Teardown: DMM old ProsKit 03-9303C  (1988) - Page 1
Forum section: electronics / general

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 16, 2011, 11:42:25 pm
content: This was my first DMM that I took for professional use at 1988, at the age of 18.The price of it at that time, it was equal with the 1/4 of the lowest monthly fee of one worker.I had never open it up completely, I was just opening it to replace the batteries for the last 23 years.Even today is still on calibration, and the only range out of calibration is the 200M ohm.With this DMM came and my first K-type thermocouple, and I still have the Users manual and the box of the DMM.The strong point on it was the capacitor measurement, it can read all those tiny Pico-faradLarge display & all the ranges for more tasks.It did hit the concrete about 10 times in those 23 years, it had not travel that much in my tool box,as it was my main bench meter.It was my trusty helper for 21 years , but I started to have the feeling that it was unable to handle the latest AC sources like any voltage that comes out of one inverter, like for a example an uninterruptible power supply.By joining in the EEV I found all the details about the averaging reading meters, and so I understand the value of the True RMS DMM.And this is the cause that forced me to upgrade.The up come is that I did got the Fluke 28II ( as the top of the cream for the 2010), plus three old bench type Fluke,plus the Agilent U1272A, this one had fall from the sky as gift, due the invitation that I got for product review by the Agilent marketing organization.Even so, that today I own one complete basket team of DMM, I still find it attractive and useful.And I decided to take it apart for one in-depth cleaning / PCB and exterior.And also I want to see if it is possible to calibrate the 200M ohm range.The tear-down revealed one re-badged Mastech with PCB code M940 D1-21plus the secondary PCB for the range switch with PCB code M940 D2-21The construction quality is good for it age, the double sided PCB and the hidden solder in those parts holes,prevents damage due the oxidation of the solder, and any signs of oxidation its just in the surface.This device belongs at the time that the CAT I , II , III regulations was not even there.And so the phase plate is quite honest about that matter.About finding the schematic and calibration information about it, I had contacted Mastech and ProsKit in Taiwan.Mastech totally ignored me,and ProsKit wrote to me that they do not have the schematic of it.Not to say that the Proskit representative suggested to me another fresher Proskit DMM if I care to upgrade.Dear ProsKit, from a company that does not have the schematics of the products that it had sell, I never buy anything from it !!Well this DMM is still on sale from other makers, and found it to called as 303-9303C.Even so if you have see it by other brand name please let me know about it.I have not lost the hope yet, about calibrating it at the only range that is off.Pictures.Professional Multimeter:303-9303C32 rangesLarge LCD display 1999 counts 3 1/2Display : 30x60mm LCDDC Voltage : 200m/2/20/200V0.5%, 1000V0.8%AC Voltage : 200mV1.2%, 2/20/200V0.8%, 700V1.2%DC Current : 2m/20mA0.8%, 200mA1.5%, 10A2%AC Current : 2m/20mA1.2%, 200mA2%, 10A3%Resistance : 200/2K/20K/200K/2M0.8%, 20M1%, 200M5%Capacitance : 2n/20n/200n/2/20uF2.5%Temperature : -50C~1000CContinuity checking with buzzerDiode TesthFE: 1~1000Power source: 6F22 (9V) x 1Size Weight: 88 x 170 x 38mm 340g (including battery)

author: ciccio
author_rank: Frequent Contributor
timestamp: September 17, 2011, 12:03:28 pm
content: The images don't open correctly.Sorry

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 17, 2011, 12:12:10 pm
content: The forum is on transition to a new server, it will be some issues for the next hours, about avatars and pictures.

author: EEVblog
author_rank: Unknown
timestamp: September 17, 2011, 12:18:13 pm
content: Quote from: Kiriakos-GR on September 17, 2011, 12:12:10 pmYep, sorry about that, it will take a few hours at least, and will hopefully get fixed!Downloading from the old server now (pretty quick), and then need to re-upload to the new server (pretty slow), I'll leave it running overnight.New uploads will work now though.Dave.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 17, 2011, 07:22:34 pm
content: Pictures restored, avatars too.I think that we are ok .As I said and above, if any one see any identical DMM from any brand, write a word about it here.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 19, 2011, 09:44:02 am
content: Well, after few hours ago, I got an email from Mastech and actually from their sales engineer Mr. Zang,he replied to me that they are unable to locate the schematic of my re-badged Mastech.But he made to me one proposition, that if I accept to ship back my unit to Mastech,he will make the arrangements so to receive an modern bright new Mastech :for praising your 20 years using of our product.And now I am in a big dilemma, should I keep the old and the memories? or accept the generous Mastech offer ?By the way, I visited just by curiosity the new Mastech web site, the old Mastech changed their old structure,now comes with an new brand name ( new cooperation) , called asV&A Instruments.With a truly modern DMM series under theV&A logo, by keeping active and the older Mastech product line.Fresh DMM with single and dual display, plus lots of interest about safety, with another specialized line of products,that shares the same safety mechanism that blocks the probe holes, like the Gossen Metrawatt does.Not to say that this latestSafe - DMM line, has an clean key design, that comes close to Fluke 87-5.More here..http://www.mastech.com.cn/html/en/products.htm#4Damn I am confused .. and excited too, I just email them for one schematic, and they offer to me a new modern unit !!

