Okay, Howard, let's troubleshoot this isolation transformer behavior. It's definitely not what you'd expect from a proper isolation transformer, but the readings you're getting can happen due to a few common causes.

Understanding the Expected Behavior of an Isolation Transformer Purpose

An isolation transformer electrically isolates the load (your radio) from the AC mains. This prevents a direct conductive path to ground, reducing the risk of electric shock.

Expected Readings

Ideally, you should measure very high resistance (megohms) between either side of the secondary (output) and the primary (input) windings, and between either side of the secondary and ground. When energized, you'd expect no significant voltage between either side of the secondary and ground.

Possible Explanations for Your Readings1. Capacitive Coupling

This is the most likely culprit. Even with good insulation, there's always some capacitive coupling between the primary and secondary windings. This is inherent in the construction of the transformer. The amount of capacitance is typically very small (picofarads), but at 60 Hz AC, it can allow a small current to flow. That small current, combined with the impedance of your voltmeter, can produce a measurable voltage to ground. Think of it like a very tiny capacitor connected between the primary and secondary.

How it explains your readings

The amount of voltage you measure depends on the capacitance, the frequency of the AC, the impedance of your voltmeter, and any other resistances or capacitances to ground in the circuit. One leg might show a higher voltage due to slight imbalances in the capacitive coupling.

Testing for this



High Impedance Voltmeter

Use a very high impedance voltmeter (10 Megohms or higher, like most modern digital multimeters) and see if the voltages change significantly. A lower impedance voltmeter will "load down" the capacitive effect and give a lower reading.

Parallel Resistor

Try connecting a high-value resistor (e.g., 1 Megohm) between each output leg and ground one at a time. If the voltage drops significantly, it supports the capacitive coupling theory.2.

Leakage Current (Insulation Breakdown)

While you checked for continuity, it's possible there's very slight insulation breakdown. This could allow a tiny current to flow, again creating a voltage potential to ground. This is more serious than capacitive coupling because it indicates a potential safety hazard.

How it explains your readings

A small leakage current flowing through the impedance of your voltmeter creates the voltage drop.

Testing for this



Megger (Insulation Tester)

The best way to test for this is with a megger. This applies a high DC voltage (e.g., 500V or 1000V) and measures the insulation resistance. A good transformer should show hundreds of megohms or even gigohms. A lower resistance indicates insulation breakdown. This is the proper way to test the insulation.

Current Measurement

Carefully (and with appropriate safety precautions!) measure the AC current flowing from each output leg to ground with a known load connected to the transformer. A very small current might be normal due to capacitive coupling, but anything more than a few microamps could indicate a problem. Use a sensitive AC current clamp meter if you have one.3.

Faulty Transformer

There could be a manufacturing defect or damage to the transformer. Perhaps there's an internal short to the core (which is often grounded).

Testing



Visual Inspection

Carefully examine the transformer for any signs of damage, overheating, or oil leaks.

Smell Test

Does it smell burnt or acrid?

Comparison

If you have another isolation transformer, compare the readings.4.

Grounding Issues

The voltage readings might be influenced by grounding issues in your electrical system or equipment.

Testing



Verify Ground

Ensure your electrical outlet is properly grounded.

Floating Ground

Try plugging the transformer into a different outlet on a different circuit to see if the readings change.

Safety Considerations Electric Shock Hazard

Always be extremely careful when working with AC mains voltage. Even a small leakage current can be dangerous.

Isolation Transformer as the Only Protection

Never rely solely on an isolation transformer for safety. Always use proper grounding techniques, wear appropriate safety gear, and use a GFCI outlet whenever possible, especially when working on old or potentially faulty equipment.

Unplug

ALWAYS unplug the transformer before making any measurements or connections.

Recommendations1. Start with the simplest explanation

Try the high impedance voltmeter and the parallel resistor test to check for capacitive coupling.2.

Visual Inspection

Carefully examine the transformer for any damage.3.

Megger Test

If you're serious about using the transformer for safety, get it meggered. That's the only way to be reasonably sure the insulation is good. If the insulation is questionable, do not use the transformer.4.

Current Measurement

Carefully measure the leakage current to ground.5.

If in doubt, discard the transformer. Your safety is paramount. A faulty isolation transformer is worse than no isolation transformer at all because it gives a false sense of security.Let me know what you find when you try these tests. Good luck, and be safe! Flag for review