PT Primary Impedance - 1.4 ohms?!

[waspclothes]

Hey guys,
I have a Leslie 22H amp on the bench. Blown mains fuse, haven't tried to power it on yet. I decided to measure the impedance of the power transformer primary, an I'm getting only 1.4 ohms!

Would that indicate a shorted PT?

[nickb]

Hey guys,
I have a Leslie 22H amp on the bench. Blown mains fuse, haven't tried to power it on yet. I decided to measure the impedance of the power transformer primary, an I'm getting only 1.4 ohms!

Would that indicate a shorted PT?

It's the primary inductance rather than resistance that matters, but, for a 40W amp I'd still expect the resistance to be quite a bit higher than that. So, yes, that transformer is bad.

[Enzo]

To be sure, COMPLETELY disconnect all secondary wires and power it up.

Use a light bulb limiter instead of blowing more fuses.

[R.G.]

I have a Leslie 22H amp on the bench. Blown mains fuse, haven't tried to power it on yet. I decided to measure the impedance of the power transformer primary, an I'm getting only 1.4 ohms!
Would that indicate a shorted PT?

Unless you used an AC bridge to measure the impedance your measurement didn't tell you much.

Particularly, if you measure the DC resistance with your meter, you may or may not have anything worth knowing. As noted earlier, that sounds awfully low for the resistance of a primary winding (which is all your normal DMM or analog meter can read), but many simple meters are notoriously poor about accuracy in reading low ohms.

If you want to know if the transformer is good or bad, you use your meter to see if all windings are isolated from one another and from the chassis **with NO connections to any primary or secondary circuitry at all**. If they're isolated where they're supposed to be and measure a low DC resistance, they may be OK. There are only two things that can be wrong if the windings test OK to this test. Those are a shorted internal turn, and a shorted internal turn that only happens when the transformer gets hot.

You can test for the first one with the shorted turns tester: http://www.geofex.com/FX_images/xform_test.gif. But there are probably better ways to go about this.

First, make up a light bulb limiter, and while you're at it, snag several replacement bulbs, as our government has outlawed their manufacture because they think we spend too much electricity. Use the limiter to debug your AC-power side circuitry with an open secondary. You're looking to isolate whether the secondary load is causing a blown fuse, or whether the transformer causes the blown fuse with no secondary load, or whether the AC power wiring causes the blown fuse without even the PT primary connected. Only after that can you do some more detailed debugging.

While you're there, read about testing PTs and OTs.

[waspclothes]

To be sure, COMPLETELY disconnect all secondary wires and power it up.

Use a light bulb limiter instead of blowing more fuses.

I went ahead and connected it up, before I saw your post to disconnect the secondaries. I have a 1.5A fuse in there, and everything powered up fine. I've never measured the impedance of a PT's primary before, and didn't know what to expect.

All the voltages seemed fine, no fuses blew. But, the amp was drawing close to 140W which seems like a lot to me.

[elipsey]

while you're at it, snag several replacement bulbs, as our government has outlawed their manufacture because they think we spend too much electricity.

Will the new 43 or 72 watt incadescent halogen bulbs work instead of standard 60 or 100 watt incandescent? They cost an extra 75 cents, but they sell them everywhere and I don't think they're going to stop...

[g1]

I'm not sure the halogen resistance reacts the same way to the temperature increase, however I have seen them used as tweeter protection so they may be fine.
As far as I know, certain incandescents will remain in production though they may get harder to find. Like "rough service" bulbs, I believe.

[Enzo]

The label tells the story, that 75 watt equivalent (in lumens) is only a 23 watt bulb, so it will act like a 23 watt bulb.

wasp, you STILL have not measured the impedance of the transformer winding, you have only measured its resistance. This is an important difference.


If the transformer was drawing 140 watts with the secondaries completely disconnected, that is a bad sign. it ought to draw very little by itself.

Try RG's transformer tester.

When a winding shorts, it usually shorts turn to turn, not end to end. And so a meter will not note much difference.

[waspclothes]

The label tells the story, that 75 watt equivalent (in lumens) is only a 23 watt bulb, so it will act like a 23 watt bulb.

wasp, you STILL have not measured the impedance of the transformer winding, you have only measured its resistance. This is an important difference.


If the transformer was drawing 140 watts with the secondaries completely disconnected, that is a bad sign. it ought to draw very little by itself.

Try RG's transformer tester.

When a winding shorts, it usually shorts turn to turn, not end to end. And so a meter will not note much difference.

Thanks Enzo, will try RG's transformer tester. The 140W was with everything connected, tubes in.

[trobbins]

How were you measuring '140W' ?

[waspclothes]

How were you measuring '140W' ?

With a kill-a-watt at the plug. ( P4400: P3 INTERNATIONAL: Test, Tools & Supplies )

[trobbins]

That meter won't be accurate because of the low power factor. It will be measuring the VA.

[Jazz P Bass]

Something is off kilter if a 40 watt output amp is consuming 140 watts from the mains at idle.

[nickb]

That meter won't be accurate because of the low power factor. It will be measuring the VA.

Those kill-a-watt things can report both VA and Watts.

[trobbins]

Apologies, I stereotyped that meter - due in part to the almost complete lack of specs on the P3 site. There are more specs on ?????????? Prodigit Electronics Co.,Lid--??. If it is a version with VA and W option then yes it is likely to be suitably accurate, even with high-crest factor current:
Measuring standby power - Page 1

I can only suggest to be careful in choosing the model - it may be quite easy to think you are getting the better version, and end up getting a less functional model.