BK precision 389A Meter fix? - Page 1
Forum section: electronics / general

author: Kryoclasm
author_rank: Regular Contributor
timestamp: May 13, 2011, 07:47:19 pm
content: Hi folks,I was wondering how hard or easy would it be for one to fix the cap range on the BK precision 389A multimeter if it was damaged.I had bought it used and the multimeter's capacitance test is the only range not working. I looked inside, other than a blown 20A 600v, fuse all looks well.Thanks for any ideas.ShawnBTW I got it for $49 US.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 13, 2011, 08:29:43 pm
content: Well , you got it at 1/3 of the price ,the capacitance range of it, looks very small . (Max 40mF )Theoretically the meter haves an Over voltage protection up to 1000V DC or AC rms.The 20A blown Fuse, translates to , that some one had connect it to Mains .As second hand there is no warranty for you.And even if there was an way to repair the capacitance range,if it is unprotected , it will stop working again.My advice , get an capable capacitance tester, and move on.

author: saturation
author_rank: Super Contributor
timestamp: May 14, 2011, 12:00:40 pm
content: It depends on what is broken; I agree with Kiriakos that if a fuse is blown, worse case it was not fast enough to protect the DMM and something else was damaged; I would make sure the DMM is still rated to its proper specification before I would consider only the capacitance range damaged.The DMM may seem to read properly, but if you test its accuracy against another DMM, it might have other things subtly damaged too, if so it makes the DMM unreliable and thus, unfortunately, useless unless fully repaired. I think this model cost about $100, so I'm not sure it worth repairing in that case.I would first inspect the PCB, make sure its clean and nothing is shorted; check the jacks and clean them. Performance test the unit per the service manual.If all is well, DMM usually measure capacitance either as an RC time constant or making calculation vs a filter network or comparing it against a known good cap, the last is unlikely. It is possible to repair anything but ask yourself:Can you solder/desolder SMT components, as this is likely what's in the DMM?Can you trace the PCB and find the capacitance subcircuit without a schematic, or with a schematic and if so, do you have access to the service manual?Do you have the time to do this?If the answer is yes to all, then its worth repairing.

author: Kryoclasm
author_rank: Regular Contributor
timestamp: May 14, 2011, 03:19:12 pm
content: Well, I did do a thorough inspection with a full tear down of the meter.No burned traces, magic smoke smells, lifted components or obvious damage other than the blown fuse.SMD work is no problem for me as I have the equipment and ability.I don't think I can trace / troubleshoot the circuit with out a service manual, but I have had BK send me a service manual for an oscilloscope recently. So getting one for this meter should be possible.I am hoping it is a minor component that is trashed and not the main chip.It is difficult for me to compare BK 389A meter with a calibrated source as I do not currently have access to such an item. I do not think using my el'cheapo radioshack or super cheapo china multimeter's as any kind of useful standard.As it is, I just might order a few very low tolerance components from mouser next time I need to get some parts and use them as a reference.Does anyone care to see pics of the meter as it was taken apart?Shawn

author: saturation
author_rank: Super Contributor
timestamp: May 14, 2011, 03:48:18 pm
content: I'd love to see it, if you can post.Since you are just checking basic performance not really calibrating it, you can compare it against your lesser quality but known good DMM to insure the readings are as it should be, at least in the ball park.Its unlikely the main ADC is damaged if truly at least, the Vdc function is working properly. I'm not sure what type of chip it uses, but if its similar to the commonly used ICL7106, with built in LED/LCD drivers, knowing the readout is ok is already a good sign.Quote from: Kryoclasm on May 14, 2011, 03:19:12 pm

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 14, 2011, 04:32:15 pm
content: Quote from: Kryoclasm on May 14, 2011, 03:19:12 pmNo ThanksCurrently I use limited bandwidth connection from GSM network.Even so I have one last suggestion for you ..From your words .... you have an oscilloscope that needs repair , your DMM needs repair ,you have nothing as reference to test them ..But still you can do SMD work .Well something does not stick from this story .( Just my thoughts in out laugh mode.)And now my suggestion , get one truly new DMM ,accurate and all , so to have something to count on it.Good luck

author: Kryoclasm
author_rank: Regular Contributor
timestamp: May 14, 2011, 10:48:34 pm
content: Quote from: Kiriakos-GR on May 14, 2011, 04:32:15 pmSorry, I can't afford new and shiny equipment like some people. I am a just hobbyist.I get what I can afford and make the best use of it.If I have to fix, tweak, or hack the hell out of it to fit my needs then I am happy to do so.I was just asking for some insight.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: May 14, 2011, 11:17:01 pm
content: Quote from: Kryoclasm on May 14, 2011, 10:48:34 pmYou should had say so, from the start.I was thinking that you are making your living, with those tools.And I just reminded to you , the correct practice.Nothing more - nothing less.Welcome on the forum.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 15, 2011, 03:54:28 am
content: i dont think 20A fuse got anything to do with capacitance. something else must have been broken. since you indicated the other range is doing fine. pls post a picture if you can (smaller <=1000 pix width), better if you can sketch/trace the capacitance circuit track on it for easier view by others.

author: saturation
author_rank: Super Contributor
timestamp: May 15, 2011, 11:07:29 am
content: For future reference, look for test equipment via eBay, I see you are in the USA. If you have the space, consider lab quality gear for ~ < $100 each, made from the 1980s, particularly from Hewlett Packard. There is a thriving community in keeping these items working. They are cheap because they tend to be big and heavy but they have specs that blow many new gear out of the water.FWIW, you can get Fluke 80 series DMM for between $50-200 used, the price depends on age and condition [ there are 4 generations of 80 series, stretching from 1989-2006].Quote from: Kryoclasm on May 14, 2011, 10:48:34 pm

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 15, 2011, 12:44:05 pm
content: i learnt my lesson. better get $100 new shiny unit than having $50 used and then later broken unit.

author: Unknown
author_rank: Unknown
timestamp: May 15, 2011, 01:32:34 pm
content: Quote from: Mechatrommer on May 15, 2011, 12:44:05 pmI also learned my lesson. Better spend $50-$200 on a second hand professional tool that's build like a tank than spend $100+ on a shiny new device that may die soon, doesn't have any service documentation, and has much worse specs.