Brymen BM257s 0L on Ohms - Page 1
Forum section: electronics / repair

author: RobertRupa
author_rank: Newbie
timestamp: April 20, 2021, 06:16:23 pm
content: Hi i've today noticed that my BM257s have problem with Ohms, when You try to measure something it always shows 0L.I've tested all safety features, path from banana plugs through switch are going to pin 5 of 74HC4053D and pin 2 of Q10 VLR. (sot23?) (2SC5866TLQ?).other functions are working fine, continuous mode, V, A, Diode, Capacity.

author: ogdento
author_rank: Regular Contributor
timestamp: April 21, 2021, 12:23:47 am
content: If you put it into Ohms mode and use another good meter to measure the voltage at the V/com terminals, do you get any output? I think it should be about 0.26 voltsif it was functioning properly.You're basically using your bad meter to measure the input impedance of the good meter... but I'm just curious if you're getting any voltage at all out of the bad meter. If not, then the source for reading ohms could be damaged... sounds like q10 might be part of that.

author: RobertRupa
author_rank: Newbie
timestamp: April 21, 2021, 05:02:28 am
content: 0.070V measured with Aneng AN8008. It will be hard to measure it on the board. how this could happen?

author: ogdento
author_rank: Regular Contributor
timestamp: April 21, 2021, 05:34:18 am
content: Hard to say how it could have happened... maybe a high voltage at the input when in ohms mode? Or some kind of transient? Somebody here smarter than me will have better ideas.You say you checked the "safety features"... are you talking about the movs and ptcs? Make sure the mov's are open of course, and I think the ptc should be 1-1.5k. Check the input clamps to make sure they're all ok.Also, have you made sure the input impedance is around 10-11M? (Do the reverse of what I suggested before - set your bad meter to dcv and connect the v/com inputs to a good meter set to ohms)If I were debugging this I'd draw up a schematic with the parts I know, and then check them out. I'd test Q10 on-board and if it looks weird I'd pull it and test it off-board. The 74HC4053 is an analog multiplexer (switch) so it's likely being used to route different signals around, and it's possible that could have died too.

author: ogdento
author_rank: Regular Contributor
timestamp: April 21, 2021, 05:39:28 am
content: I forgot to say... 0.070 volts is not goodThe datasheet says the open-circuit voltage for Ohms feature is 0.45volts. I get 0.49 when I measure my bm257 with a Fluke 8060 in super-high impedance mode (>1000M), and about 0.26 when I measure in "regular" impedance mode (~10M).

author: RobertRupa
author_rank: Newbie
timestamp: April 21, 2021, 07:00:03 am
content: Movs, ptcs, resistors and diodes are ok.On BM257sVoltage on specific Ohm rangeMOhm = 0.252VKOhm = 0.437VOhm = 0.497Vimpedance on specific Ohm range on BM257s(positive on positive)MOhm =AN8008 MOhm 0L (on BM257s display 0L)AN8008 KOhm 0L (on BM257s display 0L)AN8008 Ohm 0L (on BM257s display 0L)KOhm =AN8008 MOhm 9.792MOhm (on BM257s display 0L)AN8008 KOhm 0L (on BM257s display 0L)AN8008 Ohm 0L (on BM257s display 0L)Ohm =AN8008 MOhm 9.777MOhm (on BM257s display 0L)AN8008 KOhm 0L (on BM257s display 0L)AN8008 Ohm 0L (on BM257s display 0L)impedance on specific Ohm range on BM257s(positive on negative)MOhm =AN8008 MOhm fluctuating from 3.2MOhm to 0.030MOhm (on BM257s display 5.535MOhm, when below 0.070 0L)AN8008 KOhm 0L (on BM257s display 5.535KOhm)AN8008 Ohm 0L (on BM257s display 5.535MOhm)KOhm =AN8008 MOhm 0.000MOhm (on BM257s display 0L)AN8008 KOhm 0.000KOhm (on BM257s display 0L)AN8008 Ohm 0L (on BM257s display 5.535KOhm)Ohm =AN8008 MOhm 0.000KOhm (on BM257s display 0L)AN8008 KOhm 0.000KOhm (on BM257s display 0L)AN8008 Ohm 0L (on BM257s display 553.5KOhm)

author: ogdento
author_rank: Regular Contributor
timestamp: April 21, 2021, 03:11:23 pm
content: Hey Robert,Those numbers don't look right... I noticed you're listing the Ohm values that are displayed on the 257, but when you're measuring the input impedance of the 257 it should be set to DCV (not Ohms), and the AN8008 should be set to Ohms. It sounds like you've got them connected correctly though - pos to pos and com to com.

author: ogdento
author_rank: Regular Contributor
timestamp: April 21, 2021, 03:26:19 pm
content: Here's a shot of using the 8060 to measure the input impedance of the 257... the 8060 is set to Ohms (Meg range), and the 257 is set to DCV (I could change the ranges with the range button but it's on auto). inputs are pos to pos, com to com.

author: RobertRupa
author_rank: Newbie
timestamp: April 23, 2021, 05:32:34 am
content: not the same result as Yours ;/

author: ogdento
author_rank: Regular Contributor
timestamp: April 23, 2021, 03:54:08 pm
content: Hmm, that picture puzzled me for a second... I expected to see something around 10.xx or 11 M ohms. I looked up the An8008 specs and resistance measurement tops out at 9.999M