Huh? Isn’t this a hobby blog? What’s with this electronics stuff? Well over the last 18 months I’ve become quite reliant on my Sherline model 1000 lathe (and its attachments) for a large portion of my modelling work. It has an electronic speed controller built onto the motor and I had noticed that the maximum speed had been gradually getting slower. Not surprising since the machine is about 10 years older than I am!

Well a couple of nights ago the speed controller gave up and would only give two speeds: slow-crawl and insane-fast. So this morning I pulled it apart to have a look at the problem.

The setup is a basic DC motor running on half wave rectified 240VAC, with an SCR chopping the wave to control the speed. You can see a schematic I reverse engineered from the PCB below. It’s using pretty old-school components, including some carbon composition resistors. I at first suspected one of the semiconductors but some basic testing with a multimeter proved that the diodes and SCR were most likely fine. So I checked out the speed control pot, thinking maybe the wiper had gotten dirty. But that too seemed fine. Capacitors checked out OK so the last things left were the resistors.

I can’t recall ever having a faulty resistor, but I figured since these ones were so old I’d best stick the ohmmeter across them. Lo and behold, the 82k resistor in series with the SCR anode had crept up to approaching 1 meg! I pulled it off the PCB and checked again. The expensive Fluke meter never lies! I had no idea what the power rating of the failed resistor was, but I found a similar sized part with the same value in my box of odd resistors and stuck that in. I’ll check it again in a few weeks and make sure there’s no signs of overheating that would present is the power rating of the replacement resistor was too low.

So that’s the story! I decided to post it here in case anybody else has the same problem and is Googling it

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