Saturday, 28 July 2018

Heed Orbit Series 2 power supply repair.

Colin dropped in to bring me an LP or two, and dropped off a mysterious black box.

"It's a Heed power supply, you plug it into your LP12, and it allows you to switch speeds without swapping the belt over, I think it's been got at. Care to take a look?"

Yeah ... why not...

It takes mains in, and spews mains out ....

The little switch on the front alters the output frequency between 50Hz and 67.5 Hz, thus changing the speed of the LP12's motor.

Mains comes in and is supplied to the larger transformer, where it is rectified into +/- 32V supplies. There is a crystal divided oscillator (with 2 crystals, one for each frequency), and this oscillator, after some signal shaping, drives a push-pull amplifier, which drives the primary of the output transformer, which turns the waveform back into 230V AC, at the frequency of the oscillator.

This unit had no output, and everything was getting hot. The heatsink, both transformers. Something was very wrong.

It didn't take long to find the negative rail was being heavily loaded, and the BD242C output transistor was short circuit.

I replaced this, and the other half of the pair, a BD241C just to be safe...

Replacement of the transistors provided the cure. I left the unit on, running into a small load to soak test it.

Although the 67.5Hz is a shade high...

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