Can't measure AC/Frequency w/ BM869 , must be damaged

Thread Starter

DarthVolta

Joined Jan 27, 2015
446
This thing always seemed pretty useless at measuring freq (I love it besides that), but now it must be damaged. So far it will still display 60Hz, w/o probes attached, but probing anything from the sig.gen, it just sits at 999.99Hz, and the Vrms seems stuck at root 2, 1.414V, or a little lower. Regardless of the wave, like sine/sqr/pulse, and the amplitude/freq settings/duty cycle.

And it's doing it on the main VAC setting, both displays (there's a dual display), and on the various mVDC and mVAC dial settings, same thing 999.99Hz, stuck at basically root2 volts, and stuck at 30.85% duty cycle, regardless of input.

The only mistake I made lately, (that I'm aware of) was on mVDC range, I momentarily connected just under 1VDC, BACKWARDS, and the low battery light came on. I guessing that did it. I have done something like that before, and seen the low-batt light come on.

The only other mistake I make 1/2 often, it being on Ohms still , and probing a low voltage, usually under 12VDC. I hardly ever work over 20V, and hardly ever on mains AC stuff.

So yup, AC reading is stuck at 1.41V and 999Hz, with the probes attached. W/O them, it's at 0.0043Vrms and 0.000Hz
the VFD reads 0.43V, 0.000Hz
on DC+AC, I just get the 1.406V as the AC part, and no DC from the DC offset on my AWG
on DC dial, a sqr wave with any duty cycle, reads 0V, so it's not being averaged or anything, pretty sure it used to average stuff like that.
mVDC-999.99Hz/ stuck at 1.48mVDC and 30.83% dutycyc
mVAC- 999.99Hz/ voltage OL at even 9mVpp
Diode/cap-works
ohms/buzzer-works
Pure DC on DC dial works fine, but it won't average any AC

This only happened in the last 2-3 days, and I only noticed 1st thing today. As I know, it was fine before, I momentarily connected just under 1VDC, BACKWARDS, and the low battery light came on. I don't remember that last time I can say for sure the AC or Hz reading was working.......but the Hz reading on this always were easily fooled, so I rarely used it . But it never was pegged at 999.99Hz, and since the 60Hz reading stil shows up when the probes are not connected to anything, I never noticed until today.


This is terrible, I hope it's an external part that needs fixing, and not some impossible to find chip or processor. Any tip's on taking it apart to look around ? It was new from an ebay seller a few years ago, so I highly doubt there's any applicable warranty

 
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Thread Starter

DarthVolta

Joined Jan 27, 2015
446
This goes in repair

The only other weird thing lately, might be very bad, was I had something connected to my isolated PSU, or 5V USB from PC, and I was soldering something, and if it was the isolated PSU, ok dinw, it was just 1 or 2 things I soldered.

But something went haywire, either the arduino reading, or the DMM ? IDK remember what that was about, except, Disconnect everything is the safe bet. I usually do. I know the soldering iron is gnded, and my scope, and PC chasis/5V-GND

 
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Thread Starter

DarthVolta

Joined Jan 27, 2015
446
It's a lithium 8.4V, I charged lately, and it was at ~3.5V today, so that's low. The 10A 1 fuse is good, I never checked the 440mA before i lost it already today.

The other day, I was trying to figure out the voltages on a computer speaker system, that's when I must have shorted the soldering iron tip to about 10.5V iirc, from a maximum of 18VDC in circuit. So at worst, my DMM COM was on main's-EG, and I was trying to measure a pin voltage, so tacked a wire on and saw the DC voltage drop right down, as it was a short-circuit.

So IDK, that shouldn't affect the DMM ?? Also I had the current limiter on the PSU, powering the stereo I'm fixing , for just over what it required.

I'd say I fried something with up to 18V, when I was on mVAC or mVDC range by accident.

I think the chip is still behind there, reading out the noise that makes it in, like 60Hz mains, the voltage for that reads a few hundrends of mVAC and changes like normal.

I'm having a hard time getting it to turn on, but that'srelated to it's logic and th switch, and the way it turns off.......it never likes having the battery yanked, it will auto-off it's-self after


I've taken it apart, I'm new to the inner workings of a DMM, besides the input protections, which seem fine, I've yet to check the 1st transitors and other silicon, just mapping the dial...looks a maze to me.
 
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jpanhalt

Joined Jan 18, 2008
9,338
It's a lithium 8.4V, I charged lately, and it was at ~3.5V today, so that's low. The 10A 1 fuse is good, I never checked the 440mA before i lost it already today.
Not quite sure how to translate that.

