Top speed or nothing from speed control box!

  Diemmess 16:30 31 Jul 12

My son has a small control box, Chinese made, using mains to low voltage, supplying DC for a hand-held drill. Like a Dremel but more robust.

The machine has suddenly ceased to run at any speed other than top speed either when switched to foot control or speed control knob on the box.

Since the controls are switched and each consists of only a variable resistor I assume the fault lies inside the box which has a separate transformer, but everything else on a reasonable circuit board.

No circuit diagram, I'm out of touch with theory and practice, so little use me having ago to draw my own diagram.

One piece of luck which could solve the problem --- while looking the thing over, (electrolytics look OK) I found that one transistor in the(control circuit perhaps) has literally split a corner and most of one side is missing.

As far as I can make out this transistor TO92 package)has a label saying MCR 100-6 P89

This is as close as I could come to a description Data Sheetbut most of that data sheet is gobbledygook to me.

What I hope someone will do is to look at the specs and name an equivalent or better still say where a suitable replacement is in stock.

I tried Maplin but hardly surprising, their stock of yesterday's technology is not comprehensive!

  Diemmess 09:18 02 Aug 12

End game

I found a data sheet for the replacement, it is an SCR. Very clued friend found several on eBay and I paid $2.89 USD including postage for 10 pieces! Other sites were offering @ up to £10 single plus postage.

Seller today says item despatched today. Doesn't say whether by air or container ship from - China

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