help replacing an old ac power transformer.

[[email protected]]

Hi. I have a very old transformer/power supply that blew that i'm
trying to replace.

All it says on the outer box is:

120vac
75 watts
50-60hz

The transformer has 4 leads...two reds and two blocks.

The transformer was made sometime between 1974-1979.



I went to Rat shack to buy a new transformer which has three blacks and
two yellows. Based on the way the wiring *was* setup I rewired the
transformer to the new one and flipped it on.. Nothing..not even a hum
or anything. The radio shack lists "center-tapped secondary" if thats
any help.

There is one black wire "loose" and I am unsure where it goes or if it
should go anywhere. There's nowhere else for it to go unless its
supposed to also connect to the ground (the ground already existed for
the plug to the box the transformer is in). Is that last wire a
ground?

Any ideas?

Thanks!

 

[John Popelish]

Hi. I have a very old transformer/power supply that blew that i'm
trying to replace.

All it says on the outer box is:

120vac
75 watts
50-60hz

The transformer has 4 leads...two reds and two blocks.

The transformer was made sometime between 1974-1979.



I went to Rat shack to buy a new transformer which has three blacks and
two yellows. Based on the way the wiring *was* setup I rewired the
transformer to the new one and flipped it on.. Nothing..not even a hum
or anything. The radio shack lists "center-tapped secondary" if thats
any help.

There is one black wire "loose" and I am unsure where it goes or if it
should go anywhere. There's nowhere else for it to go unless its
supposed to also connect to the ground (the ground already existed for
the plug to the box the transformer is in). Is that last wire a
ground?

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.

 

[[email protected]]

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..

 

[Pooh Bear]

Hi. I have a very old transformer/power supply that blew that i'm
trying to replace.

All it says on the outer box is:

120vac
75 watts
50-60hz

The transformer has 4 leads...two reds and two blocks.

The transformer was made sometime between 1974-1979.

I went to Rat shack to buy a new transformer which has three blacks and
two yellows. Based on the way the wiring *was* setup I rewired the
transformer to the new one and flipped it on.. Nothing..not even a hum
or anything. The radio shack lists "center-tapped secondary" if thats
any help.

There is one black wire "loose" and I am unsure where it goes or if it
should go anywhere. There's nowhere else for it to go unless its
supposed to also connect to the ground (the ground already existed for
the plug to the box the transformer is in). Is that last wire a
ground?

Any ideas?

Is this a troll ?

Graham

 

[John Popelish]

John I can email a small digital photo ...

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.

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).

That really narrows the possibilities a lot.

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.

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.

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

Hope this helps more..

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 http://www.digikey.com/
http://rocky.digikey.com/WebLib/Hammond/Web Data/166 Series.pdf

 

[John Woodgate]

I read in sci.electronics.design that John Popelish <[email protected]>

Unfortunately, 24, 25 and 28 volt transformers are all more common.

28 V plus a 0.33 ohm. Perhaps Watson would wind him one. (;-)

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?

 

[martin griffith]

Is this a troll ?

Graham

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


martin

 

[fotoobscura]

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:

12.6v AC, 3-amp
center-tapped secondary

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!

 

[John Popelish]

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:

12.6v AC, 3-amp
center-tapped secondary

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.

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

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).

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

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.

 

[Pooh Bear]

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:

12.6v AC, 3-amp
center-tapped secondary

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

You just said the bulb is 27V.

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

Are you completely clueless ?

Graham

 

[fotoobscura]

Are you a complete asshole?

Go away troll, some people are actually helping me.

 

[fotoobscura]

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.

 

[Pooh Bear]

Are you a complete asshole?

Go away troll, some people are actually helping me.

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

 

[fotoobscura]

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!

 

[John Popelish]

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.

Sounds like a plan.

 

[Rich Grise]

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.

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?

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:

12.6v AC, 3-amp
center-tapped secondary

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

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

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

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.

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

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

 

[Rich Grise]

Are you a complete asshole?

Go away troll, some people are actually helping me.

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

 

[Ken Taylor]

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

He likes Tiggers though.

Ken

 

[Spehro Pefhany]

He likes Tiggers though.

Ken

And honey.

http://www.webfitz.com/lyrics/Lyrics/1968/111968.html



Best regards,
Spehro Pefhany

 

[fotoobscura]

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!