Sunday 18 October 2015

Powermaster Rectifier Replacement

After last week's RSGB Convention I called in at my Father-in-Law's, to be rewarded by a brace of vintage train controllers...


These devices, made by Hammant and Morgan Ltd, formerly of Watford, are the boat-anchors of the UK train controller world. They are old and heavy and made-to-last, unlike modern equivalents (in at least the first two attributes).

Fortunately I have a quiet obsession for collecting and operating old, heavy toy trains which have demonstrated their ability to last for over half a century...


so these controllers were ideal for my applications.

Of the two newcomers, the Duette offers two "channels" of control, whereas the Powermaster offers only one controllable output (plus some sector switching gear). It might appear that the Duette wins hands-down. However, the Powermaster is an entirely superior device, offering a genuine variable output voltage (rather than a single output voltage with a variable source impedance). This is achieved by a transformer with exposed secondary, over which a wiper can pass to tap off a desired voltage - rather like the way an antenna tuner with a "rollercoaster" inductor works. The result is something like a Variac (but not, in this case, an autotransformer).

There's a (flawed, but useful) description here, along with some interior photos of a newer generation of Powermaster, showing the variable transformer. Having linked to that site, it is important to repeat - the Powermaster is NOT an autotransformer. There is isolation between the mains voltages on the primary side and the SEPARATE secondary.

On test, the Duette still worked as intended. However, the Powermaster worked on its "reverse" output range, but not on its "forward" voltages, producing an output voltage reading on a meter, which quickly collapsed to zero under load (a 50 Ohm dummy load). Obviously it was not doing its job of working as a voltage source having low source impedance.

In light of the asymmetry described above, I suspected a fault in the old Selenium rectifier or the polarity change-over switching and associated wiring. Unfortunately, I could find no detailed descriptions of the Powermaster on the internet, so these notes describing my repair may be of interest to others caught in the same boat...

I removed the rectifier and figured out what the connections did (by visual inspection)...


To check my diagnosis, I lashed-up a temporary replacement outside the case - all the wires were sufficiently long to pass through the many holes conveniently available in the back of the case...



Having confirmed that this was all working fine, I made a (slightly) more permanent solution on a scrap of copper-clad board, with some pads formed by whittling away Cu in judicious places and a sprinkling of diodes (I had 1 Amp 1N4003s to hand and have used those for the moment)...


The holes in the PCB are arranged such that I can just screw this new home-brewed rectifier in place of the old selenium unit.

The wiring is self-evident - the only possible confusion being in connecting the Half/Full-wave rectification switch identified here on the front panel by a dashed red circle...


The switch is connected as follows...


The result? A perfectly-operating Powermaster, giving my 50-year-plus old toy trains slow speed performance which they never had before - all from a "period" controller.

Anybody seeking to make such a repair in their own vintage controller should attempt it only if they are appropriately qualified and then only at their own risk. I don't need your burnt-out armatures, house fires or electrocution on my conscience.

These controllers have another nostalgic significance for me - back in my childhood, they were the most convenient power supply to hand. A perfect place to start with some electronics experiments. Guess where the young m0xpd used to get his LT from!

In fact - I'm pretty sure the Duette IS one of my childhood controllers, inherited from my cousins, passed on to Father-in-Law - and now passed back with interest in the form of the Powermaster.

Now - time to play with some of that new slow-speed performance. Thanks Malcolm!

...-.- de m0xpd

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