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Serviceman's Log

It's not often these days that electronics gear is faulty out of the box but it does happen. Usually, we return the item for an exchange or refund but what happens if you've left it too long and lost the receipt into the bargain?

by the Serviceman

Items Covered This Month

• Faulty digital mains timer

• A shocking winch

• Faulty marine transceiver

• Timing the doors on a lift

• Sorting out a PA system

In this business, I get volunteered for all sorts of things – especially by Mrs Serviceman

Recently, at her request, I bought two digital mains timer switches for an elderly relative. Unfortunately, he left them unopened for several months and when he finally got around to trying them out, it soon became apparent that one of them was faulty. By then, the receipt had been well and truly lost and so Mrs Serviceman thought it would be a good idea for me to fix it.

After all, I was the one who had bought the timers, so it was my fault in the first place. That’s logical.

Click for larger image

While the digital timer portion of the device seemed to work fine, the actual mains switching did not. The working unit’s relay would click each time the output was switched on or off via the manual control pushbutton. By contrast, the faulty device would click twice when it was switched on but there was no output at any time. In addition, there was no sound from the relay when it was switched off.

Luckily, this wasn’t one of those plastic cases that is glued shut and has to be mangled to be opened. After removing a few screws from the back, it snapped open to reveal two PC boards. One of these was attached to the inside front face and carried the LCD, pushbutton switches and control electronics. The other board was attached to the rear of the case and carried the power supply circuitry and the mains relay.

I traced the layout of the main board and discovered that the power supply consisted of a 330nF X2 mains capacitor in series with the Active wire, followed by four 1N4004 diodes in a bridge rectifier configuration, then a 100µF 50V capacitor in parallel with a 36V zener diode to Neutral. A low-value 1W resistor is in series with the Neutral side of the circuit to limit the inrush current, while the X2 capacitor has a parallel high-value resistor to discharge it when the power is switched off.

In addition to the power supply and relay, the only remaining circuitry was an NPN transistor to drive the relay and a PC-mount trickle-charged 1.2V NiMH cell for powering the control board. The boards were connected via a 3-pin header and cable – two wires for powering the control board and one for driving the transistor that powers the relay.

Another interesting discovery was that the relay was a 48V type, while the supply seemed to be regulated to 36V. Most relays are guaranteed to switch at approximately 75% of their rated voltage, which is why this circuit still works but I think I would have designed it a little more conservatively to allow more room for component tolerances.

Not wishing to risk applying mains power with the case open and not having a convenient source of 36VDC, I turned my bench supply up to its maximum 30V output and connected it across the zener diode. This proved to be enough to switch the relay but only when it was orientated so that gravity would assist its switching.

Fortunately, that was enough to allow me to determine that with this new power source, the unit worked correctly. Pressing the manual on/off button toggled the relay normally. So it seemed that the fault was in the power supply section.

Because they have a reputation for unreliability I decided to first replace the 100µF electrolytic capacitor. My reasoning was that it may be low in value and not providing sufficient current to keep the relay energised. However, that didn’t help and the capacitor that had been removed tested OK.

The only other component that could explain this fault in the power supply was the 330nF X2 capacitor. However, these are designed to be reliable enough to be used across mains conductors, so surely that couldn’t be it?

Despite my doubts, I removed the X2 capacitor and checked its value. It read 120nF which is just over a third of what it should have been! So now it was clear why the device was behaving as it was. While the supply was able to charge up to 36V initially, the voltage dropped significantly when the relay was turned on because of the higher impedance of the X2 capacitor.

In fact, it was dropping so far that the relay could not be kept closed. This explained the double click at turn on and explained why the output did not switch on properly. I replaced the X2 capacitor with a new one and it worked perfectly.

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