Fluke DMM repair

[Phil Allison]

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 shite 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

 

[Tom Miller]

"Phil Allison" <pallison49@gmail.com> wrote in message
news:a9aa4fd1-8bef-446f-ad2a-e07693b13fc2@googlegroups.com...

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 shite
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

++++++++++++++++++++++++++++++++++++++++=

Nice story. Thanks for sharing.

 

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

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 ?

 

[Phil Allison]

jurb

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

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 ?

** 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.

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 ?

** 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

 

[Trevor Wilson]

On 21/09/2014 4:28 PM, Phil Allison wrote:

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 shite 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.

**Urk. We all make mistakes, but we don't often tell the world about
those mistakes. Kudos.

--
Trevor Wilson www.rageaudio.com.au

 

[Phil Allison]

Trevor Wilson wrote:

**Urk. We all make mistakes, but we don't often tell the world about

those mistakes. Kudos.

** When repairing something, making at least one mistake along the way is common. The trick is in finding that mistake quickly and rectifying it before returning them item to the customer.

In this case, the error was rather subtle and the problem it created non obvious.

Two cheap Chinese 9v batteries was no biggie.


..... Phil

 

[Trevor Wilson]

On 22/09/2014 12:48 PM, Phil Allison wrote:

Trevor Wilson wrote:



**Urk. We all make mistakes, but we don't often tell the world
about

those mistakes. Kudos.


** When repairing something, making at least one mistake along the
way is common. The trick is in finding that mistake quickly and
rectifying it before returning them item to the customer.

**Tell me about it. An incorrectly installed electro just last week.
Scared the crap out of me (when it exploded). I picked a 2,000uF/16Volt
one. I intended to pick a 100uF/75Volt one. Same can size. Dunno how the
2,000uF/16V one ended up in the wrong place, but I hope it doesn't
happen again.

In this case, the error was rather subtle and the problem it created
non obvious.

Two cheap Chinese 9v batteries was no biggie.

**Yeah, no major problem.



--
Trevor Wilson www.rageaudio.com.au

 

[Jeroni Paul]

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 different cause. A very small electrolytic leaked and the sometimes conductive mess 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 occasionally doesn't, well, better than nothing.