Isolation transformer / amp potiential

[mamm]

I built the AA5 amp in Tino Zottola's book one. I used an isolation transformer for a power transformer as told to in the book.

I first powered up the amp. with a seperate isolation transformer in case there were shorts or wiring problems. There seem to be none that I could find and the amp works as promised.

The problem is that if I measure the voltage between the grounded case of the isolation transformer and the grounded amp. case I get a 105 volt difference. If I put one hand on the amp and one hand on the isolation transformer (I haven't done that yet) I would get a shock.

Both grounds test perfect. The isolation transformer is plugged into a grounded (tested) outlet. Both cases should be at 0 volts since they are connected by the ground wire.

What am I missing here?

[techineer]

The transformer probably has a coating of varnish on it that is insulating it from the chassis.

With the power cord disconnected, use an ohmmeter to measure the resistance between the chassis and the transformer case. I'm betting it will be very high.

Next, use clip leads to connect a 1500 ohm resistor across your voltmeter probes. (1K - 3K is fine.) Power the amp on and measure the voltage between the chassis and the transformer case.

If the resistor smokes, the transformer has a low-resistance short to the case, and you would indeed be shocked by it. DON'T put one hand on the transformer and one hand on the chassis!

If the meter measures less than a volt or so, you won't feel more than a slight tingle (if that). Disconnecting the resistor will probably again cause a high voltage reading. If you are brave enough to try the touch test, do so with the other hand in your pocket. (Just in case!)

More than likely there is a high resistance leakage path in the transformer, or you are reading the voltage induced in the core. The insulating varnish is preventing it from being shorted to the chassis. The high resistance of the voltmeter doesn't put enough of a load across the gap to drop the reading to a low level like the resistor does.

---------------------------- Update -------------------------

I see I misinterpreted your situation.... I was interpreting that the isolation transformer used as the power transformer was showing the voltage on the case. I see now that the voltage reading is between the amp chassis and the transformer between the mains and the amp.

However, the measurement test above should still be valid.

The issue depends on how the ground path is handled by the isolation transformer (IT). Does the amp use a 3-prong grounded plug? Does the isolation transformer also have a 3-prong cord plug and outlet? It's important to find out if there is a continuous connection from the amp chassis through the power cable ground pin, to the IT outlet ground pin, through to the IT power cable ground pin. Use an ohmmeter. Start between the ground pin of the IT power cord and the amp chassis to test the whole chain at once. If it shows high resistance, test each segment and connection to find out where the path is broken.

Some ITs don't have a grounded socket, either because they were made before grounded plugs became common or because they want to be certain that there is no possibility that the secondary may develop a voltage with respect to ground.

While the AC line is hot with respect to earth ground, the secondary side of the transformer is hot only between the slots and not to the ground pin. (Unless there is a defect.) So if you are getting 105 volts between the transformer case and ground, it sounds like it may be an issue with leakage from the primary winding combined with a high resistance ground.

Good luck! I'd like to know what you find out.

[melvin]

Need more information. but double check your wiring. same thing you described happens to an amp with no ground, a "death cap" and the mains polarity reversed. check resistance from each chassis/case to outlet ground. if the outlet ground is not very good and the varnish thing proves correct, the whole ground could be at mains potential if miswired...