Disc sander - speed control mystery

I have had a Lidl disc sander for some years. I regularly use it and its been great. A few weeks back it suddenly went slow and adjusting the variable speed pot made no difference at all. I dissembled it and could see nothing broken or disconnected. I assumed the speed control unit had failed. In fact I removed the mains cable completely to keep as a spare. I left the unit in bits

Today I thought I would give it another go and reassembled it all and refitted the cable. When I tried it I got the same result as before. In fact the motor would only start to turn if you gave it a little flick to get it going. Adjusting the vari-speed pot didn't alter the speed, just a slight variation in a very faint buzzing sound.

I turned it off at the mains and then (just as a last resort) turned it on again and tried it. It worked perfectly. Motor working normally and speed control functioning just as it should. My question is what happened?

Picture of the circuit board below for interest.

I'm not an electrician or familiar with that tool but my first thought looking at your photo would be to check for dust within any movable component.

Do you have compressed air available or dust blower aerosols?

Is it a DC or AC motor? Any dirty brushes etc.

If you have had it a number of years and used it a lot it is highly likely the brushes are worn especially as it burst into life although that will only be temp.upshot change the brushes

My first thought was worn/dirty brushes and second thought is a loose connection.

Third thought would be a worn/dirty control pot. Twist it back and forth a few times to clean the contact.

Could the "buzzing" sound be arcing ?

Yes the buzzing sound is almost sure to be arcing from the brushes to the comutator

Check for dry solder joints under the circuit board too.

Didn't sound like arcing chaps, more a quiet hum than a buzz.

I did check the underside of the board but couldn't spot any dry joints. However, that doesn't mean there isn't one of course!

It is definitely an intermittent fault as after using it for 15 mins (on / off use) today it reverted to slow speed only again. So next step is to take it all to pieces again. Will try to get into the motor to examine the brushes. I will examine the pcb again; test the pot; maybe a drop of switch cleaner and test the mains switch.

Here is a (very poor) video that shows the "problem". I hope!

Disc sander problem

Having seen that then thought number 3.

Check the resistance across the pot while turning the knob. If it's a standard variable resister then centre connection to either end connection should prove it.

Remember to unplug from the mains first.

Then thought number 2 would be a loose wire or dry joint. Moving and twisting individual wire should show that.

Thanks John,

I start disassembly and testing tomorrow. I will report back if I find the cause of the problem.

Did some testing today and looks like the 500k pot is going faulty, so I will be ordering a replacement. Whilst at it I will fit a new mains switch as the original has seen quite a lot of use too.

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Yes, actually I nearly always run it at less than max RPM when using with balsa (which is what I use it for most). I have ordered a new pot so should be back in action soon.

Ordered the parts to repair the disc sander yesterday. What happens today? I was shopping in my local LIDL when I spotted a single brand new "sealed in its box" Parkside PTSG disc sander in the reduced price "end of line" bin. It was the latest 2020 version, and not an "old" model. So I now have a new one and will get the old one fixed as a back up when the parts arrive.

Oddly I had emailed LIDL to ask when they would be stocking these again. I got a reply today saying they had no idea and couldn't tell me when any would next be available in their stores.

Strange how life works out sometimes!

Edited By David Ovenden on 26/11/2020 21:20:40

David, you're not alone. I've seen Lidl own brand stuff in the reduced bin that hasn't been in any of their recent leaflets. Some months ago I bought one of their scroll saws for £59.99 by that method. Still not got around to using it yet though, but I will!
Some of the stuff in the reduced bin I've never seen advertised at all, despite it being from one of their own brands. Where does it come from?

Not just me then Robin!

I guess stores send unsold stock (after the end of the offer week period) back to the central warehouse and then it gets redistributed to other stores. Or maybe, it goes out the back into the storeroom until the local store manager clears it out via the bargain bin.

Problem solved. The new pot arrived and I have now soldered it in place. Everything now working again as it should. Thanks to all who contributed. Hope you all have a good Christmas.

Hi David , I have a Chinese micro mill that has a similar problem, it runs ok flat out but in the middle range it fluctuates. Not too much but not good for milling, these days it gets used more as a pillar drill as I have gone from making brass loco's to aeroplanes. One day I will strip it down and check the pot, funny the only part that has given trouble over the last ten years is the circuit board and this was a selling point in the ad. Circuit board MADE IN AMERICA. I wonder what Mr. Yeager would have said about that..

Merry Christmas to you too and El Condor Pasa soon I hope.

Eric,

The Condor wing is now started, so a flypast is just a little bit closer. But not as close to flying as your Sea Fury!

Well done David, I'm glad you had a successful outcome and thanks for reporting back with the solution. I hope the sander now gives you many more years of service.