EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: argg on February 01, 2021, 03:40:33 pm
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since two weeks ago the multimeter displays zero in both ACV and ACmV ranges, no matter the input voltage. Will not display frequency also. All other functions like VDC, resistance, capacitance, diodes, A, mA, etc. work all right.
Really disappointed with Brymen as I went for this expensive model expecting to be reliable enough but it proved otherwise despite having seen very little and careful use.
For so many years I never had a multimeter fail on me, even some cheap ones, and it is ironic that I went for this supposedly "premium" model for extra reliability :palm:
I do not think I can do anything myself for repairing and since it is now out of the 2-year warranty, Brymen proposes to send it to them for diagnosing (~60euro back and forth) and then they will say if it is repairable and at what cost. I think I'd rather trash it and try to forget about it :--
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Well if you are willing to trash it, you might as well try to fix it.
How about testing the following ranges to start:
Amps AC
mA AC
VDC+AC--the one with the switch set to VDC and then you select VDC+VAC
mVDC + AC
And does it truly read all zero or is there a residual noise of a few counts?
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I'm happy to pull mine apart and measure components, if it helps.
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In another thread discussing the Brymens, it has been surmised that models like the BM869 and BM789 with the higher bandwidth AC RMS specification probably use a separate RMS converter IC (probably an Analog Devices part) to increase the RMS conversion performance from that which appears to be built into their main chipset.
It sounds like your RMS chip isn't feeding the signal into the main controller, either because the converter IC isn't getting the signal, the chip is inoperable or defective somehow, or the output signal is not getting to whatever auxiliary input pin on the main controller IC that it is connected to...
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I just tried these
Well if you are willing to trash it, you might as well try to fix it.
How about testing the following ranges to start:
Amps AC
it shows zero
mA AC
it shows zero
VDC+AC--the one with the switch set to VDC and then you select VDC+VAC
when in DC is shows the DC component
in DC+AC it shows 0 for AC while for DC in shows momentarily (1-2") the DC component and then it zeroes that too
mVDC + AC
same as above but in DC+AC it maintains the DC component (does not zero)
And does it truly read all zero or is there a residual noise of a few counts?
all zeros
A strange thing I noticed is that in mVAC it always displays zero mV but above ~80mV it displays the frequency, while in VAC it never displays frequency
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I'm happy to pull mine apart and measure components, if it helps.
thank you for the kind offer but I do not think that I will try to fix it - i am terrible at working with smd's :) besides it could be one of the custom chips
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I'm happy to pull mine apart and measure components, if it helps.
thank you for the kind offer but I do not think that I will try to fix it - i am terrible at working with smd's :) besides it could be one of the custom chips
If it is a problem actually in an IC, it could well be the RMS converter IC, which is probably only like an 8 pin chip, and will probably be a standard off-the-shelf part available in onesies for like $4.
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I guess I will need a service manual to see what the RMS converter is
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As an aside, I wonder how much shipping would be from Greece to Canada? ;)
If you do decide to just toss it, I'd consider paying the shipping to check it out as it's quite possibly fixable and I've been considering buying a BM789 or something to replace my aging 30-year old daily-driver meter, although your experience doesn't exactly inspire my confidence in Brymen. :-\
Still, it might make for a good sleuthing / repair video though, if shipping isn't completely insane. 8)
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I guess I will need a service manual to see what the RMS converter is
Or, you could pull the back off and see what it says on the chip. :)
I think Brymen is pretty tight-lipped about service info, I don't expect service manuals to be available.
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It would be interesting to see what failed. I think this is the first failure I have heard of with the BM869s.
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indeed, me and my luck :( I've searched the internet and couldn't find any such failure. And what bugs me is that it happened out of the blue - for no reason at all
Oh, well
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Could someone have dropped your meter and not told you?
It does sound a bit like there might be a bad solder joint on an IC.