1) Did you actually lose the low-current fuse? I suspect it is required. Was it OK before you lost it?
2) If I had an 2S lithium pack that read 3.5V, I would not try to recharge it. I would replace it.
 

Thread Starter

DarthVolta

Joined Jan 27, 2015
446
Human speech is a thing of the past for me, only computers now.

I misplaced the 440mA fuse, so IDK it's condition. Everything around it is fine. So are the caps from Vbat +/- to the COM

I already put the battery in the charger, I'm not up on 8.4V Li batteries, is it dead, or a fire hazard ? I'll go see what it is now. Thanks

Also I've been powering it off the PSU since I took it apart, it ran like as described, and only it draws 7mA so that sounds normal enough
 
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KeithWalker

Joined Jul 10, 2017
877
Human speech is a thing of the past for me, only computers now.

I misplaced the 440mA fuse, so IDK it's condition. Everything around it is fine. So are the caps from Vbat +/- to the COM

I already put the battery in the charger, I'm not up on 8.4V Li batteries, is it dead, or a fire hazard ? I'll go see what it is now. Thanks

Also I've been powering it off the PSU since I took it apart, it ran like as described, and only it draws 7mA so that sounds normal enough
The low input ranges will not work without the 400 mA fuse. You can test it temporarily with a higher current rated fuse but make sure you replace it with the correct one before you use it again.
Keith
 

Thread Starter

DarthVolta

Joined Jan 27, 2015
446
I found the fuse and it is open, the stereo used about 306mA, and I set the current-limit to 355mA. And the fuse is 440mA. I haven't measured current with it in a dog's age, so I'm not sure what happened. I used ali-clip's to bridge the fuse not long after I removed it, and it didn't seem to work then.

I'm been making progress on mapping the PCB. But all I did on the A/uA area was check the diodes, which are fine.

So back to mapping the circuits, I want to do that anyway. Everything looks fine, and with the large SMD sizes, and the easy to see-through PCB, it's fun. Too bad some chips would be near impossible to get though. Then I'll have to wait until I can program a MCU to be the new brain over the analog.
 

Thread Starter

DarthVolta

Joined Jan 27, 2015
446
Actually the current on the 5.1ch amp jumps to near 1 Amp at some volumes I used, and maybe 1Vpp input was too much anyways.

But some GND loop probably is the mistake I made. I even did it twice, the bad condition happened, and I said, hmm, my DMM doesn't like that, it makes it think the battery is dead, so touched that point again (on the 10.5V powered area) , just to be sure something was wacky.

I put to much faith in the workings of my DMM, and not enough science LOL.
 

rsjsouza

Joined Apr 21, 2014
244
I agree with others regarding the DMM power: 9V or bust.

I couldn't quite get if you have DC offset or not, but my BM857 (older design, but still Brymen) does not like DC offset when I use the "Hz" button on the V~/mV~ ranges - only when I use the exclusive "Hz" range. I looked at the BM869 but couldn't find this range on the dial. Hopefully this may help.
 

Thread Starter

DarthVolta

Joined Jan 27, 2015
446
No trust me it's not the battery, it did the same thing from the bench PSU. I'm close to finishing mapping the lower PCB, I forgot the main MCU is on the upper board, the big IC on the lower board is the ADC, and it also supplies power to some chips.

I sure hope it's source-able part. At least they are "only" around $250 new, I love it, I'd get a replacement for sure, I think it's a great DMM
 

Thread Starter

DarthVolta

Joined Jan 27, 2015
446
Well I soldered on jumpers for the rotary switch set to VAC, and I'm sweeping up to 100kHz, and it's reading the voltage and the freq just fine. I don't get it ? It's powered off 7V from my PSU, but I powered it off that before when it didn't work.
Now see solder up the mVdc/ac and see if they work again
 

rsjsouza

Joined Apr 21, 2014
244
Perhaps the rotary switch contacts are bad or intermittently contacting with the PCB? That would be my first guess.

Given you mentioned a fuse was opened, perhaps the contacts suffered some arcing that degraded its operation.
 

Thread Starter

DarthVolta

Joined Jan 27, 2015
446
holy cow it is the 9V batteries. The low battery light is not on, but they can't provide enough juice to power the AC section, but with the PSU it is working.

I never noticed this before tho. But it's also hard to turn on sometimes, it\s related to the STBY mode.
 
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