Bad OT

[GainFreak]

Hi,

I have a 100 Watt OT that appears to be bad. The question is what exactly?
The primary measures 36 Ohms - 16 and 20 Ohms for each half primary.
When I measure the inductance it shows ~5mH which is impossible.
I have the same OT which is OK and it has an inductance of 5.9H across both ends of the primary.
Any idea what could be the problem?

[tubeswell]

The primary measures 36 Ohms - 16 and 20 Ohms for each half primary.

That doesn't sound out of the ordinary. One half of the primary has a higher DC resistance because the amount or wire is longer there (because one winding is 'stacked' over the top of the other). What is important is that the number of turns on each side of the primary is the same.

When I measure the inductance it shows ~5mH which is impossible. I have the same OT which is OK and it has an inductance of 5.9H across both ends of the primary.

Why is that unusual? HT Chokes can have similar inductance. (The thing is a coil wound over some iron after all.)

When you say problem, does it perform ok?

[R.G.]

Hi,

I have a 100 Watt OT that appears to be bad. The question is what exactly?
The primary measures 36 Ohms - 16 and 20 Ohms for each half primary.
When I measure the inductance it shows ~5mH which is impossible.
I have the same OT which is OK and it has an inductance of 5.9H across both ends of the primary. Any idea what could be the problem?

An internal shorted winding would do exactly this. The few millihenries is the value of flux that leaks around the short. Shorting one winding and testing inductance on another winding is in fact the way you do measure leakage inductance.

You can see if there is a short, any short, by doing this:
http://geofex.com/FX_images/xform_test.gif

It works. I've had grizzled amp technicians call me a liar for proposing it then being as apologetic as they can get when they try it.

[GainFreak]

An internal shorted winding would do exactly this. The few millihenries is the value of flux that leaks around the short. Shorting one winding and testing inductance on another winding is in fact the way you do measure leakage inductance.

You can see if there is a short, any short, by doing this:
http://geofex.com/FX_images/xform_test.gif

It works. I've had grizzled amp technicians call me a liar for proposing it then being as apologetic as they can get when they try it.

Thanks for the diagram but unfortunately I don't have any neon bulb around.
Are you suggesting the problem is primary shorted to the secondary or some of the secondaries shorted?

[R.G.]

Thanks for the diagram but unfortunately I don't have any neon bulb around.

Then find one. They have been made by the multiple millions for over 50 years. They are *not* rare. Before LEDs they were the *primary* way of indicating AC power on. Huge numbers of coffee pots, toasters, almost anything that indicated "power on" used a neon as long as it didn't have tubes inside to give 6V for indicator lights.

Are you suggesting the problem is primary shorted to the secondary or some of the secondaries shorted?

No way to tell, other than you could tell with an ohmmeter whether primary is shorted to secondary. A shorted turn *anywhere* would do it. Inside the primary or inside the secondary, either place would cause the same result.

[GainFreak]

Then find one. They have been made by the multiple millions for over 50 years. They are *not* rare. Before LEDs they were the *primary* way of indicating AC power on. Huge numbers of coffee pots, toasters, almost anything that indicated "power on" used a neon as long as it didn't have tubes inside to give 6V for indicator lights.

If you're talking those small ones that glow in red/orange I can get those. I conpletely forgot about them - you can't see them around very often nowadays and that 6V battery got me confused as well. Can I use one of those inside a Marshall type rocker switch from JCM800/900?

[R.G.]

If you're talking those small ones that glow in red/orange I can get those. I conpletely forgot about them - you can't see them around very often nowadays and that 6V battery got me confused as well. Can I use one of those inside a Marshall type rocker switch from JCM800/900?

Yep, small red-orange bulbs, pointy end, thin wires and no wire connection between electrodes inside. They appear to be open circuit to an ohmmeter.

They are filled with... drum roll, please... neon gas. The conditions are such that the gas doesn't ionize and conduct until about 65-90V appears across the terminals. At that point, the gas ionizes, light emits, and the resistance between the two terminals drops radically. They normally use a 100K to 470K resistor in series with them to limit current, and are always used directly across the AC power line to emit light.

The reason they work is that loading up any winding of a transformer charges up the core magnetically with DC in this circuit. When you open the DC connection, whatever magnetic field was in there does the inductive flyback thing and makes a pulse of high voltage. In a good transformer, the flyback bounces up to over the neon bulb threshold, and causes it to flash. However, if one turn is shorted, this "eats" all the magnetic energy and keeps it from ever causing high voltage. The bulb does not flash.

It's a remarkably good test for such a simple setup. I wish I could say I thought of it, but it was old before my time. The common wisdom among amp techs is that you can't tell this kind of thing from outside the transformer. Wrong... you can. But you have to be very careful that there is no load on any winding, or it will give you a false bad result (no flash). All those windings have to be unloaded.