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Jeff Liebermann Jeff Liebermann is offline
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Default Flakey Molex(?) connectors

On Sat, 15 Jun 2019 16:09:17 -0400, Bob Engelhardt
wrote:

Part of my Habitat volunteering has involved repairing their power
tools. They get Milwaukee tools from the Milwaukee refurb/repair
facility, through a Milwaukee charitable program.

A common problem with these tools is the trigger. Which, of course,
isn't just a switch but is the trigger with 4 control wires as well as
the 2-in, 2-out power lines, a tiny PCB, and the battery connector -
also with control wires. I.e., a nightmare to diagnose and sometimes
ends with us junking the tool (a new switch assembly is much more than
what we pay for the tool).

I was able to definitively diagnose the current tool's problem as a
flakey connector on the signal cable between the battery and the PCB.
The connector appears identical to the ones used for running PC fans
from the MoBo. Like this:
https://i.imgur.com/Q7kfqQG.jpg

I found these contacts that look to be identical to the ones in the tool:
https://www.digikey.com/product-deta...7CT-ND/5287249

I'm puzzled that Molex connectors, if that's what they are, can be so
unreliable. And that Milwaukee would use such unreliable connectors.
Has anybody else seen problems with these connectors?


Yes, but not in power tools. I've had to deal with intermittent
square pin connectors in commercial 2-way Motorola radios and
repeaters for quite some time. Motorola used larger versions of the
connectors to interconnect PCB's to backplanes during the 1970's and
1980's. In general, they were fairly reliable. However, they were
not perfect. Occasionally, I would get a repeater that had an
intermittent connection. After manual percussive remediation (beating
on the box with a rubber mallet) the repeater would fix the
intermittent connection for a few months, when the problem would
re-appear. Sometimes, sliding the PCB's in and out would fix the
problem, but sometimes the intermittent would re-appear on another
pin. Contact cleaner or Cramolin did nothing useful. It was quite
maddening.

Motorola apparently also knew about the problem and later switched to
round stake pins in the PCB, with matching square receptacles. That
worked well and there were no intermittents. It also provided me with
an important clue as to the cause of the problem. Take you best guess
now, and continue reading.

I eventually decided to analyze the problem. When the next Motorola
repeater failed, was careful not to jar or move the box, an found the
intermittent pin and socket. I removed the PCB and carefully
inspected both under a microscope. Sorry, no photos.

The edges of the square pin had a burr along two adjacent edges. Like
the rest of the connector, the square pins and burrs were duly tin
plated, making this a manufacturing defect. Instead of making contact
on the allegedly flat side of the pin, contact was being made to the
sharp edges of the burrs. These sharp edges would initially make
contact, but did not have enough surface area to handle much current.
An amp or so would cause them to burn off the plating and expose the
underlying copper. The copper expanded when it started to arc, moving
the pin away from the side of the receptacle and breaking the
connection. It was difficult to see the problem, but the presence of
a burr, with microscopic dark black spots along the sharp edge of the
pin and along the flat part of the receptacle, were sufficient to
assign the blame.

In the case of your power tool, the problem might have been an
"insulation crimp" where the wire was crimped to the receptacle by the
insulation instead of the exposed copper. This sometimes happens when
the wire was not properly stripped and insulation is still on the end
of the wire.

BTW - I fixed the tool by removing the contacts from the shell and
soldering them to the pins on the board.


Yech. Whatever works.


--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558