Touch Control Lamp

My neighbour brought me this lamp for repair. It's a touch-control lamp. You touch the base once and the lamp lights dimly. Touch again and it's brighter. Touch a third time and it comes up to full brightness.

The label states "max 60 Watt" but he had fitted a 75 Watt bulb. Now the lamp wouldn't turn off.

A plate underneath was held by four screws - easily removed. Inside was a plastic box. I unclipped its lid to see inside.

The BT136 TRIAC was likely to be the culprit so I desoldered it and tested it.

As you can see from the reading, below, it is measuring 1.54 Ohms - a short-circuit, which explains why the lamp is permanently lit.

Here's a good triac under test, for comparison. The tester recognises it as a triac.

Replacing the triac fixed the fault. Some circuits use a diac with the triac. Had this been the case, I would have replaced that as well, before applying power.

Note: I bought this MK-168 tester via eBay. It is one of the most useful tools in my workshop, alongside the ESR tester.

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