Electronics

Diagnosing and repairing a Trend Router tt/127

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About 2 years ago I came to use my router and when I pulled the switch, nothing happened. My initial thought was that maybe I had blown a fuse in the plug, but after checking the fuse and finding it fine, I gave up. 

Some have suggested that in this situation it is easy to give up, because you now have a reason to buy a new tool. In my case my always amazing wife bought me a bosch palm router for my birthday.  

The majority of my routing needs are easily covered by the smaller router, but sometimes I run it a little hot and realise that ideally I would have a larger machine available. 

Last week I was watching a youtube video about using a router to thickness pieces of wood, and I was struck by a new desire to get my router working again. I'm not really sure why I didn't do it when it first broke, but this time I decided to break out the multi-meter and figure out exactly where the fault was.

I decided to film my efforts, so if you want to watch me bumble about making some wrong assumptions, then see the look of dawning realisation when I figure out how the switch is supposed to work... you can find that video here:

 

For those that prefer to read. 

I had found from a quick search on line that hte switch is often the first thing to go in a lot of this kind of tool. So that seemed the obvious place to start. Afterall, I know the plug fuse is fine, so power must be getting as far as one side of the switch. 

Two screws held a cover over the switch, and another 2 screws held the trigger/switch assembly into the casing.

I remove it and ran some quick tests with a multi-meter over the terminals. My first tests were based on an assumption that the power came in from the plug at one side, and went out to the motor at the other. I checked for resistence between the opposite sides with teh switch open and closed and found it all to be inifite resisitence. No connection. Looks like the switch is broken. Turns out it was, however I later discovered my initial tests were a false positive. Or rather a false negative.

The plastic casing of the switch is in two halfs clipped together with a couple of points on each side. A Quick pry with a flat head screwdriver popped it open. Inside you find that the trigger causes a piece to slide up and down inside. There is  small stiff spring which rests on a little nub on the trigger slide slide.  

The other half of the switch has a little 'raft' which also had a nub point on it for the spring, when lifted out of the switch the raft carries two little plates with conductive pads which clearly make contact to a parallel set of pads inside the switch. At this point I realised that the switch doesn't bridge one end to the other, it bridges one side to the other. The little plates bridge the connection between a terminal and the one next to it. Once I had realised this, I saw that I needed to check how the resistence changed between adjacent terminals when the switch operated, rather than oppoiste terminals. 

I replaced the raft and tested the resistence as I slid the raft back and forth. This showed that the connection is made and resistence drops to zero, but only with a reasonable amount of force pushing the pads together. 

This suggested that the problem is just that the trigger slide is not really moving the raft hard enough up against it's contacts. Which brought me back to the little spring and the idea that maybe it hassimply become unseated from its little nubs and was no longer pushing the raft when you operated the trigger.

The tricky part was getting the switch housing back together, with the stiff little spring in place. I let hte raft sit in it's 'off' position, and brought the two halfs together slightly askew, then used a small g-clamp to help me press the two pieces toegther against the resistence of the spring until the little clips reseated themselves. 

Operating the trigger at this point there was a distince and different sound compared to before I started. This type of switch is a 'rocker' switch, and you could really hear and feel at the raft was clicked back and forth with the trigger.  

I attached the trigger back into the housing, and did a final quick test with the multi-meter. And lo, pulling the trigger now reduces the resistence between the terminals to zero. 

Quick reattach the outer casing, plug int it... and the soft-start motor kicks ing and runs up to speed for the first time in 2 years. Hurrah! 

Conclusion

Having figured out what was wrong with the switch, I'm not almost certain I understand why it failed. I had been using the router in a home made router table.  In that configuration I needed to have a strap around the trigger switch to hold it on, so that the power would be controled from the main plug. It was left like this for quite a while, and through changing temperatures. I guess that under these conditions the spring simply pinged loose of the small nubs and stopped driving the mechanism.

I should have put this on my workbench at the time, and done a proper diagnosis before leaving it to gather dust. However better late than never.