Mastech MS8251B Multimeter Problem - Page 1
Forum section: electronics / repair

author: M.najari
author_rank: Contributor
timestamp: February 18, 2023, 01:45:37 pm
content: I have a Mastech MS8251B Multimeter.It was nice and accurate.I used it with a Rechargeable 9V Battery.Some days ago when battery discharged, I powered up the multimeter by lab power supply on 8.5 volts.immediately noticed that the measurements are incorrect on all functions and renges, voltage show high, ohms show lower....I put another battery in it, but results not changed.I Opened it and can't find any obvious problem on it, except a little heat effect on a tantalum Capacitor.I replaced all Capacitors on the board, but nothing happened.I tried calibrate it by jump connectors on the board and recived Error 4.There are 2 chips on the board DTA0660L and HY11P14 from HyCon, power regulator output is OK 3V.Why did this problem occur with external Power?Any suggestions to fix it?What's Error 4?

author: wasedadoc
author_rank: Super Contributor
timestamp: February 18, 2023, 04:21:21 pm
content: Did you try to use the meter when powered by battery to measure its own battery?Or when powering from lab power supply was one side of that lab supply grounded and did you then try to measure some supply (maybe the same lab supply) which was grounded?If yes to either question you have made the mistake of using a DMM to measure its own supply. That is a known way to endanger the life of a DMM.

author: M.najari
author_rank: Contributor
timestamp: February 18, 2023, 04:35:04 pm
content: Yes,unfortunately I used DMM to measure it's own battery when connected. But a long time ago.I didn't know it was a dangerous job.This problem happened when I was connect it to external power supply.My power supply is not grounded.It's repairable?What should I do?

author: M.najari
author_rank: Contributor
timestamp: February 18, 2023, 06:09:26 pm
content: Quote from: wasedadoc on February 18, 2023, 04:21:21 pmWhy?What happening?

author: wasedadoc
author_rank: Super Contributor
timestamp: February 18, 2023, 06:48:15 pm
content: In order to measure negative and AC voltages/currents the Common (black) input terminal of a DMM is often not at the same potential as the negative terminal of the battery. Whatever mechanism is used to arrange that is short circuited when you connect the Common input of the meter to the battery's negative.However if you did that some time ago and the meter had been working OK since then, it is probably not be the cause now.Could the lab power supply have supplied significantly more than 8.5 Volts. For example a brief switch on transient?

author: M.najari
author_rank: Contributor
timestamp: February 18, 2023, 07:22:43 pm
content: Quote from: wasedadoc on February 18, 2023, 06:48:15 pmLet me tell the story in another way. The battery of the multimeter had finished charging, and while the battery was connected to the device, I connected the power supply to the battery, so that the battery could be charged while using the multimeter. After a few hours, I removed the battery from the multimeter to test the charge level, and the device only stayed connected to the power supply , and while testing the battery, I noticed the wrong measurement, and from then on, it stopped working properly.My power supply is accurate,on the other hand there is a power regulator ic on DMM board.What is your suggestion about this problem?Which component on the board damaged?

author: wasedadoc
author_rank: Super Contributor
timestamp: February 18, 2023, 07:37:31 pm
content: Sorry but I have no idea what component(s) to check or replace.You might be able to find a data sheet for the chips which contains a typical application circuit and then compare that with your meter.

author: abdulbadii
author_rank: Frequent Contributor
timestamp: February 19, 2023, 01:15:05 am
content: Quote from: M.najari on February 18, 2023, 01:45:37 pmIs it including the tantalum ? are they faulty ie. shorted ?

author: rsjsouza
author_rank: Super Contributor
timestamp: February 19, 2023, 03:12:06 am
content: Quote from: M.najari on February 18, 2023, 07:22:43 pmWas your power supply operating in constant current mode?In other words, if you set the voltage and current limiters of your power supply to specific setpoints but the load exceeds that current, the power supply will compensate the excess current by reducing the output voltage. At this point the correct course of action is to increase the current limiter knob/potentiometer until the setpoint is set above the load current, allowing the power supply to operate in constant voltage mode.One common mistake is to confuse the two (sometimes four) potentiometers of the power supply and inadvertently move the voltage limiter setpoint above the rated voltage of the equipment. This has no effect in the output of the power supply if it is operating in constant current mode, but it is dangerous if the load changes as the output voltage will increase.In your scenario I think that might have happened. When the DMM had an increased load of a discharged battery, the power supply might have been operating in constant current mode. The moment you removed the battery, the load decreased and the voltage increased as the current limiter was set "free". This might have damaged your DMM, even with an input voltage regulator.Unfortunately I don't have this meter and therefore can't provide much more information about its repair.

author: floobydust
author_rank: Super Contributor
timestamp: February 19, 2023, 03:35:31 am
content: NEVER power a handheld battery-powered multimeter using a bench power supply.Why? Because Input COM(-) is not the same as Battery(-) and any forced difference in potential will fry the DMM chip.Running the multimeter from a battery, with another multimeter measure voltage between BATT(-) and vdd, vss and input com(-).I suspect the IC is blown which shows up as the mid-voltage on AGND being way off. It should be around 1.0VBut your tantalum C6 looks a bit darker. Not sure what pin it goes to but it might have cooked.A DTM0660 datasheet translatedhttp://www.kerrywong.com/2016/04/03/dtm0660-datasheet-translated/Attached is generic DTM0660 multimeter schematic, you can see the IC power connections and charge-pumps.

author: TheMG
author_rank: Frequent Contributor
timestamp: February 19, 2023, 04:05:21 pm
content: Just another theory. Even if your bench supply is floating, it might still have a bit of leakage and some capacitance to ground. Depending on how sensitive the multimeter chip is to this kind of thing that could be enough to cause some problems.