EEVblog 121GW Multimeter Issues

[ChrisG]

I re-tested in my early morning and in the LowZ position I could not change the range and it indicated 1000v in the lower right hand corner. So assume yes this is the highest 600V then too. Upping the Vpp to 5.0v at 389Khz the result was a staggering 367v on the display.

[Kean]

@ChrisG What tpw_rules was suggesting I believe was to input the 1Vrms 100+kHz input into the VAC mode (instead of LowZ), and then manually select 1000V range.  And thus see if the same wrong readings were displayed as in LowZ.

Well I just did that, and got some interesting - but not consistent - results.

With 2.8Vpp (~1Vrms) sine at 300kHz:
LowZ AV 1000V range read 161V - consistent
VAC auto range (5V)  read 1.63V 299.9kHz - once only, see below - otherwise ~1V 299.9kHz
VAC 5V range read 1.63V 299.9kHz - once only, see below - otherwise ~1V 299.9kHz
VAC 50V range read 12.6V 299.8kHz - once only, see below - otherwise 0V 0Hz
VAC 500V range read 149.2V 299.8kHz - once only, see below - otherwise 0V 0Hz
VAC 1000V range read 149.3V 299.8kHz - once only, see below - otherwise 0V 0Hz

I actually tried going through these VAC ranges (and back and forth to LowZ) several times.  The first time gave the 0V 0Hz on the AC 50V ranges and above (can't recall the 5V range reading).  The second time the above high readings.  And all subsequent times back to the 1V or 0V.

Then tried 1kHz and 10kHz inputs, and got slightly low readings in the higher ranges (1V in 5V range dropping down to 0.6-0.7V in 1000V range), but also interesting was that it couldn't display the frequency any more in the 50V and higher ranges.  It then just showed 0Hz.  The meter is specified to 5kHz in the VAC mode, but I guess in those manual ranges the attenuation is too high for the frequency measurement.  But how does it "work" then in LowZ mode (i.e. measures correct frequency, but incorrect voltage) if also using the 1000V range attenuation?

I tried several things including increasing my output to 5Vpp and I couldn't get those strange readings again.  The fact it happened once does point to tpw_rules theory about the whacky adjustment for frequency being potentially correct.  Or I could have been hit with the rotary switch wobble issue that one time.  Or maybe there is a specific sequence needed to reproduce...   

While doing this I noticed another strange reading on AC mV manual ranges too.  Same 2.8Vpp 300kHz sine wave.  On 50mV range it read 23.0mV, and on 500mV range OFL.     At 10kHz both showed OFL.  Limited bandwidth coming to play here I guess - way beyond the 5kHz spec, and not as worrying as the weird LowZ readings.

[exe]

If the switch is wobbly, but the measurements are not affected by moving it, is this still a defect?

[dr.diesel]

If the switch is wobbly, but the measurements are not affected by moving it, is this still a defect?

I'd argue the wobbly switch has a questionable longevity. 

[IanB]

I'd argue the wobbly switch has a questionable longevity.

Possibly, but it could be by design. There should be some slack between the switch knob on the case and the switch mechanism on the PCB, so that rotating the switch does not put excessive stress on the PCB.

[tpw_rules]

@ChrisG What tpw_rules was suggesting I believe was to input the 1Vrms 100+kHz input into the VAC mode (instead of LowZ), and then manually select 1000V range.  And thus see if the same wrong readings were displayed as in LowZ.

...

I tried several things including increasing my output to 5Vpp and I couldn't get those strange readings again.  The fact it happened once does point to tpw_rules theory about the whacky adjustment for frequency being potentially correct.  Or I could have been hit with the rotary switch wobble issue that one time.  Or maybe there is a specific sequence needed to reproduce...   

You are correct about what I intended. I've been calling it the 600V range because that's what it's referred to in the calibration chart.

As for the behavior, ...huh. I'm not sure what to make of that, if it proves or disproves my theory. I've made another patch which removes the software frequency compensation entirely. If you can't replicate the behavior in any mode, that compensation must be the problem. If this is what happens, can you dump your calibration data using the patched firmware and upload it for me? I don't know much about the meter circuit, so someone else will have to take over if that doesn't fix it. It may end up being fixable in software though.

[Candid]

With the patched no frequency compensation 1.02 version I have identical behavior as with the 1.02 (autorange patched) version and as described by Kean.

