help replacing an old ac power transformer.

John I can email a small digital photo ...

Does this help: This is a photographic enlarger and has a 27V GE bulb inside it (thats the only thing thats being powered by the transformer/power supply).

I am not absolutely sure the transformer is bad, I guess. I presumed, perhaps incorrectly that the relatively loud buzzing sound when turning it on was the culprit but that may be presumptuous? The power supply/transformer are separate from the photographic enlarger.

The bulb doesn't look *physically* blown so I doubt its the bulb.

The wire looks like regular household wiring gauge (12ga?) but is much more pliable.

Hope this helps more..

Transformers can be made in almost infinite variety. We have to figure out the approximate output voltage and current rating of the old unit. We may get a clue of the voltage from the big electrolytic filter capacitor voltage rating after the bridge rectifier. The rectifiers or capacitor may also be bad. In fact, your transformer may be fine. Once the output voltage rating has been figured out, the current rating can be approximated based on the power rating. As a last resort (after you are absolutely sure the transformer is bad) would be to take it apart and count the turns on primary and secondary and compute the output voltage that way. The wire gauges are also an indication of the current rating needed.

You are welcome to do so. There is also a newsgroup that welcomes picture attachments called alt.binaries.schematics.electronic but you can't post to this through Google groups.

That really narrows the possibilities a lot.

Do you have a multimeter (a digital one cost less than $20 at Walmart)? This will let you check the output voltage when the power is on, and the winding resistance when the line cord is unplugged. You can also check that the filament is intact with the ohm meter.

It does. Is this a color enlarger (where the bulb temperature is really important? Are you sure there is no electronics between the transformer and the bulb? If not, you are probably looking for a 27 volt 3 ampere transformer. Unfortunately, 24, 25 and 28 volt transformers are all more common.

For instance these from Digikey

Hey John,

Thanks. Yeah it is a color enlarger with the bulbs costing $30/each...yup...I am posting the picture to the alt.binaries.schematics.electronic group after this message.

The picture is of the old transformer on left and the new on right along with the box that houses the transformer and a power switch. Very basic little box.

The specs of the new one are:

Its quite possible the new one I bought is just plain the wrong one...Maybe not enough voltage?

I do have a digital multimeter. But I'm a little sketchy on playing with AC unless I know exactly how to measure it safely. And (where) to measure :)

I am not sure there are electronics between the transformer and the bulb. I can take it apart to check, though.

Thanks!

Is this a troll ?

Graham

Are you a complete asshole?

Go away troll, some people are actually helping me.

Yeah i'll take a look at the path from the power supply and see if I can't narrow it down more.

I'll test the wires as well...

Thanks John.

I read in sci.electronics.design that John Popelish wrote (in ) about 'help replacing an old ac power transformer.', on Sun, 9 Oct 2005:

Buzzing does suggest shorted turns. Maybe the lamp is shorted. Does it buzz if you take out the lamp and then switch on? Does the transformer rapidly get hot?

Sounds like enough voltage to get a nice red glow out of your lamp.

If you don't get that, then it should make you suspicious that you don't very well understand the situation.

The 4 lead transformer should have 120 volts AC (don't touch those wires with your fingers when measuring) on the line side and something like 27 or so on the lamp side. the transformer probably puts out a volt or two extra when it is unloaded (no lamp connected).

It is possible that there is some sort of voltage regulator function between the transformer and lamp, but I haven't any experience with enlargers. If there is, that allows for several additional failure modes.

What? From Phil the failed criminal from the underland? Dont think so

martin

You just said the bulb is 27V.

Isn't it obvious that 12.6V *isn't* 27V ????

Are you completely clueless ?

Graham

Hi John,

Thanks for all your help. I traced the wires into the enlarger and the wires go straight into the lamp assembly. There are no electronics between the transformer/power supply and the lamp assembly.

I am going to meter the thing hot and let you know what I get.

Thanks!

By the sound of it you shouldn't even be let loose with a soldering iron !

Your hostility noted. Typical reaction from a know-nothing idiot.

Graham

Sounds like a plan.

Apparently my server is a little slow to catch up - what subject line did you use, and did you sign it "fotoobscura", like this one?

Well 12.6V is certainly not going to light up a 27V bulb!

First, you put your meter on the ohms range, to find out which winding is which; but an RS tranny should have stamped on it which is supposed to be the primary. But I've never heard of a tranny with three black leads

- are you sure one of the black ones doesn't have some kind of stripe?

Well, I can be more helpful after I've seen the picture.

Might as well just take a look - as others have said, it could be one of several factors. Did it just start buzzing recently? That could be just a cheap transformer. Did it smell like something was burning? Are you only replacing it because of the buzzing, or does the light not light at all?

Thanks, Rich

Don't go too hard on Pooh - after all, this is a question that's more suitable for sci.electronics.basics - a lot of us self- appointed experts ( ;-) ) hang out there as well, and sometimes Pooh can be a little impatient with beginners.

Good Luck! Rich

And honey.

Best regards, Spehro Pefhany

He likes Tiggers though.

Ken

Ok I'll move my question to sci.electronics.basics. Point is, I don't need to be chastised for asking a question by some obvious jerkoff. A simple "post this here" would suffice.

Thanks Rich!