repair fluctuating DMM VC860

[rve]

Hi,

I have an old DMM (voltcraft VC860) which fluctuates a lot when measuring voltages. For example, I connected 7.4 volts DC to it (which is stable, I've checked it with a scope and another DMM) but the readout on the VC860 keeps fluctuating between 7.413 and 7.458. The last 2 digits keep changing and never settle. The value keeps increasing and at some point it starts decreasing. This repeats itself over and over making the DMM pretty useless.

I've attached the schematic. If I short the input the DMM displays 0.0000 and does not change. The fluctuating happens in all voltage ranges.

Measuring resistors seems to work fine. Although the resistor value on the display keeps increasing over time at a much slower rate compared to voltage measurement. But I think this could also be normal thermal influences.

What could be wrong? Is this a somewhat common failure mode of DMMs?
Thanks.

[Labrat101]

Did you check the board for dry joints . or tin whiskers as this meter probably has been in service for many years . As you mentioned that it does zero when the inputs are shorted .
So the last 2 digits floating are caused by noise .
I would be inclined to clean the entire board with IPA and a soft brush . Even a hair line crack
in the pcb, as it may have well have been dropped some time in its 15+ years .
The very last digit may well float between 3 and 5 . but the other digits should hold reasonably stable.

[rve]

Good idea, I'll check all joints. Thanks.

[Labrat101]

Don't forget the rotary switch or any moving parts  first as they always get abused . I don't think you have any major problems as it works. In ohms etc and noisy voltages. 

[The Soulman]

Likely a bad adjustment pot, VR 2 sets the reference voltage, maybe give that a little wiggle and see if that changes things.
Note the position before you touch it, you need to return it to the same position after wiggling,
otherwise your voltage readings will be incorrect.

[rve]

Hi,

I finally found the issue. I've cleaned everything and I found that at least one contact on the rotary switch was misplaced. After putting all contacts in the correct position it works again.

Thanks for the suggestions!

[Labrat101]

Hi,

I finally found the issue. I've cleaned everything and I found that at least one contact on the rotary switch was misplaced. After putting all contacts in the correct position it works again.

Thanks for the suggestions!

  Glade you found the bad connection .