And for the frequency thing:
At about 15Vpp/1kHz I can switch between 5V and 50V range without OFL in 5V range. Then I get correct Hz reading on both ranges. Going to 500V and 1000V range voltage reading stays ok but Hz again is 0.

[ChrisG]

you patched the firmware or which patch did you apply? If you're still in version 1.02 of the firmware what would be different then?

[Candid]

I tested the tpw_rules 1.02 with removed frequency compensation (see one post above my last one).

[ChrisG]

you patched the firmware or which patch did you apply? If you're still in version 1.02 of the firmware what would be different then?

Apologies, I now see the download firmware link.. 

[EEVblog]

With 2.8Vpp (~1Vrms) sine at 300kHz:
LowZ AV 1000V range read 161V - consistent
VAC auto range (5V)  read 1.63V 299.9kHz - once only, see below - otherwise ~1V 299.9kHz
VAC 5V range read 1.63V 299.9kHz - once only, see below - otherwise ~1V 299.9kHz
VAC 50V range read 12.6V 299.8kHz - once only, see below - otherwise 0V 0Hz
VAC 500V range read 149.2V 299.8kHz - once only, see below - otherwise 0V 0Hz
VAC 1000V range read 149.3V 299.8kHz - once only, see below - otherwise 0V 0Hz

I don't understand why you are testing at 300KHz? That's way outside the frequency spec.

[EEVblog]

The Low-Z reading problem at high frequencies is confirmed and has been reported.

[ChrisG]

all, has it been pointed out here on this list the duty cycle:

• on an a-symmetrical square waveform is displayed inverse on the 121GW? I noticed that yesterday evening and re-tested it 10 times. so 40% on the SDG2042x is displayed as 60% on the 121GW, 70% duty cycle is then 30% on the 121GW and so on.
• It needs 1 Vpp or more to actively announce the duty cycle. 500mVpp or below nothing happens. 600mVpp or more I might be lucky.
• where the % sign is oddly placed next to the Temperature.... AARHHHHHGGG).

[amspire]

121GW Manual revision 24 Jan 2018 (minor issue):
http://www.eevblog.com/files/EEVblog-121GW-Manual.pdf

On page 57, the description of the Setup Option Multimeter ID has been added, but it was not added to the list on page 53.
The correct order should be:

1. Temperature
2. Battery Voltage
3. Auto Power Off (APo)
4. LCD Contrast
5. Year
6. Month-Day
7. Hour-Minute
8. Multimeter ID
9. Logging Interval (ln 1)
10. Additional settings dependent upon the function selected.

[Candid]

I made some measurements with my FeelTech FY6600 and RD JDS6600 function generators concerning the phantom voltage to earth. Both of them use simple SMPS and I can measure about 80Vac to earth in normal mode.

In LowZ mode I would expect only some Vac. I checked with my Brymen 235 and the 121GW. Both behave like expected but with the 121GW, when I wait some seconds suddenly it measures about 70Vac in LowZ mode but only with the FeelTech FY6600 not with the RD JDS6600?

The Brymen 235 (and a labeled UNI-T the Voltcraft VC890 from Conrad Electronic in Germany) always give me the correct(?) LowZ behavior.

Can someone confirm this and does anybody know why the 121GW behaves like this?

[HKJ]

LowZ is often a PTC that draws current, depending on the current it will heat up and increase resistance. It sounds like the PTC in the 121GW heats up faster or at lower current.

[amspire]

Capacitance in Manual Ranging not properly detecting Overflow
121GW Ver 1.01

In Auto Ranging mode, if you attach a 100uF electrolytic capacitor, the meter steps through the ranges to the correct range in less than 5 seconds, so the meter can detect the OFL state on a range fairly quickly.

However in manual ranging modes, if you step through the ranges with the same 100uF electrolytic capacitor attached, the period can be 10s of seconds to show OFL to never showing OFL in the low ranges.

It seems the oscillator measuring the capacitance has no timeout, and so it will happily wait forever for the capacitor to charge. With electrolytic leakage, it may never reach a voltage on the lowest ranges to complete a single reading. There is no sense in this in that in all ranges, there is a maximum possible time for a reading, and so there is no reason not to time out the reading and show OFL.

While you are waiting, the meter shows the last successful reading - usually zero.

For the same reason, if you short the leads in capacitance mode with manual ranging, the meter just shows the last reading forever. Usually zero.