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It might be easier to fix than you think. Both of the components I would first suspect are through-hole. Look at this picture and see if it matches yours. The RMS converter is in a can and what I think is the AC blocking capacitor is the large mylar film cap--check it's ratings, it should be fairly high voltage. You can use the AD636 datasheet to figure out where the signal should be traced through to so if you are losing it along the way, you can find it.
https://www.analog.com/media/en/technical-documentation/data-sheets/AD636.pdf (https://www.analog.com/media/en/technical-documentation/data-sheets/AD636.pdf)
[attachimg=1]
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no way anybody messed with it. As regards the cold joint this is the best case scenario and something I can try myself. I will only have to buy or borrow a hot air gun as I do not work with smd's and only have a standard soldering station
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thank you bdunham7, I will open it tomorrow and have a.look
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It might be easier to fix than you think. Both of the components I would first suspect are through-hole. Look at this picture and see if it matches yours. The RMS converter is in a can and what I think is the AC blocking capacitor is the large mylar film cap--check it's ratings, it should be fairly high voltage. You can use the AD636 datasheet to figure out where the signal should be traced through to so if you are losing it along the way, you can find it.
https://www.analog.com/media/en/technical-documentation/data-sheets/AD636.pdf (https://www.analog.com/media/en/technical-documentation/data-sheets/AD636.pdf)
I opened the meter today and the board is exactly the same as the one attached. I carefully reflowed the pins of AD636 to rule out any possibility of a cold joint but it made no difference.
Also took AC measurements on the pins (including the IC input and output) and found no difference with or without an AC signal fed to the meter, therefore the problem must lie at an earlier stage :(
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It might be easier to fix than you think. Both of the components I would first suspect are through-hole. Look at this picture and see if it matches yours. The RMS converter is in a can and what I think is the AC blocking capacitor is the large mylar film cap--check it's ratings, it should be fairly high voltage. You can use the AD636 datasheet to figure out where the signal should be traced through to so if you are losing it along the way, you can find it.
https://www.analog.com/media/en/technical-documentation/data-sheets/AD636.pdf (https://www.analog.com/media/en/technical-documentation/data-sheets/AD636.pdf)
I opened the meter today and the board is exactly the same as the one attached. I carefully reflowed the pins of AD636 to rule out any possibility of a cold joint but it made no difference.
Also took AC measurements on the pins (including the IC input and output) and found no difference with or without an AC signal fed to the meter, therefore the problem must lie at an earlier stage :(
That's a good thing. The closer to the jacks the fault is, the easier it is likely to be to fix it.
Did you try to reflow the capacitor pins, or measure the signal either side of it?
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I can trace the input ac up to both pins of the mylar capacitor. But after that it never reaches the AD636 and with no schematic I cannot track down the problem in between
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The next thing to do is take that shield off and take a hi-res photo and post it. Try and read the numbers off of the individual ICs, look them up and figure out their function. Then use the reference circuit(s) from their datasheets and sort of figure out what the circuit might look like in the meter. It may sound daunting, but that's what it takes to fix modern, 'undocumented' devices. Post good photos of both sides of the board with shields off and IC #s if they are hard to see and we'll help.
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i was just looking at some 869 teardown photos posted by iloveelectronics over at mjlorton's forum.
below is one with the shield off... it looks like the mylar cap connects to a through-hole that runs underneath to the precision resistor at/near the R9 silkscreen mark on the following photo:
https://www.mjlorton.com/forum/index.php?action=dlattach;topic=122.0;attach=433;image (https://www.mjlorton.com/forum/index.php?action=dlattach;topic=122.0;attach=433;image)
there's also another through-hole at that R9 connection, but I can't tell where it goes. there are 2 caps and 3 resistors (C2 and R52a/b/c) just below that precision resistor... are those for an rc filter on the input?
here's iloveelectronics's shot of the back:
https://www.mjlorton.com/forum/index.php?action=dlattach;topic=122.0;attach=435;image (https://www.mjlorton.com/forum/index.php?action=dlattach;topic=122.0;attach=435;image)
note that the above photos are from an 869 (not an 869s) and it doesn't have the ad636... and there are other unpopulated pads where yours may have parts.
