[SOLVED] Help repair a Greenlee/Brymen DM-810A

[giosif]

Hi all,

I've got myself a Greenlee DM-810A DMM off eBay and all is ok with it, except for the resistance measurements.
When I switch to resistance, even without connecting the probes to anything, I get a 300 kohm reading. 
Funny thing is, for lower value resistors, it is still able to measure the resistance accurately.
Only when going to higher value resistors (I tested with a 1 Mohm resistor), it start displaying wrong values (a few hundreds kohm's for that 1 Mohm resistor, from what I remember).

Checking the internals of the DMM (to the best of my abilities), all seems to be in order, so don't think the analog part has an issue.
Also, since I can still get good readings with lower value resistors, I believe the ADC should be alright.
So, my initial suspicion would be that someone played with the calibration of the meter and screwed the resistance values.

What do you think?

Also, is there anyone who happens to have the service manual for the meter?

Thanks!

Regards,
George

[saturation]

Check the input stage protection MOVs, when they are partially open due to an overvoltage event, they appear as high impedance across the inputs rather than 'open' and make ohms scales appear as if they are loaded.

Check the input impedance in volts mode, if its not 10M ohm or more it points to the MOVs as failed.

[giosif]

Hmm... That's an interesting thought!
Thanks, saturation!

Now, silly question: how do I check the input impedance?

[Lightages]

Use another multimeter to measure the input resistance. It should be 10Megohm when set to measure volts. As already said, the MOVs could be bad, or there could be contamination on the circuit board. It could also be out of calibration. If someone started the calibration routine and didn't complete it correctly it will show incorrect values.

Based on the value it is showing you I would guess someone played with the calibration procedure and screwed up the calibration. To do the calibration for resistance you need the following values at 0.1% accuracy minimum: 300R0, 3k000, 30k00, 300K0, 3M000, 30M00 and possibly 30.00nS.

[giosif]

Thanks for that!
I guessed that may be the procedure and already tested it: input resistance showed 10 Mohm or above for all volts measurement settigs, except for AC voltage setting (a separate switch position on this DMM) which measured as open (i.e. OL).
So, I am guessing everything is alright on the MOV side of things.

And many thanks for the information on the resistor values & accuracy percentage required for calibration!
Is there a calibration documentation available somewhere for this meter?

[retiredcaps]

I've got myself a Greenlee DM-810A DMM off eBay and all is ok with it, except for the resistance measurements.

Link to auction please?

Also, is there anyone who happens to have the service manual for the meter?

I have searched on/off for a while for a Brymen multimeter schematic, but never found one.  I can't even find a datasheet for their multimeter chipset.

[giosif]

I've got myself a Greenlee DM-810A DMM off eBay and all is ok with it, except for the resistance measurements.

Link to auction please?

What would you need it for?

Also, is there anyone who happens to have the service manual for the meter?

I have searched on/off for a while for a Brymen multimeter schematic, but never found one.  I can't even find a datasheet for their multimeter chipset.

I have identified a calibration document that may apply to this meter as well on MJLorton's forum (search for amprobe calibration; you also need to be registered to be able to see and download the file).
Now if only I had those resistors...

[retiredcaps]

What would you need it for?

It helps us to know what condition it was advertised in and if the seller has competence/knowledge in selling test equipment.

I found your auction.

The meter was listed as in "used but good".  Since you already left positive feedback saying "Item is as described", you are now screwed in any hopes of getting a refund back from the seller.

[giosif]

Ah, ok.
Yes, that was the auction.

And, yes, I am aware I can't return the item, but I didn't intend to anyway: the meter seems to be ok otherwise and the resistance measurement issue looks to be something reverseable.
Maybe I have too low expectations from products on eBay... 

[Lightages]

I cannot disclose the calibration procedure as per agreement with Brymen. Martin Lorton has used a calibration procedure and you can follow that but using the values I provided. I am not giving away anything that hasn't already been provided or can be encountered by following Martin's video. I am pretty sure this will fix your problem. To get it working close enough for now you can just use some 1% resistors to get it close and usable.

[giosif]

Did not intend to make you reveal information you are not allowed to disclose.
The information provided so far is very useful and I couldn't ask for more.

Thanks again!

[giosif]

Just to let everyone know I've performed the calibration for resistance and all looks good now.
Thanks to all who helped!
I now have a fully working DMM.