A tale of two meters

[joeqsmith]

With the new Extech, the battery indicator never comes on.  It could be another problem with this particular meter.  Even at 5.2 volts the display looks very good.   In the first video where I show decoding the RF link, the battery was so low by the time I was done the display would really dim when it would transmit but it kept on running. 

I repeated the test at 40MHz. It's a little more sensitive. Roughly 6.6Vp-p is needed.

[joeqsmith]

Someone was asking about the peak detect of the Fluke and I forgot I had ran this on the Extech.     

[joeqsmith]

Looks like there is a bug in the data logging.

[morato]

Hi,

Same equipments in world

CEM DT-9939/DT-9929 (original manufacturer)
EXtech EX540/EX530 (Americas/Europe)
Digitech/Jaycar QM1575 (Australia)
Hikari HM2920/2900 (Brazil)

Some information for calibration and repair

VR1  - voltage reference -200mV (bottom left pot in shield)
VR3  - AC+DC opamp offset
VR4  - Temp offset
VR5  - adjust for 10x gain used in temperature measurement (top right pot in shield)
VR6  - milivolt adjust
VR7  - true rms zero offset
VR8  - true rms converter
VR10 - DC component for AC+DC true rms

IC3 - Voltage offset for temp measurement
IC8 - frequency signal amplifier. Defective circuit here result in poor frequency detection and double frequency value
IC17 - Buttons encoder
IC18 - Switch encoder


The fuse lock bits in microprocessor are not set and is possible to read the original firmware (USBasp).

After download the original firmware I'm trying to make a new. The original is poor implamentation for capabilities of frontend and usability.

Any posibilities:
- Auto hold (touch hold)
- Auto-range in relative mode
- Min/Max/AVG with auto-range
- 10 or 5 reads per second
- Best bargraph sample time
- Use 10x feature to low burn voltage in uA (removing R57).
- Short beep for good diode
- Flash backligth for continuity test
- Fast response in continuity test
- Extended setup options
- Best average calc routine
- Smooth mode for AC Volts
- Compensation for true-rms converter error in 400 mV RMS operation. AD636 is disigned to 200 mV full scale and error grow significantly above 300 mV
- Press any button for weekup

I'm working with Hikari HM-2900 (without RF option = DT-9929) and I not have intention to implement communication with PC.

The meter hardware is capable to implement good and useful features, but the worst software implementation result in poor usability.

In the next few days I will publish the beta version of the new firmware (without RF communication).

[joeqsmith]

I went to use my EX540 today and it looks like it was borrowed.  I mentioned how one time a the person who borrowed it, turned the knob past the dead stop which broke the plastic hex.   Here's what I found today. 

[tautech]

I went to use my EX540 today and it looks like it was borrowed.  I mentioned how one time a the person who borrowed it, turned the knob past the dead stop which broke the plastic hex.   Here's what I found today.

Hmmm, I'll add DMM's to my list of never to be lent which is now: DMM, chainsaw and wife !
Otherwise they all have the risk of being given back shagged !

[MacGyver0606]

Hi, I'm looking for the calibration/adjustment procedure for any of these Multimeters, which are pretty much the same. I couldn't find it anywhere. Any help??

CEM DT-9939/DT-9929 (original manufacturer)
EXtech EX540/EX530 (Americas/Europe)
Digitech/Jaycar QM1575 (Australia)
Hikari HM2920/2900 (Brazil)

[Bobson]

My CEM DT-9979 has strange problem: sine wave voltage readings are approx 10% low (proper voltage seen using oscilloscope, APPA 506 and Fluke 8846A) at 2KHz-70KHz range, it becomes  closer to actual at 70-100KHz. Triangle and pulse are much more exact. Also, CF (Crest Factor) readings are very random: it shows them sometimes, but rarely. There is calibration menu, but absolutely no docs. Some calibration features are rather straightforward, but I found nothing dealing with this problem.

Will try to do the same test with DT-9989, which is much newer, but has very similar DMM part (AFAIK).

[morato]

two years later...  I am back 

New firmware in beta version

Any posibilities:
- Auto hold (touch hold) - OK
- Auto-range in relative mode - OK
- Min/Max/AVG with auto-range - OK
- 10 or 5 reads per second - OK
- Best bargraph sample time - OK
- Use 10x feature to low burn voltage in uA (removing R57). - OK
- Short beep for good diode - OK
- Flash backligth for continuity test - OK
- Fast response in continuity test - Yeah! Better
- Extended setup options - OK
- Best average calc routine - OK
- Smooth mode for AC Volts - OK
- Compensation for true-rms converter error in 400 mV RMS operation. AD636 is disigned to 200 mV full scale and error grow significantly above 300 mV - No!
- Press any button for weekup - OK

https://youtu.be/Z0y45DUREvE

[morato]

Hi, I'm looking for the calibration/adjustment procedure for any of these Multimeters, which are pretty much the same. I couldn't find it anywhere. Any help??

CEM DT-9939/DT-9929 (original manufacturer)
EXtech EX540/EX530 (Americas/Europe)
Digitech/Jaycar QM1575 (Australia)
Hikari HM2920/2900 (Brazil)

I don't have one.

But... Don't touch in VR3, VR5 and VR10

For DC
Select VDC, input precise 3V and adjust VR1 to correct read
Select mV range, input precise 300mV and adjust VR6
For AC
Select VAC, short input and adjust VR7 to zero reading
Input precise 3V AC and adjust VR8 to correct read

It's basic calibration procedure.