author: Lightages
author_rank: Unknown
timestamp: September 19, 2011, 09:22:48 pm
content: Quote from: Kiriakos-GR on September 19, 2011, 09:44:02 amI think you are very lucky to be offered a new multimeter for almost nothing. The trade for an old meter seems like a good thing to me. But as you said, you have had that meter for many years. So the question is: Are the memories and the pride that you have kept good care of something outweigh the chance of having a new meter that may or may not actually perform better? This especially when you take into account that you already have many new and very nice meters already. Personally I would hold on to the old memory as it cannot be replaced. I know we have had our differences, but think about it.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 21, 2011, 02:53:12 am
content: Its true that on this case , there nothing to do with value in money.And so I did something different, I had invited Mastech to visit the EEV, and suggested to receive something top of the line,that I will do for it a complete product review.If they accept, we will just exchange a piece of the glorious past with something equal that belongs in the future.The all idea probably sounds crazy, but its not.Currently I am waiting for their marketing organization to make their minds about this suggestion.And honestly I prefer to have in my hands something stamped with the Mastech logo (Original manufacturer),than a re badged as ProsKit.They deserve the fame, they made it.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 24, 2011, 04:00:15 pm
content: I got passionate to explore the history behind this specific model, and found that the specific model description was something chosen exclusively from ProsKit.The rest of the world identifies this model as: M890C+ or M890C or DT890C.Mastech was the true Dad of it, and there is even a M890C with the Mastech name on it.The interesting part of the story is that the design of this unit had been replicated by lots of known and unknown DMM makers, the known: UNI-T & MetexThanks to the passion about DMM of some people from Poland and Russia,I found many details about the M890 series C+ …D…F…GI found many forum messages in Poland, saying that the no-name versions of this design suffers from issues just in a year time, due the inferior parts in them.And the info was not about few resistors or electrolytic cap, in some cases even the main IC 7160 was turning bad.By reading all those pages about this model, I found and the schematic of it.And even hi-res pictures of the PCB made by others than Mastech.The useful discovery from those pictures are that they had printed on the PCB the naming of the trimmer pots, and soon enough I will combine the info from the service manual and the pictures that I found, so to create a new document with easy explanations of what its trimmer pot does and the location of it on the PCB.With the schematic in my hands I will also trace the problem at the 200M Ohm range,By crossing the leads I get 1M Ohm reading, and by measuring a resistor I get the value of it plus the 1M Ohm reading, I have the feeling that some one at the assembly line had mistakenly select the value of the R59.If this is true? I forgive him, the 24 years that this unit is healthy, makes me to forgive him.Either way the plan is to spend my evening by finding all the truth about it, and have some fun too.For now I will add as attachment the schematic of it, in order to help and other lost souls, with a 03-9303C in their hands.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 25, 2011, 07:09:26 am
content: After lots of hours of reading schematics and testing, I came to the point to scream Victory.I had calibrated ACV / DCV / CAP / K-Probe / by having as reference the Agilent U1272A .The rest ranges like ACA / DCA / Ohms / are fixed, but spot on.The 200M Ohms range hides a surprise, there is poor translation on the manual,and a major mistake that the translator who wrote the Users manual in English did.I got some large resistors, and connected them in series so to create the bigger value of 15.4M Ohm.Then started to explore the behavior at the 200M range and at the 20M range.The 20M range is almost spot on, or better in the specifications.And after solving the puzzle of what really happens and with what the user manual advices,the 200M range looks to be in the specifications too.It feels really good, that even after 24 years, I have the upper hand on it.The first picture that shows what those VR pots do its second to none worldwide.I found the complete data-sheet of the IC UMC 7106, amazing info in there, plus I found that this chip was ready to be used even with True RMS circuitry.Check the pictures.

author: pez
author_rank: Contributor
timestamp: January 04, 2022, 03:35:42 pm
content: Quote from: Kiriakos-GR on September 19, 2011, 09:44:02 am.http://www.mastech.com.cn/html/en/products.htm#4-It was archived and now it can be found at:.https://web.archive.org/web/20190416083524/http://www.mastech.com.cn/html/en/products.htm.