Topic: Transformers - Robotsin disguise ?

[Miles]

How Do FW's, hope you're all grand.

Annoyed of Canterbury ... Looks like my Meopta Enlarger (Magnafax4a) transformer has died.

Looking at it, it's just a 240c AC - 12v DC (11.75) 100w jobbie, sowould anything similar suffice, rather that a genuine fellow ?

A trip to Mr. Cad where Igot the enlatger unless anyone has any other good sources ?

[Francois]

Any transformer with similar specs would work with a bit of tinkering. DC lamps are not polarity sensitive so it's an easy replacement.

Around here, enlargers have dropped in price so much that getting an original replacement can be cheaper than fixing one from scratch.

I'm really surprised that the transformer went. These are nearly bulletproof. Have you opened the case of the Meopta transformer to see what's inside? Sometimes, there's a hidden fuse inside the case of power supplies.

If not, you can check into computer power supplies. They have four leads on each cable that supply both 12 volts and 5 volts. The two black wires in the centre are neutral. The externals are the voltage. One external and one centre will give the 12-5 volts range. Both externals give 7 volts.

[Miles]

Thanks chum.

The box is rivited so I can't get in.  There is an 8amp fuse next to the cable that goes to the lamp housing, that checkout out ok.  I tested the lamp and the cables from the trans to the lamp housing, all grand.  I then put a Std. UK 13amp plug on the trans rather than the mini one and plugged it in direct to eliminate the timer from the equation and the output pins don't give out the 11 - 12 volts I need to see.  Only a little trickles out and the the metre stops reading.

Some Halogen lamps are 240v to 12v DC 100W so one of them might work.

I agree, the things would normally go on forever and it was fine last week.

[Francois]

I don't think there's a riveted box that held me back before!
It's not like we're afraid to void the warranty by now
Just drill the suckers out. You need a drill just slightly larger than the rivet's post. If the drill won't fit in the rivet hole by a fraction, it's usually big enough.

When inside, look for smells, mystery blue smoke residue, arcing and all the usual.

Does the timer output 240vac to the tranny?

[Miles]

Yep 240vac from timer as per pic > Trans >Lamp Housing

I've direct from the mains to the trans too and nothing.

[Miles]

Also, hitting the focus or red button for timed power/light and you hear the power going to the transformer, that hum and it gets to the windings. 

[Francois]

Looks like you haven't busted the rivets... the faulty part is inside that box.
On a transformer, there's never much danger unless it's plugged-in.

If you see a big capacitor, just use a voltmeter to check if it still has power before touching anything. If it does, use a 100 watt incandescent bulb in a socket with two leads to drain it. They sell those pre-made at home improvement stores... they're designed to go outside and are pre-wired. makes things easy.

My guess is that the power goes to the transformer but some problem prevents the juice from going out. There could be a short right after the transfo. Also check the plugs and the lead to the enlarger to see if there is continuity. The problem could be in the line that goes to the enlarger. That wire move around quite a bit more than you might think.

[Miles]

Checked cont on wires from plug that goes into the trans to the bulb holder and all good.

Alex @ Mr. cad have be the details of the chap that does all their electrical fixing but the postage won't be cheap.  Thinking about just getting a general purpose 240vac > 12dc 100w trans from ebay and trying that ?

Something like this ?

http://www.ebay.co.uk/itm/12V-DC-Power-Supply-Adapter-LED-Strip-Lamp-Driver-Waterproof-IP66-/171174633398

[Francois]

I think that could work. Given the price, it's probably not so bad a deal.

You don't know someone with a spare computer PSU? That can deliver a whole lot of power and can often be found in people's basement.
It's also very clean power so the light's stability is pretty much guaranteed.

[Miles]

I'll have a look, thought they were only 5.5v DC though rather than 12 and the wattage lower, I'll have a dig around.

Thanks chum.

[Francois]

Only the laptops are of the lower voltage. Desktops have both.

When you look, try and find one that was made for an old 486 or early Pentium. These came with a mechanical toggle switch and will be easier for you to adapt. The newer models are controlled by the motherboard and require that a resistor be inserted in some of the pins. Not that it's any harder to do... just different.

Check this out. http://makezine.com/2011/06/20/turn-a-computer-power-supply-into-a-desktop-power-supply/
Now I don't want you to freak out at the soldering and wires. It explains the basics of how to convert the box. Since you don't need all the power buses that they put on theirs, your build will be infinitely simpler.

I just found this too
http://makezine.com/2008/04/15/turn-an-atx-power-supply-into/

[Miles]

Ooooooooo I have a Sun Sparc Ultra 5 under my desk ... :-)