In autoranging, a short will cause the display to blank and it will autorange to the maximum range, but it may be that it has no timeout in this maximum range either. At least the display is blank rather then zero.

The problem with this is that you cannot tell whether you are reading something that has no capacitance or something that has a lot of capacitance when in a manual range. I would expect to see OFL if the capacitance is too big for a manual range.

[ChrisG]

Hello all, it's been quite the time there has been a post here, 11 days. I'm just wondering if there isnow an overall issue or backlog list. HW and SW related. I assume there must be and is Dave able to share it then here?

[bitwelder]

I hope this thread would work also for issues regarding the documentation.

Regarding the manual (revision 24.1.2018), I would suggest:
- add a reference to the meter f/w version, not only the manual revision date (e.g. setup menu has a paragraph about buzzer on/off, but that option doesn't exist in fw v1.01)
- the Auto Power Off function is mentioned, but it doesn't describe after how many minutes it triggers.

[lowimpedance]

 In diode test mode the temperature display still remains on instead of showing the currently set test voltage. Pressing the RANGE button will toggle to the test voltages,  but switch to another function then back to diode test and the test voltage is replaced with temperature again.
 Also when the beeper is disabled and you select the continuity mode the beeper symbol on the LCD is on but the beep is off. As others have noted in the discussion thread the beep mode should have multiple options for the functions that actually need the beep.

[lowimpedance]

Minor manual correction, the 500mA text on the front cover picture is still missing next to the A text over the current input jack.

[lowimpedance]

 When in Ohms (auto range) mode and shorting the probes the display will show some random numbers for a short time before ranging down to display the value of the shorted probes.
So 95 % of the time the random numbers come up instead of blanking the display while changing ranges. It only occurs on the top M Ohm range and will blank as soon as the auto ranging starts.
Do others see this effect on theirs with firmware version 1.07. ?.
Not a major issue, but hopefully will be corrected at some point.

[morrone]

So 95 % of the time the random numbers come up instead of blanking the display while changing ranges. It only occurs on the top M Ohm range and will blank as soon as the auto ranging starts.
Do others see this effect on theirs with firmware version 1.07. ?.

Yes, I see the same thing with 1.07.  When the probes are applied to a resistor the screen blanks for a moment, then shows a random bad value and pauses for a second or two with no sign that it is still working to find the correct value, then it blanks again briefly before showing the correct value with relatively rapid updates to the lest significant digits.

[bicycleguy]

minor Manual, 07 March 2018,

Page  kind  description
7  spelling 'VERION'
29 grammar (twice), 'for the is jack'
59 grammar, 'and is can be'

Starting to look pretty good !

[Candid]

I already posted in the discussion thread about a problem I have with Vac readings. Now I will switch to the correct place for this.

In 5Vac mode I get wrong readings.

Today I checked the ZERO OFFSET CALIBRATION and had to find out, that only in 5Vac range mode with shorted inputs I get a reading of 0,499xVac. With open input I get nearly the same value of about 0,5Vac.

In all other Vac (and Vdc) ranges I get 0,0xxV. So I tried to calibrate only the zero offset in Vac 5V range mode. During the count down it displays the 0,499xV and after the calibration I still have this offset with shorted inputs on the 5Vac range. At the end of the calibration countdown I see "OFF-E" for about half a second what is not mentioned in the manual.

I have two references from voltagestandard.com. The 121GW is spot on for all voltage measurements I can do (Vdc 0,350V up to 10.000V and Vac 5V).

So there must be something wrong with the 5Vac range mode of my 121GW.

Another thing happened today while testing for the first time. When I switched from left side off to V mode the meter did not turn on. I taped on the mode switch still in this mode and it turned on. So there must be another problem with my 121GW.

You can read my postings and realted answers if you want to get the complete story:
https://www.eevblog.com/forum/testgear/eevblog-121gw-discussion-thread/msg1446232/#msg1446232
https://www.eevblog.com/forum/testgear/eevblog-121gw-discussion-thread/msg1447697/#msg1447697
https://www.eevblog.com/forum/testgear/eevblog-121gw-discussion-thread/msg1447957/#msg1447957
https://www.eevblog.com/forum/testgear/eevblog-121gw-discussion-thread/msg1465469/#msg1465469
https://www.eevblog.com/forum/testgear/eevblog-121gw-discussion-thread/msg1469130/#msg1469130
https://www.eevblog.com/forum/testgear/eevblog-121gw-discussion-thread/msg1469890/#msg1469890