Baffled by cordless tool problem

[Bob Engelhardt]

I have a Milwaukee 12v cordless jig saw. It was working fine then
stopped, dead. Then started again, with no symptoms. It did this back
& forth a few times, then stayed dead. It never stopped while being
used - it just wouldn't start at all.

I took it apart to diagnose & as soon as I put it on the battery, it was
OK! I reassembled it and periodically tested it over a week. Always
OK. Today I put the base/shoe back on & it was dead!

As I took it apart again, I kept checking it, but it didn't come back.
And then, sitting there, it did come back, spontaneously.

It has a small PCB for speed control. The PCB also controls an LED
"work light", which comes on when the battery is connected. Except when
it's dead - making the LED a good go, no-go indicator. And showing that
the problem is more basic than speed-control circuitry.

The PCB has a number of wires soldered in through holes. I have checked
all of them. I have stressed the board with mild twisting; I've cleaned
it with carbon tet; and have warmed it with a hair dryer. Nothing had
any effect.

The board is SMT (with a 48 pin chip!), except for the wires, 1 cap, and
the output FETs, which are TH.

Is there anything else that I can try?

Thanks,
Bob

[mrob...@att.net]

Bob Engelhardt <BobEng...@comcast.net> wrote:
> I have a Milwaukee 12v cordless jig saw. It was working fine then
> stopped, dead. Then started again, with no symptoms. It did this
> back & forth a few times, then stayed dead. It never stopped while
> being used - it just wouldn't start at all.

One bad winding on the motor, a bad spot on the commutator, really
worn brushes? (Assuming it's not brushless.)

To check: Get it to the dead state again. With your fingers off of
the trigger, use a stick of wood or something that's not your fingers to
turn the blade a few degrees. Then try the trigger again. If you can
reliably make it start that way, look at the brushes and commutator.

Matt Roberds

[Bob Engelhardt]

On 12/28/2015 7:13 PM, mrob...@att.net wrote:
> One bad winding on the motor, a bad spot on the commutator, really
> worn brushes? (Assuming it's not brushless.)

> ...

When it's dead, even the LED does not come on. Which it normally does
even without the trigger being pulled. So it can't be commutator, etc.

[amdx]

Intermittent battery connection.

Mikek

[Bob Engelhardt]

I want to say that I've checked that - that there is voltage on the
connector terminals when it was dead. But I've gone around in so many
circles that I can't remember if I was even able to do that. I.e., if
it was ever dead after I opened it.

I have wiggled and stressed the battery connector without consequence.

When it dies again, that will the first thing I check.

Thanks,
Bob

[Bob Engelhardt]

Update: I put it together & it died. It took it apart & it stayed dead.
There is voltage on the connector and on the board. No poking,
prodding, twisting brings it back.

That pretty much clinches it for me: the board is bad. It is curious
how intermittent it was without reacting to anything I did.

The saw is under warranty - if I can find the receipt for it (unlikely)!

Bob

[John-Del]

If you can't find the receipt, reflow the board (including the chip). I've seen a lot of reflow repairs where flexing or thermal cycling is inconclusive. Make sure you don't miss the crystal/resonator. Thoroughly clean the board afterwards as some unseen residues can wreak havoc with processor ICs.

[Bob Engelhardt]

On 12/29/2015 2:52 AM, Charlie+ wrote:
> On Mon, 28 Dec 2015 22:48:42 -0500, Bob Engelhardt
> <BobEng...@comcast.net> wrote as underneath :
>
> snip

>>
>> The saw is under warranty - if I can find the receipt for it (unlikely)!
>>

> You might have baught it with a credit card, the account would give you
> proof of purchase.. C+
>

Oh ... good idea! I almost certainly did use a card. Thanks

Bob

[Bob Engelhardt]

On 12/29/2015 7:46 AM, John-Del wrote:

> If you can't find the receipt, reflow the board (including the chip). I've seen a lot of reflow repairs where flexing or thermal cycling is inconclusive. Make sure you don't miss the crystal/resonator. Thoroughly clean the board afterwards as some unseen residues can wreak havoc with processor ICs.
>

Thanks. I'll definitely try that if I can't get it fixed under warranty.

Bob

[gyro_john]

Might this be a battery issue? You see, a lot of these modern battery packs have a Battery Management System circuit board inside the battery pack, and its job is to properly charge the batteries and shut off the battery when it's drained. If the battery pack and the saw have to communicate with each other, maybe the BMS says it's time to shut off the battery, yet the battery is not yet dead.

You might be able to check by swapping in a different battery pack.

Good luck and Happy New Year.

[Bob Engelhardt]

On 12/29/2015 3:19 PM, Bob Engelhardt wrote:
> On 12/29/2015 7:46 AM, John-Del wrote:
>
>> If you can't find the receipt, reflow the board (including the chip).

>> ...

>
> Thanks. I'll definitely try that if I can't get it fixed under warranty.
>
> Bob

Long story short: warranty does not apply & I'm going to try to reflow
the board.

On cleaning the board I found some stuff that looked like the rubbery
caulk/adhesive that is used to stabilize components. On removing it I
found a TH device that has me puzzled. I'm worried that it might not
handle the reflow heat.

The pictures show it next to a brass (?!) heatsink that the power FETs
sit on and a close-up of it. The rubbery stuff covered it and bonded(?)
it to the heatsink. Thermal stuff?

https://www.dropbox.com/s/nf2odbubz0pgd78/DeviceOnBoard.jpg?dl=0
https://www.dropbox.com/s/axu84cbjlpqgh1q/Device.jpg?dl=0

What is it? Do I need to remove it for reflow?

Thanks,
Bob

[Bob Engelhardt]

On 1/1/2016 12:17 PM, gyro_john wrote:
> Might this be a battery issue? ...

> You might be able to check by swapping in a different battery pack.
>

I have tried different batteries - it is not a battery problem. Thanks
anyhow.

[Bob Engelhardt]

On 1/2/2016 12:44 PM, Bob Engelhardt wrote:
...

> https://www.dropbox.com/s/nf2odbubz0pgd78/DeviceOnBoard.jpg?dl=0
> https://www.dropbox.com/s/axu84cbjlpqgh1q/Device.jpg?dl=0
>
> What is it? Do I need to remove it for reflow?

I think that it's a thermistor. It _looks_ exactly like this one:
http://www.dxmht.com/products/thermal-resistor.htm

Since it's soldered in, it should be OK for reflow.

Bob

[John-Del]

Huh.. learn something new every day. Never saw a thermal resistor in that package, but it sure looks like the one in the link.

It seems the board has a conformal coating on it. When resoldering a board like this, it's much easier to accomplish with the coating off.