# How do I safely test a transformer?



## keeperofthegood (Apr 30, 2008)

*How do I safely test a transformer? Update*

Hey oh

In line with the other thread here on an out-put transformer, I decided it was an opportune time for a similar thread on power transformers.

I have this power transformer. It was from an Admiral TV. The numbering is a house numbering, and I have found no cross references on it anywhere on-line. I can purchase a schematic of the original TV for 20, but that is kinda overkill I am thinking.

SO, how do I 'black box' determine what this transformer is? I have measured the ohms of the windings. The reds are 24 ohms, with the yellow striped red being in the middle of that. The blacks are 2.5 ohms. The greens and yellow get complicated, The yellow to either of the greens is 3.5 and between the greens it is unreadable on my analogue meter (no, I don't have digital).

Logically, I would assume the blacks were power, the reds high tension, and the greens the filament with the yellow the center tap for that. I think this is wrong. SO, how do I go about testing this transformer out?

Screwdriver for reference. 
The wires are

Red
Red/Yellow
Red

Black
Black/Red

Green
Green/Yellow
Yellow





















EDIT: I know, it is a pre-win-key keyboard. Someone tossed it new in the bag in the blue bins. My other keyboard was due for replacing since the last coffee I spilled all over it >_> so, I replaced it >_>


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## nonreverb (Sep 19, 2006)

Why would you think this is wrong? Sounds right to me....
But if you're gonna play it safe, get out the Variac and apply 1 volt AC to the two black wires and see what you get on the red wires....don't bother with the greens as I suspect they're filament and you won't see squat on them...
If you get 2 or 3 volts from center tap to either of the red wires then you can figure out the ratio.


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## WCGill (Mar 27, 2009)

You're right on with your deductions I'd say. The best way to test it is to put some ac voltage through the primary, but small ac, say from a wall-wart and then measure the other ends. The secondaries should measure higher, red to red likely twice as much as red to red-yellow. The filaments should measure much lower than your input voltage and again, from the outer legs the voltage should be twice as high as from each leg to centre-tap. As it's a bit of an odd colour scheme for the filaments, this will help determine which is which here.


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## keeperofthegood (Apr 30, 2008)

nonreverb said:


> Why would you think this is wrong? Sounds right to me....
> But if you're gonna play it safe, get out the Variac and apply 1 volt AC to the two black wires and see what you get on the red wires....don't bother with the greens as I suspect they're filament and you won't see squat on them...
> If you get 2 or 3 volts from center tap to either of the red wires then you can figure out the ratio.


In my "logic" of what to expect, I would make the assumption that the filament winding would be smaller than the primary in impedance. So a primary near 2.5 ohms, should not the filament winding be unreadably small rather than higher?

What I almost get the feeling of is the yellow is the neutral, and one green is 110, the other is 117 on a 'voltage' switch. (BUT then, the lengths of those green and yellow suggest they were not power leads too, which are almost always short compared to the rest).

The other confusion is that TV's were typically series run sets. The tube voltages of the set would seem to suggest this as there is (6v * 9) + 23v + 38v = 115v not including the 1v on the 1AY2 HT Diode.

:/ otherwise yes, I would jump to the idea that the greens were filament leads too.

I have a lot of little transformers hanging about, 6v and up. I will give them a set up and go.

Keeps


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## Wild Bill (May 3, 2006)

Wire resistance means very little, I'm afraid, except in the sense that the HV winding will have many turns and show a higher resistance and the filament is just a few turns so it will show a lower one.

The colour codes are actually close to standard for old tube device transformers. The primary is black and black with a stripe of some colour. The HV is red, with a red/stripe centre tap. The filament winding is green and there's a different colour for the filament centretap.

This is assuming the transformer came from an old tube tv. If it powered a transistor telly the primary is likely the same but you won't get useful voltages from the other windings.

There's only one way to know the voltages and that's to feed a voltage to one winding and then measure them all. The suggestion to get a low AC voltage from a wall-wart supply is a good one. Try to find something 6 VAC or less. Don't use a DC supply or you'll burn something up for sure.

Feed the low voltage onto the green wires. I suggest that you use 'gator leads or something to pre-connect your meter leads to each winding. That way you don't have to fool around with the probes to take a reading. The voltages might hurt! Don't take the voltage from the wall-wart for granted. It might say 6 vac but that's just a ballpark that varies a lot with the load. Measure it too!

Once you've measured all the windings you now will know the ratio's involved. You can plug 120 volts into your ratios on the black wires and figure out what you'll have on the HV and filament windings.

Don't get upset if you get 7 vac as your number for the filament winding. Windings with no load on them tend to read about 10% high.

If you're lucky you'll end up with useful voltages for a tube amp project. Still, you don't know how much current you can draw from each winding. There's no easy way to know this with an unknown trannie but we can make some educated guesses.

A tube tv would have had LOTS of tubes! This means that the filament winding would have had to supply a BUNCH of amps! The wires are probably a thick gauge because of this. Since most guitar amps use just a handful of tubes or less you can assume that the filament winding will be more than beefy enough.

