another meter for it. I use sine waves for this purpose, so TRMS is not strictly necessary, but makes a quick calculation a bit quicker. It's all pretty rough measurement work, nothing crazy. I do not need 10 digits of accuracy. I need a large bright display that will give me a decent reading within 200V range.I love the discussion, though and various tangents. I find a digital scope tangent particularly useful as I have been considering such a purchase for a while now. Thank you, all for contributing. I love this place!

author: tooki
author_rank: Super Contributor
timestamp: July 21, 2025, 04:27:28 pm
content: Quote from: bdunham7 on July 21, 2025, 04:10:47 pmWell those were just examples. LEDs would be another common PWM application, but which isn’t reactive.