How important is 0.5% tolerance to my VOM/DMM - Page 1
Forum section: electronics / testgear

author: gooseEL34
author_rank: Regular Contributor
timestamp: February 28, 2013, 07:45:11 pm
content: Hey guys.I am wondering the following....For checking +15vDC/-15vDC in a solid state guitar amp, is my crappy Fieldpiece Lt16A sufficient with its 0.5% +/- 1 and only a 1/10th of a mV resolution?Reason I ask is this. I am a tinkerer with mostly Tube Amps. I use a multimeter for checking B+ voltages on the plates of tubes (Valves for you across the pond). I also use it to check resistances and to check if a component is short or open.I recently started tinkering with a 1970s Lab Series L5 guitar amp which has a bunch of 741 and 4558 ICs.It seems to have a 15vDC rail and a -15vDC rail.Assuming my meter is at worst, 1.5% off. Will I care? What I mean is: If I read 15.3vDC and it is actually 23mV off,Can I assume that the rails are supplying normal voltages?ThanksTimAnyone suggest a meter with 0.05% DC accuracy that isn't over $250??

author: IanB
author_rank: Super Contributor
timestamp: February 28, 2013, 07:53:05 pm
content: I don't think you need to worry too much about absolute accuracy here. Consistency and repeatability will be more important.For instance, it doesn't matter much if a 15 V rail is at 14.5 V or 15.5 V or somewhere in between.However, if you have +15 V and -15 V rails they should be balanced. If you measure 15.3 V on the +ve rail, you should measure 15.3 V on the -ve rail also. This only requires that your meter has repeatable measurements and sufficient precision to compare two measurements.

author: KJDS
author_rank: Super Contributor
timestamp: February 28, 2013, 08:03:28 pm
content: I suspect that so long as the rails are between 10V and 18V then the amp won't care, so 0.5% will be more than good enough.In the 70's the person who designed it probably only had an analogue meter for checking voltages that would have been at best 2% and possibly far worse.

author: Unknown
author_rank: Unknown
timestamp: February 28, 2013, 08:50:15 pm
content: Unless you move both leads when you go from measuring the positive rail to measuring the negative rail, you also need the meter to be symmetric, ie read the same if the leads are reversed. This is more than just repeatability.

author: IanB
author_rank: Super Contributor
timestamp: February 28, 2013, 08:52:48 pm
content: Quote from: alm on February 28, 2013, 08:50:15 pmThis is true. I think all of my meters are asymmetric to some degree, but it is more noticeable on the higher precision meters with more digits.

author: zaoka
author_rank: Frequent Contributor
timestamp: March 01, 2013, 02:21:30 am
content: Voltage on rails does not tell you much, its more important to run amp on max load and measure ripple for both voltages. This will tell you if capacitors are OK as well as voltages.You should use scope for this, you can also use multimeter to measure ripple by using MinMax (Peak) mode, you capcure Min Peak and Max Peak, it will tell you Peak-to-Peak. its not preferable and accurate way, however, it can be done that way.If voltage is not regulated it does not have to be accurate + - 15% is usually OK. (if schematic says 15V it usually means that your 120V or 220V input voltage must be accurate as well). If its regulated we usually tolerate + - 5%.For $250 you can get used Fluke 87V.Dave recently reviewed Brymen BM257, it could be a choice..

author: retiredcaps
author_rank: Super Contributor
timestamp: March 01, 2013, 08:17:28 am
content: Quote from: gooseEL34 on February 28, 2013, 07:45:11 pmIf you are in the USA, you have a lot of options if you don't mind buying used off ebay or locally.An used Fluke 87V can be purchased easily under $250. The Fluke 87 III is 0.05% + 1 digit and can be found in the $125 to $175 USD range.Many older bench multimeters are 0.05% + 1 digit and can be found in the $50 range. Examples, Fluke 8050A, Keithley 175, etc.I don't know if you need this feature or not, but some multimeters (like the Fluke 8050A and Keithley 175 above) also read db/dbm which might make working with amplifier troubleshooting a bit easier?PS. I own a Fluke 8060A (handheld version of the bench 8050A) and just got a Keithley 175, but I do no stereo equipment repair.

author: EEVblog
author_rank: Unknown
timestamp: March 01, 2013, 09:22:27 am
content: 0.5% is plenty for what you want to do.People got by (and some still do) on +/-5% analog meters for what you want to do.Dave.

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: March 01, 2013, 09:41:03 am
content: Quote from: EEVblog on March 01, 2013, 09:22:27 amHaa Haa ,Someone Just Hurt Some Antique sentiments

author: SLJ
author_rank: Frequent Contributor
timestamp: March 01, 2013, 12:24:28 pm
content: Quote from: EEVblog on March 01, 2013, 09:22:27 am+1Tube circuits are usually not that critical. You would even be hard pressed to find two tube testers of the same model that always give the same readings within one or two percent or even the same tubes that were that close. Of course if you are an "audiophool" electrical laws and the laws of physics are different for you and you need to spend all your money attempting to get everything matched as close as possible (usually within .005 percent).

author: c4757p
author_rank: Super Contributor
timestamp: March 01, 2013, 02:50:37 pm
content: Quote from: SLJ on March 01, 2013, 12:24:28 pmWait - you're not serious, are you? 0.005%? Nah, it can't be...

author: SLJ
author_rank: Frequent Contributor
timestamp: March 01, 2013, 04:51:26 pm
content: Quote from: c4757p on March 01, 2013, 02:50:37 pm

author: gooseEL34
author_rank: Regular Contributor
timestamp: March 01, 2013, 09:43:12 pm
content: Thanks guys.Tim'

author: G7PSK
author_rank: Super Contributor
timestamp: March 01, 2013, 10:27:38 pm
content: Quote from: c4757p on March 01, 2013, 02:50:37 pmIt's that last electron that slips by your meter that get's you and ruins the sound.

author: PA4TIM
author_rank: Super Contributor
timestamp: March 07, 2013, 11:33:54 pm
content: Yes, that is true, i just repaired my Twin reverb and at first I forget to first play in my directional multimeter testleads made of golden ferryhair, woven by elves under a fool moon.So I had to do it all over again.

author: tom66
author_rank: Super Contributor
timestamp: March 08, 2013, 12:57:44 am
content: Can't forget reading somewhere that cutting off the top plastic ring of the main electrolytic caps in the audio amp can significantly improve audio quality.

author: gooseEL34
author_rank: Regular Contributor
timestamp: March 09, 2013, 03:28:03 pm
content: Found a Fluke 189 ridiculously cheap, so now I can in fact, capture each stray electron that may affect that magical guitar tone!!!