Brymen BM837RS - Teardown / Repair

[dardosordi]

Got this Brymen BM837RS meter for under $ 10, the Volt-Ohm-Amp ranges don't work but the mV range is spot on and the frequency counter also works. The 10 amps fuse was blown, so I think it got overloaded.

This unit is also rebadged as the Amprobe AM-91.

Attached are some pictures from the teardown (thanks to my wife). I desoldered the shield can from the bottom board in order to take pictures and inspect it.

I also removed the range switches (not in the pictures) looking for arching but its clean.

I would appreciate any help or directions into repairing this beautiful meter.

Better res pictures here: https://imgur.com/a/BhwOMBf

[dardosordi]

Further testing, reveals it measures fine on mV scale and volts scale under 400mV, after that it overloads. So I think its an issue on the range switching.

[retiredcaps]

Questions so we can further diagnose.

1) With the probes shorted and rotary dial set to ohms, what does the LCD show? Do the annunciators look correct?
2) With the probes open and rotary dial set to ohms, what does the LCD show? Do the annunciators look correct?
3) With the probes shorted and rotary dial set to volts, what does the LCD show? Do the annunciators look correct?
4) With the probes open and rotary dial set to volts, what does the LCD show? Do the annunciators look correct?
5) Did you check the input protection components like the MOVs, PTC and fusible resistors, given that this is an used and abused meter, to make sure they are okay?

[dardosordi]

Questions so we can further diagnose.

1) With the probes shorted and rotary dial set to ohms, what does the LCD show? Do the annunciators look correct?
2) With the probes open and rotary dial set to ohms, what does the LCD show? Do the annunciators look correct?
3) With the probes shorted and rotary dial set to volts, what does the LCD show? Do the annunciators look correct?
4) With the probes open and rotary dial set to volts, what does the LCD show? Do the annunciators look correct?
5) Did you check the input protection components like the MOVs, PTC and fusible resistors, given that this is an used and abused meter, to make sure they are okay?

1) The analog bargraph shows 1 bar up and down. The reading is blank, never settles.
2) On first power up, it goes straight in OL, then slowly starts creeping down, to a around 6 Megs, this is with no probes. Connecting probes it jumps all over the scale and autorange never settles.
3) First overloads (OL), then displays around 1000 volts and starts going down (looks like a capacitor discharging) until it reaches 0.000 in about 10 seconds.
4) Displays -300 volts, steady if not touching probes. When touching probes, lecture jumps around.
5) This is what each component measures (using my trusty Fluke 77 IV):

- 10 Amp Range Fuse: open, shorted it temporarily for now.
- mA Range Fuse: short, its a glass fuse, will replace with proper HRC fuse once repaired.
- 4 input big resistors: each around 450 ohms.
- PTC1 670 Ohms.
- PTC2 1K94.
- SPG1, SPG2 (MOVs?): 11 Megs
- Fusible resistor R57: 1 Ohm (matches markings).
- Also checked every diode on both boards, all in the 0.6 volts ballpark.

Thanks!

[dardosordi]

When its powered up, you can hear a very high pitch noise coming from it. Its not the buzzer (since its not connected).

[retiredcaps]

- 10 Amp Range Fuse: open, shorted it temporarily for now.
- mA Range Fuse: short, its a glass fuse, will replace with proper HRC fuse once repaired.

The fuses won't matter with V/ohm measurements.

- 4 input big resistors: each around 450 ohms.
- PTC1 670 Ohms.
- PTC2 1K94.

My guess is that both PTCs should be around the same.  When you write 1K94, is this 1094 ohms?

- SPG1, SPG2 (MOVs?): 11 Megs
- Fusible resistor R57: 1 Ohm (matches markings).

Both SPG should measure infinite resistance. I would desolder both SPGs and retest the meter.

[dardosordi]

- 4 input big resistors: each around 450 ohms.
- PTC1 670 Ohms.
- PTC2 1K94.

My guess is that both PTCs should be around the same.  When you write 1K94, is this 1094 ohms?

Both PTCs are different in size, so I assume its ok for them to be different values. 1K94 = 1.94K ~ 2K.

- SPG1, SPG2 (MOVs?): 11 Megs
- Fusible resistor R57: 1 Ohm (matches markings).

Both SPG should measure infinite resistance. I would desolder both SPGs and retest the meter.

I desoldered both, no changes.

[retiredcaps]

Look under the black rotary switch.  Maybe you will find something like

https://www.eevblog.com/forum/repair/fluke-787-repair-any-idea/

or

http://mrmodemhead.com/blog/ideal-carbon-footprint/

[retiredcaps]

Test 6.

Put the Brymen into DCV and turn it on.  Use the Fluke IV and measure the input impedance of the Brymen.  My guess is it should be around 11M ohm ballpark.

PS. You can leave the SPG out for now as we troubleshoot.  Do they still measure 11Mohm out of circuit or 0L?

[dardosordi]

Test 6.

Put the Brymen into DCV and turn it on.  Use the Fluke IV and measure the input impedance of the Brymen.  My guess is it should be around 11M ohm ballpark.

PS. You can leave the SPG out for now as we troubleshoot.  Do they still measure 11Mohm out of circuit or 0L?

Test 6:

Measuring red probe to red, black to black: starts low (< 5 Megs) and slowly rises up to 36 Megs.
Measuring red probe to black, black to red: starts low (< 5 Megs) and slowly rises up to 23 Megs.

Both SPGs measure 11 Megs out of circuit (just one leg desoldered).

Thanks for the help, I appreciate this a lot, and didn't even think of measuring half of this!

[dardosordi]

Look under the black rotary switch.  Maybe you will find something like

https://www.eevblog.com/forum/repair/fluke-787-repair-any-idea/

or

http://mrmodemhead.com/blog/ideal-carbon-footprint/

Its clean, that's the first place I looked (had a Fluke 323 clamp meter with carbon track from arching between pads).

[dardosordi]

I did the test #6, it fails the test, but what could cause that ?

[retiredcaps]

Something is dragging down.  I suggest you do a thorough IPA bath, let it dry overnight and retest.

I would completely remove the SPG off the board.