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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 7th Apr 2014, 2:56 pm   #1
Nickthedentist
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Thumbs up 18V mains power supplies for Hackers and similar

Since Stontronics discontinued their 200mA and 400mA linear 18V regulated power supplies which were available from CPC, it's been hard to recommend a suitable battery eliminator for Hacker (and similar) radios which were designed to run from two PP9s in series.

Unregulated supplies have been tried, but hum seems to be a problem.

One solution is to use an 18V linear series regulator fed from a suitable scrap power supply that can found in most enthusiasts' junk boxes, or picked up from a car boot sale or charity shop for a pound or two. Printer and older answering machine ones seem to be most suitable in terms of output voltage.

These can of course be built on stripboard or similar, but if time is precious or one's soldering skills are limited, a ready-made module is a nicer solution.

I've found these, based on the tried and tested 7818 IC, from eBay seller audiowind-2005 currently priced at a very reasonable 6.90 including postage all the way from Hong-Kong:

http://www.ebay.co.uk/itm/Positive-N...item68d071aa16

A bare PCB can be had at 3.45, and other voltages (positive and negative) can be had. The seller also stocks other very useful power supply modules.

Quality appears excellent. The PCB is fibreglass and is screenprinted and components are apparently good quality (e.g. Rubycon 105degC electrolytics). Soldering is first rate. There's also an on-board LED. Input and output is via screw terminals, so ideal for newbies. A bridge rectifier is included but terminals are provided for the input to go straight to the regulator if preferred, so it can be used with an AC or DC power unit, making the choice of power sources quite wide.

They're also very compact, 2" x 2" x 1.5" high, so very easy to hide within the radio.

Delivery took less than 2 weeks all told. Packing was superb, the modules being supplied in a sealed antistatic bag protected by a stout expanded polystyrene box. No VAT or customs charges were payable because of the low value of the goods.

Thoroughly recommended.

Nick.
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Old 9th Apr 2014, 10:38 am   #2
ValvoStef
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Default Re: 18V mains power supplies for Hackers and similar

In such cases I use the universal (and much cheaper) LM317, which can be configured to any voltage between 1.2 and 37 V dc.
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Old 9th Apr 2014, 10:55 am   #3
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Default Re: 18V mains power supplies for Hackers and similar

Thanks Stef.

I don't think the LM317T is cheaper than the 7818 per se. In fact the equivalent Velleman PCB with a LM317T on it from CPC is more costly at 8.75 including VAT: http://cpc.farnell.com/velleman-kit/...18?Ntt=HK00218

But it's a fair point; keeping some variable voltage regs in stock is always a good idea.

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Old 9th Apr 2014, 12:56 pm   #4
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Default Re: 18V mains power supplies for Hackers and similar

Most 78xx series regulators have a fairly constant current down the earth leg (3mA??) so a 7815 with 1k in said leg would work, mind you so would a plain 7815 or 7915, plenty of volts for a 18V Hacker. I can't see why a plain unregulated supply wouldn't be OK, 12V, bridge rectifier and a big cap, not much to go wrong. Most new 78xx series don't need an output capacitor either.
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Old 9th Apr 2014, 1:58 pm   #5
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Default Re: 18V mains power supplies for Hackers and similar

Quote:
Originally Posted by merlinmaxwell View Post
I can't see why a plain unregulated supply wouldn't be OK, 12V, bridge rectifier and a big cap, not much to go wrong.
But have you actually tried?

I have, as have many other people on this forum and the answer in one word is... HUM.

Nick.
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Old 9th Apr 2014, 3:23 pm   #6
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Default Re: 18V mains power supplies for Hackers and similar

Some unregulated power supplies do not have much, or in some cases any, smoothing built in. Often just a transformer and 2 diodes. The radio is unlikely to have much in the way of power supply decoupling on it's input as it's meant to operate on batteries and would not be required. The official Hacker supply would have provided the smoothing.
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Old 9th Apr 2014, 3:28 pm   #7
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Default Re: 18V mains power supplies for Hackers and similar

Quite, and the official Hacker supply was regulated AFAIK.

I've tried extra smoothing and decoupling with unregulated supplies and still get a little residual hum. Not loads, but enough to make listening at very low levels (e.g. in bed) unpleasant.

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Old 9th Apr 2014, 5:22 pm   #8
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Default Re: 18V mains power supplies for Hackers and similar

I don't have a Hacker, but I run my bedside radios from D cells using external battery packs consisting of battery containers from Maplin fitted with a lead and plug to fit the radio in question. I used to have a regular supply of part-exhausted D cells from our family's cycle lamps, but now that we use LEDs, that source has dried up. I use a 3 cell box for my 4.5 V radio when using new batteries, and a 4 cell box to use up the remaining capacity of almost-exhaused cells. No problem with hum!
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Old 9th Apr 2014, 6:00 pm   #9
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Default Re: 18V mains power supplies for Hackers and similar

Hum, a huge smoothing cap would solve that, they are quite cheap these days.

Roberts radio used to make (supply!) a six C cell to PP9 adaptor (6 Cs in alkaline are chepaer than a zinc carbon PP9), are they still available? I would love two.
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Old 9th Apr 2014, 7:48 pm   #10
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Default Re: 18V mains power supplies for Hackers and similar

Have you considered using SLA batteries? I'm currently running the summer-house Roberts R707 from a 2.2AH 12V SLA and it's managed about a month of being used several hours per day and 6 or 8 hours per day at weekends with the volume turned up to 8 without needing a recharge.

Three 6V SLAs would happily give you your 18V.

SLAs have low source-impedance so are great for delivering the instantaneous current-demand of class-B output stages.

If you do go the SLA route though, I'd suggest putting a 500mA slow-blow fuse in the wiring - it might just save your output-transistors if they decide to play thermal-runaway.

Last edited by G6Tanuki; 9th Apr 2014 at 7:54 pm.
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Old 9th Apr 2014, 10:59 pm   #11
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Default Re: 18V mains power supplies for Hackers and similar

I'd also suggest recharging the SLAs regularly too- say once a week. Like any lead acid battery, they don't take well to being left for long periods in a (partially) discharged state. Loss of capacity due to sulphation will be likely.
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Old 10th Apr 2014, 6:41 pm   #12
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Default Re: 18V mains power supplies for Hackers and similar

That's a valid point - though the SLAs I'm using are 'pulls' from ~sustained luminaire~ emergency-lights which get rigorously replaced on an 18-month cycle under the site's service-level-agreement so I'm really not that fussed if they do die because there's plenty more available from the same source.

Part of me is tempted to look at a small solar-panel to stick on the roof of the summerhouse to trickle-charge the SLA-pack. I'd only need 100mA of charge-capacity over a day to replenish my afternoon/evening's load.
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