DC motor question

You can probably use a bog standard motor speed controller (of the same type used for drills, etc) followed by a full wave rectifier. Maplin probably do a suitable motor controller module (and rectifier). If it's a universal motor (i.e. field windings rather than permanent magnet), then it will run from AC too and you won't need the full wave rectifier.

Would a normal power drill be a suitable starting point, since lots of them already have speed control in them. Not sure on long term life but something like one of the green Bosch drill series, or if your really flush go for the blue industrial.

Joules

I once bought a lathe with a DC motor speed controller which had died. The original (''50s?) arrangement had DC generated by an AC motor driving an AC generator. Speed control was by varying the DC voltage. Howvever this arrangement had expired.

I found an electronic DC motor speed controller on Farnell/ CPC. They're used on conveyor belts, apparently. I'm not sure about the voltage, I've a vague idea it was 110V DC max, but can't recall the details. Try there.

And how about repairing the controller? Shouldn't be to complicated and

-even with pure guessing- the chances to get it working again are high.

I would replace everything that is big, has a cooling fin and three legs. :-)

Nick

Take a look at the site of littlemachineshop.com. IIRC they sell replacements for these speed controllers for these lathes and the mini-mills, but I think they also have some info on circuit diagrams as well. Look under the resources button on their site. They also sell the mosfets (get the part number from there) and the troubleshooting info may track the fault back to other than the parts you suspect.

Do a google search for "Mini-mill speed controller repair" or "mini-lathe circuit diagram". I know that I have seen the diagrams for these boards floating about the web.

Cheers Trevor Jones

Excellent link, Trevor, looks like they have the beast for $120 and there is someone in the States who repairs them for £25 plus bits.

I might try repairing, but apart from a pair of power transistors there are a couple of ICs. One reason I am slightly cautious about poking round with a DVM or scope is that the board is fairly tight and with 240 V on it rather than the more normal 12 or 15 it could be easy to fry something by accident. Will also try the other links, thanks all for the suggestions.

This would work if it's a shunt motor

In message , Newshound writes

I would ignore the ICs and focus on the power transistors. They don't cost that much (I guess) and even if that blind shot fails, you didn't waste too much money.

Yes, absolutely right. Unsolder the transistors, check them outside. Did you remove power while unsoldering? ;-)

Nick

LOL!

I was thinking about looking with a scope to check that the oscillator and pulse width modulation circuitry was working. One trouble is that the FETs are labelled K790 and Maplins / RS don't recognise them. Although I guess the type isn't critical as long as they'll take 240 V.

I hope that you only do that, when you know what you are doing.

Just googled for K790 and found this:

HTH, Nick

Have you tried chester, for a new PCB - when i was at there open days - they seemed to have motors and PCBs for sale for the little lathes and mills.

like the guy ^^^ if u want to play have a look at the 'FETS' i seem to remember a post on here saying that if you upgrade them to 500v each, its much better than the standard 240v fitted ones

Add "2S" to the front, to give 2SK790, and google again.

eg:

As a substitute, try 2SK2698

Well, I was working at CERN designing digital circuits for the Soviet particle accelerator at Serpukov in 1970.....

Thanks, will give that a go if my first strategy (cheap Maplin AC controller through a bridge) doesn't work. (It's a permanent magnet motor).

That's the DIY spirit!

The only difference between frying something at 15volts and 240volts is the sparks!

Steve