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Testing Transformers

From Rec.Antiques.Radio+Phono
Testing transformers is not quite as mysterious as some people think, but there are some limits on what and how to test parameters in the field unless you have proper instrumentation and know how to use it.

The type of test needed for the audio driver transformer is a simple ohmmeter "go/no-go" test. Check that each of the coils in the transformer is continuous. Generally, readings will be on the order of tens to hundreds of ohms. High resistance (megohms) is a "fail" condition. Make sure that each coil is disconnected from other circuits, then test each coil for continuity.
testing transformers
Transformers are made with all windings insulated from the core laminations and case. In a few cases, you will find a transformer that has a center tap on a coil that is internally connected to the case, but this is not common. Generally, ohmmeter measurements between any coil and the transformer core-case should show megohms of resistance. In manufacturing, a "hipot" (high potential) test is used, checking for leakage with several hundred volts between the coil and case. However, most coil-frame insulation failures will show up on a low-voltage VTVM ohmmeter, which should show "infinite" resistance on its maximum sensitivity. Note that when testing transformers and making measurements over a few megohms, holding the probes in your hands, and other leakage paths, may affect the measurements. All coils should be insulated from each other as well.

The above tests will not show shorted turns or leakage in the windings themselves. A simple test for this is to connect a 6.3 volt filament transformer or similar to one of the coils and measure the voltages on other coils. On a 10:1 turns ratio interstage transformer, you should see around 63 volts, if you energize the short winding, or 630 millivolts, if you energize the long one. Magnetizing current with the transformer coils open-circuit should be milliamperes, and you can put a small resistor (say 100 ohms) in the driving circuit and use the voltage across it to determine magnetizing current. If the coil under test loads the resistor with significant voltage drop, you've got shorted turns. Losses introduced by shorted turns in the windings are dramatic, so this will be essentially a go/no-go test.
testing transformers
The impedance ratio and inductance of transformer windings is a function of the square of the turns ratio. Thus, a transformer that has a 10:1 turns ratio has an impedance ratio of 100:1. One significant ratio to keep in mind is that the ratio of a center-tapped coil in total to that of one side to the center tap is 4:1. Impedance and inductance measurements on audio units are made a 1000 Hz. To make these measurements, you need either a suitable impedance/inductance bridge such as a GR 650 or 1650, or a test setup of some sort that can make measurements at 1Khz. and resolve the inductive reactance. I suspect that most of the readers here either don't know what I am talking about (in which case, don't try making such measurements) or have the equipment to do this and know how to use it, so I won't go into detail.

NOTE that DC resistance measurements of coils DO NOT correlate to the the turns ratio or impedance ratio. Coils are wound with different sizes of wire, and the DC resistance will vary as a function of wire size and length, not the number of turns. Inductance and impedance are a function of the number of turns and the magnetic characteristics of the core, and measurements to get these values MUST be done using AC. Measuring input and output voltage ratios will give you the turns ratios. On a balanced center-tapped winding, the DC resistance of one side to the center tap will be different from the other side, if the winding is large. Reason is that the bobbin size increases, and it takes more wire length to get the same number of turns on the outside part of the winding than on the inside.
testing transformers
You can check out AC power transformers similarly. Coils should be continuous, and all coils should be isolated from the frame. 6 or 12 volts connected to the 120 volt primary should produce outputs from the rectifier plate winding and each of the filament windings. For example, a typical old radio with an 80 has plate windings typically 500 to 600 volts center tapped, to supply around 225-275 volts of B+ With 6.3 volts on the primary, you should read 25-30 volts on the whole secondary, and half that from each side to the center tap.