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MS-5308 Power Supply Issues

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wavebits:
I am new to the forum, but I have been following this thread with interest. I purchased a MS5308 about a month ago. It failed after about 5 minutes of use and would not power up on good batteries. The eBay seller suggested trying just the AC adapter without the batteries. Bad move. The meter was replaced under RMA after a delay to hear from the manufacturer about the bad AC power supply and a recommendation to not use it. The second unit was DOA on batteries only. It makes a sound when the power button is pushed, but no display. I am awaiting a response from the seller.

I believe there is a power supply design issue on this unit. Has anyone reverse-engineered a schematic of the supply? A quick check shows what appear to be several linear regulators on board. One (U6) appears to be a 5V+ regulator, U5 may be a 3.3V+ regulator, but I was unable to get any info on the part #. Mine seems to be not regulating at 3.3V. Interestingly, if I apply about 7.5V to the battery input the meter power up and works fine. If the input voltage is raised toward 12 volts, the output of U5 starts DROPPING and the LCD dims and goes off. I suspect U5 may be blown, but what are the other two 3 terminal devices marked U3 and U4. They also appear to be 3 terminal linear regulators. If I remember correctly, U3's input is connected directly to the battery +. I wonder if they are using a virtual ground. Any thoughts? The chip set datasheet is fairly useless. No pin outs, no supply voltage requirements.

There was a comment on this thread about the 3.3V regulator having a 8V max input voltage. That would probably be correct if it is a LDO type. Can anyone check their working meter and seen what the input voltage to U5 is? That may be why these units seem to be failing. U5 is marked "32b2w" on my unit. I can't find anything on this device.
I am looking at alternative devices for a 3.3V regulator that can withstand a higher input voltage. The Seiko S-812C33SG has a 16V max input voltage and 100 mA current output. It is in a TO92 package though.

jurs:
I have the same problem with MS5308 after using PSU and you are right the U5 is fry. Next examples are on the Russian forum http://forum.ixbt.com/topic.cgi?id=48:10801#20. As says on this forum UT612 has the same IC and U5 is marked
35A
B05
After switch on U5 has 0v, 8.04v (battery), 3.49v, 8.04v (clockwise from up big pin).
I also can't find datasheet on U5. In my MS5308 U5 has
N35
2882K or 28B2K

nixxon:
https://www.eevblog.com/forum/reviews/mastech-ms5308-lcr-meter-with-esr-measurement-on-discount-at-the-moment/msg171081/#msg171081

http://youtu.be/hA_0GnNO_2A

wavebits:
I am actually referring to the internal power supply circuit inside the instrument, not the AC external power supply. Several owners including myself have apparently had failures running solely on battery power.

It appears the linear 3.3V regulator may be failing from excessive input voltage from the 12V battery supply. The regulator Mastech is using goes under a variety of aliases, none of which have been able to be deciphered by their part number. It's possible that some brands of regulators they may be using in a particular production run are more tolerant to higher input voltage than others. Several SOT packaged LDO 3.3V regulators I looked up had 7 or 8 volt maximum input voltage. Other meters using this chip set have 9V battery systems.

I actually thought I had placed my original post under the thread you linked, but it turned out I was not logged in as I had thought, and when I pasted the text it started a new thread. Damn newbies!

wavebits:
Jurs:

Are those voltages you measured on a MS5308 with a 12V battery or the other meter you mentioned? Is the battery voltage 9V or 12V? What ground point were you using on the meter - lead?

I am trying to determine if the 3.3V regulator has battery voltage minus a diode drop as input or is being fed by a pre-regulated voltage from another regulator.

Thanks.

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