Voltcraft VC 275 TRMS / alternative 276 - Page 1
Forum section: electronics / repair

author: ywnk
author_rank: Newbie
timestamp: November 25, 2020, 05:23:38 pm
content: Hello folks, I have a problem with my Voltcraft VC 275 TRMS, it has burned mosfet SMD behind the + line, not working at all. I bought it in that condition and trying to fix it. Tried to search a service manual scheme but there's no that documentation in the internet for normal users.SMD chip seems have code Q0Q with two minus above 0 nad last Q, its marked with "|" at the top from pin 1 to pin 5. Does anybody know what's the mosfet that I can replace and run the multimeter?pinology:pin 1 - is going to left Aluminum Electrolytic Capacitors 47uf 16vpin 2 -under the SMD is the + line and its connected to.pin 3 - is going to right Aluminum Electrolytic Capacitors 47uf 16vpin 4 - unconnected terminal but probably holding SMD chip to the logic board (it's not GND as it seems)pin 5 - unconnected terminal but probably holding SMD chip to the logic board. (it's not GND as it seems)I attached a few photos. Any suggestions are welcome.

author: Digorion
author_rank: Contributor
timestamp: December 02, 2020, 09:33:09 pm
content: Hello!I'm not sure, but it looks like some kind of LDO linear regulator in tsot-5 package? Is any pins grounded (try to measure on the nearby electrolythic caps, or negative terminal of battery)? The pinout may be like this: 1 - GND 2 - Vin 3 - Vout 4, 5 - NC. Look the DS for MC78LC00 Series for example.

author: ywnk
author_rank: Newbie
timestamp: December 08, 2020, 11:57:00 am
content: hello my Russian friend!thank you for a replyI checked everything one more time:pin 1: - line so thats the GNDpin 2: + linepin 3: V-outpin 4 and 5 look like holding pins the whole regulator on pcb.hmm so I just wondering now what kind of parameters that LDO should be?

author: Digorion
author_rank: Contributor
timestamp: December 08, 2020, 07:28:47 pm
content: It probably provides power to the IC's (find out if Vout goes to the VCC pin(s) of ICs). If so, find the datasheet for IC's and look for the required VCCs (+3.3V for example).I'm not sure about the maximum current, but usually this parameter is limited by the package, and is approximately the same for all types (typically 50-100mA, i think, and DMM typically consumes about 5-10 mA).By the way, i have no idea, how it has been damaged. Check the other components carefully, it could be damaged too.

author: ywnk
author_rank: Newbie
timestamp: December 09, 2020, 10:17:45 am
content: You right, take a look it powers the main CPU:DTM0660L R78LHI suppose It was damaged probably by malfunction of the battery or old battery. I bought on that condition and now trying to fix.https://manualzz.com/doc/29043796/dtm0660-datasheetpin 48 is GND so It seems we are closer to find a solutionVDD - is between -0.2~4V

author: Digorion
author_rank: Contributor
timestamp: December 09, 2020, 01:01:53 pm
content: First of all check, if the Vout and Vin pads on the PCB shorted to ground.Its DTM0660L, and i'm not shure, what L stands for (low voltage mb). According to your DS It may be 3.0v or 3.3v (also 2.5v, 2.7v, 2.8v meets the requirements), i think. Specs mostly defined for 3.0v. Maximum power draw for IC itself what ifound in this DS (for 3.0v) - 2mA.Check, if the other ICs (or other stuff) powered by this regulator. If so, find th DS for it and check for required voltages.Also check if any voltage testpoints present on the PCB.Do you have a lab PSU (preferably programmable)?Actually, i noticed that thing: the DS says "Low voltage detection: external or internal. Internal two-point voltage detection when using 3V supply, external single-point voltage detection when using 4.5V ~ 9V supply." So according to the pinout LVD pin is 13 and it not seems connected to something. Probably they use 3.0v VDD with internal low voltage detector.

author: ywnk
author_rank: Newbie
timestamp: December 09, 2020, 02:04:49 pm
content: VoutandVinpads from the mosfet aren't short to the GND.0660L maybe it means - Light version, I checked this video and it seems it could be hacked for more options:http://www.kerrywong.com/2016/03/19/hacking-dtm0660l-based-multimeters/unfortunately I don't have PSU.I assume the best option is 3.0V LAL in that case, it should fits well, what do you think?MC78LC28NTRG 2.8 LAKMC78LC30NTRG 3.0 LALMC78LC33NTRG 3.3 LAM

author: Digorion
author_rank: Contributor
timestamp: December 09, 2020, 02:58:11 pm
content: Yes, MC78LC30NTRG 3.0 LAL should work, i think. Check the voltages required for other ICs anyway.If you could take a laboratory power supply unit at least for a while, it would be much easier to check the operation of the multimeter. Set the supply voltage to 3.0V and over-current protection 5-10mA, connect to ground and Vout and see if the DMM shows signs of life.Be careful, something strange happend with previous regulator.

author: ywnk
author_rank: Newbie
timestamp: December 09, 2020, 03:05:43 pm
content: I will try to get the MC78LC30NTRG and I'll be back here with results.Digorionthanks for advice of course I will check other ICs just for sure!

author: Digorion
author_rank: Contributor
timestamp: December 09, 2020, 04:08:32 pm
content: Ok. Keep in mind, what this regulator have no overcurrent protection. Even if the multimeter starts working after replacing the regulator, if a situation arises in which the nominal current of the regulator is exceeded, it may burn out like the previous one.For example, when measuring resistance or continuity, you are supplying energy from the multimeter to the circuit being measured. If the multimeter has somehow been damaged so that its internal circuits no longer limit the current in such modes, and the resistance of the measured circuit is low enough, this can lead to an overload of the regulator.Now I can assume that the following happened: someone tried to measure high voltage in resistance mode, which damaged the measuring circuit of the multimeter somewhow (although normally made multimeters should survive similar situations). Then, when trying to measure low resistance or continuity, the voltage regulator burned out due to overcurrent. I have no other assumptions about the reasons for the failure of the regulator yet.Try to test for a short between Vout and GND with the switch position for continuity or resistance measurement with short-circuited probes.

author: ywnk
author_rank: Newbie
timestamp: December 10, 2020, 11:12:13 am
content: QuoteYou may right, I think it happened.QuoteI measured and there is 1005ohm between GND and Vout.Lets take a look at the SOT23R here:https://pl.mouser.com/datasheet/2/115/AP7333-69860.pdfWhat do you think should it work? (I'm searching for alternatives)

author: Digorion
author_rank: Contributor
timestamp: December 10, 2020, 01:10:01 pm
content: QuoteNo, this one won't work. Vin limited to 6v only. You need a regulator that supports at least 12v Vin. But, in general, this type way better than previous: "Current limit protection, Short circuit protection, Thermal shutdown protection" etc.

author: ywnk
author_rank: Newbie
timestamp: December 10, 2020, 08:49:55 pm
content: well you absolutely rightDigorion, something gone wrong during measuring and the fact about a burned mosfet that way is strange so maybe better is to find some chip that has inside protection circuit to avoid in future a burn it again. I will look for some alternative stuff.Some other friend named Tomas from Czech Republic opened his VC271 (that's a little different model) and he just sent me a photo of LDO from that PCB, it's 30A F13 but can't find any information what's the brand and also check if it got some internal protections.

author: ywnk
author_rank: Newbie
timestamp: January 14, 2021, 10:05:04 am
content: hello, I replaced the mosfet with MC78LC33NTRG 3.3 LAM, it work but unfortunately with ErrE code so probably main cpu is dead. Sad. Thanks for help anyway.