Fluke DMM repair

Hi,

the diode beeper sound from my trusty FLUKE Series 70II had been getting rather faint over time - the piezo element is attached to the back cover and connects to the main PCB via conductive rubber pads pressing on a couple of solder blobs on its underside.

In the past, cleaning all the contact areas helped, but not that much.

So, a few weeks back I decided to hard wire the darn thing to the PCB with short runs of fine, Teflon coated wire. This worked like a charm as the two metal coated areas of the piezo element solder very nicely.

Then I installed a new 9V battery ( Polaroid brand, heavy duty) and closed the meter up. Two weeks later the battery was near flat. Cursing cheap s**te Chinese batteries, I fitted another, this time an Alkaline type.

Two weeks later, it was nearly flat too, causing the battery icon to remain on.

When checked with another DMM, I found the battery drain was way high at

14mA !! Also, the current draw hardly varied if I used a fresh 9V battery or the old one. Very odd.

I suspected a leaky tantalum cap across the battery, but there isn't one. Then I removed the PCB from the case and the current went back to normal at

0.3mA. When I re-fitted the PCB, up it went again.

WTF ?

You will not likely guess the cause so I will tell you.

When I hard wired the piezo element, I reversed the polarity of the connections to the PCB - such elements are not polarised, so it could not possibly matter.

But I neglected to disable the conductive rubber pads. The crossed over wiring meant they were now both connected directly across the terminals feeding the piezo element when the PCB was installed and pressing down on them.

Those terminals have about 5V DC across them when not beeping - enough to send 14mA though the rubber.

... Phil

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Nice story. Thanks for sharing.

Actually SEEING it, would probably have figured it out. Guessing on the internet, not a chance.

But this raises the question, why did you reverse the leads ?

And another question, how come there is any voltage across that thing when it is not called to beep ? Isn't it like switched with a transistor or something ? Why would there be constant voltage applied ?

jurb

** It was not intentional, just using the shortest path from PCB to the piezo transducer. The ceramic disk is soldered to a slightly larger brass shim, so wires run to the shim and the top of the disk.

** The transducer is just a small capacitance driven direct from a pin of the CMOS computer chip at he heart of the DMM. That pin simply sits high until it beeps and presumably has a circa 14mA current limit built in.

The manual as no detailed schem.

... Phil

It is supposed these conductive rubbers you replaced were not meant to have a resistance the design may have accounted for.

My Demestres DMM buzzer also failed with similar sympthoms but very differe nt cause. A very small electrolytic leaked and the sometimes conductive mes s made a hi impedance voltage comparator misbehave. I cleaned it but no joy , tried various cleaning solutions and tried heating the area and got it to work somewhat, it beeps with a varying tone and mostly beeps but occasiona lly doesn't, well, better than nothing.