SOLVED - Brymen BM817 DMM gone wrong - Page 1
Forum section: electronics / repair

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 23, 2019, 02:41:14 am
content: Another DMM turned up, a French re-badge of a BM817 sold cheapish as "untested".On arrival the 1A fuse for the mA input was missing, other than that flickery LCD segments due to the LCD/zebra strip assembly slightly unscrewed.Once the fuse replaced a quick check shows DCA to be good, even on mA range, was not able to test below 10mA with my lab supply though.A quicktest continuity and short work well, 190k test resistor also spot-on, will have to check some intermediate values.DCV is within spec also except for the 5V range that is totally off.The reading on 5V DC range is stable but displays only half the input voltage.Measuring a 0.5V input will read 0.253V, displaying anything more than 0.301V (0.6V input) will display as an overload.The meter uses an AD-85-4 ADC, as mentioned by others here it seems identical to the Fortune FS970X.Where to look first? I made some quick in-circuit measurements at the resistor network on pins 18 to 22, all seems fine.Is there a known way to get into calibration mode? I have an I.R. interface if that's needed.

author: retiredcaps
author_rank: Super Contributor
timestamp: January 23, 2019, 06:29:01 am
content: Brymen have excellent support. I suggest you send an email to Brymen asking for calibration instructions.So, just to clarify, all the other ranges work in DCV? manual 500? manual 50? work fine?

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 23, 2019, 02:18:27 pm
content: Yes other DCV ranges (50V and 500V) are spot-on.The cal mode accessed by pressing Select and Hold and turning on toas it is on more recent models.I was able to calibrate 5V DC range fast enough with the lab supply and 50k points reference DMM.Now it looks like some of the VAC range is messed up too, this is going to be a whole lot more fun generating the reference inputs.I only hope the AAC won't need a recal.

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 24, 2019, 01:50:14 am
content: Done a little more testing on thefunction, good so far, I still have to check the 5M and 50M ranges.It turns out the whole ACV range is not working (displays 0.3V for a 280V input) the unique SMD electrolytic cap (C30 on the AD737) looks dead crusty... Turns out to have leaked all it's capacitance on the PCB.In the meantime I was tempted to think that the cal had been lost as for the 5 VDC range, tried to cal and of course messed it up.Once C30 has been changed other ranges work normally so I'm left to re-cal the 5 VAC range I've fooked-up.I was expecting to go through calibrating all the VAC ranges so the variac was unburied, I had a little fun adjusting that to 2.5V as the cal reference, but the BM817 now reads within +/-1mV of the reference Metrix MX53.That leaves the AAC to check tomorrow but it's already feeling like a fix.

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 24, 2019, 06:18:03 pm
content: To summarize, I now have a working meter after replacing the 1A fuse, replacing C30 22µF SMD cap, recalibrating the 5V DC range.I was a bit deceived by the mechanical aspects of the meter, screws, casing could be better, input protection on V/is minimal.The good points are having a great set of features for the price and a really responsive bargraph.Anyone let me know if you want the full resolution photos uploaded...

author: retiredcaps
author_rank: Super Contributor
timestamp: January 24, 2019, 11:43:39 pm
content: On most Brymen meters, including the one you have, input protection for the Volt/ohm consists of PTC, SPG or MOVs and sizeable power resistors.