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Old 19th Oct 2016, 11:13 pm   #1
Phil G4SPZ
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Default Ekco AC18 HT Unit battery eliminator (1931)

A friend gave me this rusty box, saying: "This belongs in a museum!" I had no need of such a unit in my workshop, so I donated it to the one where I work as a volunteer and have just finished repairing it.

The AC18 is one of a range of units offered by Ekco and described in their publication no. H.T.270 dated August, 1931. The AC18 is an HT battery eliminator only and has no LT output. The specification demonstrates the lack of regulation - 150 volts at 13mA falling to 120 volts at 18mA - and it has three lower outputs at reduced current plus two screen grid outputs at 1.5mA. All these currents are cumulative and the maximum combined output seems to be 18mA, hence the model name. The smaller AC12 is similarly rated at 12mA.

After removing the case I first deduced the circuit by inspection, and later found that there is lots of information on these and similar units on this Forum and elsewhere. Apart from the rusty and dusty condition, the tinplate capacitor box had literally burst open at the soldered seams. Luckily no damage had been done to the smoothing choke or mains transformer. I measured the resistances of the finned selenium rectifier at 4.5k forward-biased and 150k reverse-biased. Not being very familiar with these things, I asked around but no-one could offer any suggestions, other than "try it and see".

The bulk of the work was re-stuffing the can. I unsoldered the connecting leads and drilled out the rivets holding the can to the chassis. Using a 250 watt iron, I unsoldered the top of the can, and the contents lifted out with surprising ease - two 6uF and one 1uF wax-impregnated paper capacitors in a single block, topped with pitch. I cut the wires, unsoldered the stubs and cleaned out the terminals. A Megger test showed that the insulators were in good order. I straightened up the bent tinplate can and re-soldered it back together, then re-stuffed the can using two 8uF 350 volt electrolytics plus a 1uF 600V polypropylene (the nearest values I had to hand), adding stiff card folded inside the box to prevent the insulated leads touching the metal. I then soldered the lid back onto the box in a couple of places to hold it on.

I used 4BA bolts, nuts and shakeproof washers to hold the can to the chassis, with a solder tag on one for a mains earth connection, then resoldered the connecting leads to the can. I replaced the remnants of the crumbling two-core mains flex with some vintage-looking three-core. Creating a secure cord grip was a bit awkward, but two small cable ties around the new flex were entirely successful.

On test, I used a 10k load resistor for the HT, anticipating 10-12mA of HT current, plus 33k to simulate a screen grid load. The unit worked perfectly, so my initial misgivings about the rectifier were unfounded. The attached circuit has been annotated with the actual resistances and voltages that I measured on my unit, on-load and off-load, with the mains tapping set to 225-250 volts.

I've not yet attempted any restoration of the steel case, other than a light clean. I've seen units in much better condition, and they're finished in a dark bronze effect. If anyone knows how this was done, or how to restore its original appearance, I'd be grateful for any information. The pictures below show the unit in as-found condition.

Phil
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Old 19th Oct 2016, 11:16 pm   #2
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Default Re: Ekco AC18 HT Unit battery eliminator (1931)

And here are a couple of images of the finished job.

The circuit shows the voltages measured in operation. The overall series resistance of the transformer secondary winding, rectifier and choke is just over 6,000 ohms. Taking this into account, the off-load voltages measured with my Avo 7 on the 400 volts DC range are virtually spot-on!

Phil
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Last edited by Phil G4SPZ; 19th Oct 2016 at 11:23 pm. Reason: Schematic added
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Old 20th Oct 2016, 11:38 am   #3
HamishBoxer
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Default Re: Ekco AC18 HT Unit battery eliminator (1931)

Built to last!A another excellent job and write up.I have one of these and time I checked it out.
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Old 20th Oct 2016, 10:29 pm   #4
Phil G4SPZ
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Default Re: Ekco AC18 HT Unit battery eliminator (1931)

Thanks, Hamish! I was surprised to discover how old it was.

A further thought has since occurred to me. The high internal impedance of this supply means that some 200 volts DC appears at the outputs on no load. Many battery-operated sets of the late 1920s and early 1930s had on-off switching that simply disconnected the LT supply, leaving the HT battery permanently connected. Thus if the Ekco HT unit were left switched 'on' when such a radio set was switched 'off', 200 volts would appear across any decoupling capacitors, normally designed to withstand no more than 120-150 volts at most...

In fact, this issue is referred to rather indirectly in the operating instructions on page 3: "Switch on receiving set as usual [then] connect [HT] unit to an electric supply socket and turn on the current... Note. It is better to switch on the valves of Receiver before switching on the current to Unit, and switch off the current to Unit before switching off the Receiver." I bet many didn't bother. It's amusing to speculate how many people bought an HT battery eliminator in order to avoid the high cost and short life of HT batteries, possibly due to failing and leaky decoupling capacitors in their radio set. How many subsequently complained to their dealers that, since purchasing an Ekco HT unit, their radio suddenly went BANG and stopped working?

My other thought is, why would anyone in their right mind buy a HT-only battery eliminator and still be faced with weekly accumulator recharging? Ekco also made HT+LT eliminators, as did Amplion and others.

Phil
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Old 21st Oct 2016, 8:07 am   #5
Kevin Hoyland
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Default Re: Ekco AC18 HT Unit battery eliminator (1931)

Hello Phil.
As always an Outstanding Thred by you.
Keep the good work up.
Regards
Kevin.
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Old 21st Oct 2016, 8:23 am   #6
Phil G4SPZ
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Default Re: Ekco AC18 HT Unit battery eliminator (1931)

Very kind of you, Kevin, many thanks!
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Last edited by Phil G4SPZ; 21st Oct 2016 at 8:24 am. Reason: Typo
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Old 22nd Oct 2016, 10:11 pm   #7
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Default Re: Ekco AC18 HT Unit battery eliminator (1931)

An excellent and informative write-up Phil. Thanks again for sharing your knowledge.
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Old 23rd Oct 2016, 10:35 am   #8
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Default Re: Ekco AC18 HT Unit battery eliminator (1931)

Thanks, Gary! Hope you and the family are well.
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Old 23rd Oct 2016, 11:04 am   #9
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Default Re: Ekco AC18 HT Unit battery eliminator (1931)

Here's a battery eliminator case which I had sandblasted and powder coated.

The finish in this case is matt black, but other colours are available.
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Old 23rd Oct 2016, 12:49 pm   #10
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Default Re: Ekco AC18 HT Unit battery eliminator (1931)

That looks wonderful, puts mine to shame... okay, you've encouraged me to finish the case off properly. Rustoleum do a 'dark oil-rubbed bronze' and I've used their paints before to good effect. Other brands are available.
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Old 6th Nov 2016, 12:05 pm   #11
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Default Re: Ekco AC18 HT Unit battery eliminator (1931)

Quote:
Originally Posted by Phil G4SPZ View Post
Thanks, Gary! Hope you and the family are well.
We are all great thank you Phil.

I mentioned your battery eliminator to my Dad, he remembers using them when he was working (1950s). I think we mentioned it in the past, but Dad worked at SBC Holdsworth who were in Bridge Street, Walsall. The company repaired radios and TV's and according to Dad they had a number of battery eliminators in the workshop in order to power battery sets that were in for repair.

SBC Holdsworth is of course long gone, but the building itself survives.
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