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Got a technical problem? Can't understand a piece of jargon or some technical principle? Drop us a line and we'll answer your question. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097 or send an email to

Old Mosfet amplifier now obsolete

I’m proud to say that my Playmaster Mosfet Amplifier featured all those years ago in Electronics Australia magazine has just celebrated its 30th birthday. It hasn’t missed a beat in all that time and still sounds pretty good to me. I think the only thing I’ve had to do was flush out the pots with some contact cleaner on one or two occasions over the years.

I’ve just updated my speakers and have noticed an annoying audible “pop” when the amplifier is turned off. I reconnected my old speakers and can hear the “pop” from them as well, albeit at a slightly lower level.

The speaker protection circuit is functioning OK. At power-up, there is a delay before the relay energises and at turn-off it de-energises quickly as per design. The audible “pop” is instantaneous with the throw of the power switch.

The DC voltage at the speaker outputs is less than 3mV on each channel, so no apparent issue there. (D. W., via email).

• It’s almost impossible to nominate what might be causing the “pop” after all these years. You might try replacing the .01µF 2kV capacitor across the power switch, as it may have gone open circuit. And while it is pleasing that the amplifier is still going after 30 years, we should state that its performance is quite mediocre compared to SILICON CHIP amplifiers of the last 10 years or so.

Temperature controller for a toaster oven

Would you consider a project for a temperature controller to convert a toaster oven into a SMD reflow soldering oven? I envisage that it could be done with a microcontroller and a MAX6675 Thermocouple-to-Digital Converter chip and using a Triac to control the AC supply to the elements.

Commercial ovens can cost hundreds of dollars and this could be a good use to recycle an old oven. (T. B., via email).

• Good suggestion but it is not just a matter of maintaining a particular temperature. We had an article on this topic in the March 2008 issue, where Jim Rowe showed how to use a toaster oven for reflow soldering.

Ignition repair on a Ferguson tractor

I recently assisted a friend to repair the ignition system on a Ferguson tractor which has a positive earth electrical system. The ignition coil had failed and a new “modern” one was purchased which is designed for vehicles with negative earth.

Is there a difference in ignition coils designed for the different earthing systems? If you have to use a modern coil, is it best to retain the modern connections, that is the negative terminal goes to the points, even if this means the polarity at the spark plug is incorrect? (G. P., via email).

• The most important aspect of your question has probably already been answered: presumably, it worked.

Using a negative earth coil in a positive earth vehicle is not ideal, since autotransformer action in the ignition will subtract rather than add the primary voltage to the secondary voltage. Also, the spark polarity may be incorrect, leading to possibly reduced reliability of mixture ignition.

However, if the repair worked and you cannot obtain the original coil then these objections are more or less academic.

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