The high voltage winding is a different story. Most of the tubes in that Admiral would have been operating like preamp tubes, amplifying voltages but very little current. The only tube that would have needed a lot of current would have been the horizontal output tube. It would have needed about 20 watts or so.

So we can safely "guesstimate" that our HV winding is good for 20-25 watts. We get an additional extra "fudge factor" 'cuz the filament winding will be running much cooler than its design limits and that will help the HV current rating a wee bit. It's all about not overheating the wires, after all.

I would say that trannie would be a safe choice to power a pair of 6V6's and a couple of preamp tubes, or maybe a 6L6/EL34 SE Champ style amp.

You may be tempted to demand much more power since the trannie will seem much bigger and heavier than one used for just a pair of 6V6's, like with a Fender Deluxe Reverb or whatever. Unfortunately, that size and weight comes from having to supply 8 amps or more of filament current. As I had said, the HV winding would have needed to supply maybe 20 watts and that's the limiting factor.

Hope this makes sense!

:food-smiley-004:


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## Wild Bill (May 3, 2006)

keeperofthegood said:


> In my "logic" of what to expect, I would make the assumption that the filament winding would be smaller than the primary in impedance. So a primary near 2.5 ohms, should not the filament winding be unreadably small rather than higher?
> 
> What I almost get the feeling of is the yellow is the neutral, and one green is 110, the other is 117 on a 'voltage' switch. (BUT then, the lengths of those green and yellow suggest they were not power leads too, which are almost always short compared to the rest).
> 
> ...


Resistance, not impedance, my friend!:smile: Impedance is an AC unit. Here we're measuring the DC resistance of the wires in the windings. Impedance meters are much more rare and expensive devices and it's not likely you have one around, Mr. Keeps!

As for series vs. parallel filaments, they were made both ways. For a number of reasons, the more expensive sets tended to parallel filaments using more common 6 volt tubes. The cheaper ones were series-fed.

Don't worry so much about the colours. As I said, they seem standard. If a green was neutral then you would have no connection at all to the black wires! I'd go with the initial assumptions first and MEASURE!

:food-smiley-004:


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## keeperofthegood (Apr 30, 2008)

:food-smiley-004: Thank you much Wild Bill, I will bread board this up later tonight, and post up with pics what I find.


For completions sake, the TV this is from was (because most of the set was lost due to whatever bon-fire claimed the wood, I rescued only a few untouched parts) a:

Canadian Admiral Corporation LTD.
Port Credit, Ont
Serial No. T.V. 492799
Chassis No. G7 6X
Volts 117
Amp. 1.3
Cycle 60

The tube line up is:
Main Board
Silicon Rect. 93L5-4 (there are three other diodes on the board but this is pictured right beside the tranny)
6Y9 (6AF9)
6BF11
6EJ7
6U9
6HS8
6CG7
6JZ8
38HK7

HT 1AY2
Picture Tube 23HKP4

Tuner:
VHF
6CG6A(94M33 TUNER)
6LJ6(94C282 TUNER)
6GK5

UHF
1N82A
SE-3002 (PNP)


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## keeperofthegood (Apr 30, 2008)

*Updated*










Ok, the wee tranny is no-load 12V CT out of a clock-radio. The dirty old meter is a Micronta 10,000 Ohms/Volt AC. 

I put the no-load 6V across the GREEN wires and:

Green - Yellow - Green 6V g-g, the 6Volts measured no-load dropped to 2.5Volts when loaded by this winding.

Black - Black 44Volts no-load

Red - Red/Yellow - R-R/Y 56Volts and R/Y-R 56Volts and R-R 110Volts

Using a Green - Yellow pair cut the other winding voltages out by about half.

So, if the blacks are the primary at 120V, would that mean that I could expect:

44V/112V = 120V/xV
xV = 120V/(44V/112V)
xV = 305.454545 Volts?


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## Wild Bill (May 3, 2006)

keeperofthegood said:


> Ok, the wee tranny is no-load 12V CT out of a clock-radio. The dirty old meter is a Micronta 10,000 Ohms/Volt AC.
> 
> I put the no-load 6V across the GREEN wires and:
> 
> ...


Sounds about right! Line voltage is usually more like 122 in Ontario but you're in the ballpark.

:food-smiley-004:


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## keeperofthegood (Apr 30, 2008)

Wild Bill said:


> Sounds about right! Line voltage is usually more like 122 in Ontario but you're in the ballpark.
> 
> :food-smiley-004:


Ok, *110v* (according to my meter, is this expected load dip, or is my meter off?) across the blacks and:

My meter accuracy is as good as analogue gets LOL

Green - Green/Yellow *~ 7.5volts*
Green/Yello - Yello *~40Volts*
Green - Yellow *~47Volts

* Red - Red *280Volts*
Red - Red/Yellow *140Volts*

These are all un-loaded, what should I do to load these to see their load voltage?


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