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here are two pics of the board and one of the solder side. No further progress so far :(
It would greatly help to know the dc and ac voltages on the AD636 pins of a working unit
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here are two pics of the board and one of the solder side. No further progress so far :(
It would greatly help to know the dc and ac voltages on the AD636 pins of a working unit
You need to take the shield off, there's a good chance your problem is under there.
Look at the datasheet that I previously posted for the info on the AD636.
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and one under the shield
I did look at the AD636 and I am not sure if what I measure is correct that's why I wish I knew the correct voltages on the pins
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and one under the shield
I did look at the AD636 and I am not sure if what I measure is correct that's why I wish I knew the correct voltages on the pins
You'll need to see how far your signal goes up the chain--it should show up on that divider resistor at some point.
The datasheet has a pinout. It also has a listing of the appropriate voltage ranges for Vsupply, Vin, etc. You don't need to worry about 'correct' at this point, you have a go/no-go situation. If you have a 5VAC range and you put 5VAC on the input, I suspect you'll see the maximum (from the datasheet) of 200mVrms at the input pin of the AD636. If you don't see at least some voltage there, don't worry about the details of the AD636 because clearly that would indicate that you've lost the signal somewhere before that point in the chain.
I don't see offhand the method of scaling/switching the AC signal, it is possible that is handled by U1. U1 is likely a custom variant of this chip:
https://www.ic-fortune.com/upload/Download/FS970X-DS-44_EN.pdf (https://www.ic-fortune.com/upload/Download/FS970X-DS-44_EN.pdf)
You'd want to use the datasheet and pinout to verify that it is that chip and that it does (or doesn't) switch the AC signal to an outside RMS converter as needed.
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And here is a reference diagram excerpt from the datasheet showing exactly that--the chip does the switching and scaling, then routes the signal out to the external RMS converter if there is one. Given that and the fact that your frequency function doesn't work properly, I'd say the RMS converter is not likely to be the problem and whatever has gone wrong is most probably somewhere in the input chain or U1. I've pointed out the divider resistor and RMS converter in the diagram, keep in mind that this is a generic diagram and may not exactly match your meter, although it is likely to be very close.
[attachimg=1]
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thank you, I will start tracking from the input and see how far it goes
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the IC U13 (circled in the attached) says on top 062 / U001M / JRC and I assume it is the dual opamp NJM062. Now the strange thing about it is that I measure +180mV DC between pin4 and 8 or between common and 8. Is it possible it operates under such low voltage? And more importantly I trace a 7VAC voltage that I measure, reduced to ~250mVAC through the voltage divider, until pin 5-non inv. input but nothing at pin-7 output.
I wonder if this is the problem, or part of it
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the IC U13 (circled in the attached) says on top 062 / U001M / JRC and I assume it is the dual opamp NJM062. Now the strange thing about it is that I measure +180mV DC between pin4 and 8 or between common and 8. Is it possible it operates under such low voltage?
No, that would definitely be an issue. A TL062 (in this case NJM062) won't start to do anything at all until at least a volt or two supply voltage is applied. The problem could be as simple as a cracked solder, bad trace or via. It is unlikely that the problem would be the IC itself as it likely has little resistance back toward the supply, so if something were shorted internally it would likely be bringing down the entire power rail.
As a quick test, you could quickly check by jumpering in power, say from that neighboring xx062, and see if your AC mode comes alive with power bodged in, I suppose.
And more importantly I trace a 7VAC voltage that I measure, reduced to ~250mVAC through the voltage divider, until pin 5-non inv. input but nothing at pin-7 output.
I wonder if this is the problem, or part of it
The signal not making it through that OP AMP would certainly explain why you see no AC volts. :)