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Baffled by cordless tool problem

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

Reply to
Bob Engelhardt
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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

Reply to
mroberds

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.

Reply to
Bob Engelhardt

Intermittent battery connection.

Mikek

Reply to
amdx

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

Reply to
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

Reply to
Bob Engelhardt

On Mon, 28 Dec 2015 22:48:42 -0500, Bob Engelhardt wrote as underneath :

snip

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

Reply to
Charlie+

s

c.

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 inconclus ive. Make sure you don't miss the crystal/resonator. Thoroughly clean the board afterwards as some unseen residues can wreak havoc with processor ICs .

Reply to
John-Del

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

Bob

Reply to
Bob Engelhardt

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

Bob

Reply to
Bob Engelhardt

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

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

Good luck and Happy New Year.

Reply to
gyro_john

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

Reply to
Bob Engelhardt

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?

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

Thanks, Bob

Reply to
Bob Engelhardt

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

I think that it's a thermistor. It _looks_ exactly like this one:

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

Bob

Reply to
Bob Engelhardt

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.

Reply to
John-Del

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