Last Modified: 21 Nov 2018


Condensed from a forum topic.


Wed Dec 24, 2014 2:33 am


ust finished making this work:





It's a quite an old bench power supply and made by Marconi apparently. It was given to me by the proprietor of Danbury Electronics, before he retired, and it was given to him in turn with 3 others with a number of transformer winding machines, yonks ago. Since then it had been kicking about under his bench.

It had a number of problems. Firstly the mains lead was cut off. The Voltage selector (noval valve socket style) is underneath and had been broken, then Araldited back together, 'cause it's got a bluddy heavy transformer in it and is always sitting on the selector. It has no rubber feet but instead sits on the edges of the side panels of the top cover which extend below the base plate. Bizarre...

I put a new lead on and removed and bypassed the selector.

Turning it on gave a 60V output, and no adjustment.

Turns out someone had been trying to repair it, and had removed two transistors. Fortunately he had also written the type numbers on the board in pencil! All transistors are germanium, except one power one is apparently, silicon, 'least according to t'Internet.

I found I just happened to have two TO5 transistors to replace the missing ones, also of higher spec, and in silicon.

Just as I'd soldered them in I was poking the leads of the others, to makes sure there no breaks or even corrosion, just as well as one of them had one lead not quite through the board, so hardly attached to the solder on the track side, indeed it just came out with a tug, did this cause the originsl fault?

The Voltmeter wasn't working, took it apart and found a burnt out resistor inside. Replace this with 1% values on the basis meter is calibrated for 10V at 10mA. I.e. around 1 kilohm.

There's a big toggle switch used to turn the output on and off, continuity tests of the contacts were not convincing, fortunately I had virtually identical style in my spares so took it out and wired in a new one.


Anyway it am do now work!!


Just needed a bit more tweaking to the Voltmeter series resistance to calibrate; there is a series dropper chain for 50V scale that includes a preset to calibrate that. There's another preset on the board that sets the max. output (50V), and then it seems to be good to go.


Wed Dec 24, 2014 2:47 am

Circuit board...









Reservoir caps... with bridge rectifier bottom left...





There appears to be a second secondary on the mains trans for powering the regulator circuit, maybe this blue cap is the reservoir for it...



Smaller thing in front is not a small transformer, but a choke, believe it or not.



Wed Dec 24, 2014 2:52 am

Forgot to mention it's also got current limiting, and a setting mode which involves holding a momentary button then the current you want is displayed on the Ammeter, while you twiddle the adjuster knob. Really neat.



Wed Dec 24, 2014 1:55 pm

Oo-arr.


The mechanical construction is deceptively too plain and simple looking, almost penny-pinching, but is actually quite ingenious. Just remove 8 screws and both front and rear panels come loose to fold flat, ditto the circuit board, and the routing of the wiring looms also allow for this, so everything is accessible.

This morning I glued on 4 door stops for feet, waiting for them to set.

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