2019 June 10     Tech,     Electronics


Some components, like capacitors, are easy enough to replace in a radio. 

Others, such as IF cans or power transformers, are a lot more difficult.

And if you're trying to refurbish a piece of vintage electronics, there's not much point in re-capping or anything else until you know the transformer is OK. 

Here are five methods for testing one.

(WARNING:  Electronics repair can be dangerous.  Please read the Disclaimer.)

In This Article

Safety First


Ring Tester

Insulation Tester

Light Bulb

Watt Meter


Safety First

Don't work on electronics unless you know what you're doing.

Don't put DC voltage to a transformer;  you'll overheat it and burn up the windings.

Don't put line voltage to a transformer unless it has the correct fuse in-line with the power.

DON'T DO ANY OF THESE TESTS ON AN ENERGIZED TRANSFORMER, except where it actually says to do that. 

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This is the most basic test.  It's limited in what it can tell you, but it's still worth checking.

Be sure the transformer is unplugged when you do this.  Make sure you discharge all capacitors before working on the unit.

This test can show if a transformer is "open circuit", which is uncommon but does happen.  It can also find partial or dead shorts to the iron core or the transformer casing. 

It does not have the precision to detect a short between windings.  But it does help to know the winding resistance.  This can help make sense of other tests you might do later.

A power transformer primary might be anywhere from 10 ohms to perhaps 200 ohms.

A high-voltage secondary, the kind that might power some vacuum tubes, could be anywhere from 100 ohms to, say, 500 or 600 ohms. 

A low-voltage secondary, such as a 3- or 6-volt "heater winding", can have really low resistance:  maybe 0.2 to 5.0 ohms.  It will seem like there's a dead short, but that's not how it will act toward AC voltage (unless there really is a dead-short).  AC impedance doesn't necessarily have anything to do with DC resistance.

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Ring Tester

This one puts a low-voltage pulse to the transformer, then determines how fast the ring voltage decays.  More ringing equals higher "Q factor".  This is a fairly reliable test, because shorted windings will greatly reduce the Q factor.

The main drawback is that some transformer windings are "low Q" already.  So they will appear bad when they're not.  High-voltage secondaries could be low-Q because they have many turns of thin wire (high ohms).  A very low number of turns can also have low Q, because of low inductance.

Other components connected to the transformer secondary can also affect the reading.  For best results, just test the transformer when it's disconnected from everything else but the tester.  Or at the very least, pull all the tubes.  This may be enough; it depends on the circuit.  Check the schematic if you're not sure.

A big advantage is that it uses such low voltage, it won't fry anything even if the transformer is shorted.  (As usual, make sure the transformer is unplugged before testing.)

I'd highly recommend one of these.  Through this link you should be able to find fully-assembled ones, or kits that you can put together yourself. 

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Insulation Tester

A regular ohmmeter can find shorting to the core or the metal casing of a transformer.  But sometimes, insulation breakdown is still beyond the range of an ohmmeter.  In fact, a ring tester might not even find the problem, because it uses such low voltage.

You might still want to look for these types of issues, because they can worsen when there's current flowing.  This is why we use a megohmmeter or insulation tester.

Megohm meters used to be expensive, but there are new ones now in the $50 to $100 range.  Try this link or this one.  Even genuine Megger test meters often sell used for $100 or so.  If you test transformers, motors, compressor windings, etc., one of these can be extremely helpful.  Or if that's your daily job, consider getting a Fluke.

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Dim Bulb Tester

At first you won't know if a transformer draws "too much current" or "WAY too much current". 

The dim bulb tester is a good way to find out.  You can make one of these yourself... or save time and just buy a ready-made tester.

Disconnect all vacuum tubes from the radio you're testing.  We want to know if the transformer passes too much current, not whether the rest of the circuit does.

Use an INCANDESCENT light bulb that's 1 1/2 to 2 times the wattage rating of the radio.  This bulb, in a receptacle, is wired in series ("in-line") with the AC hot that's going to supply the transformer.  If there's a dead short in the radio circuit, the bulb will limit the current. 

The bulb should not light much, if at all, if the transformer is OK.  If the bulb stays fully lit for more than 5-10 seconds, you either used too low a bulb wattage... or the transformer is shorted. 

If the transformer passes the bulb test, it might still draw a little too much current.  That's why next, we use a watt meter.

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Watt Meter

A bad transformer could have internal shorting that just draws a little too much current.  (Unloaded, most radio transformers draw less than 200 mA).  Too much current means it will use too much power. 

The wattage rating provided by manufacturers is only for the intact radio.  The unloaded secondary will not draw as much power as the whole radio.  It really should not draw more than about 10 watts, ever.  Typically it's more like two or three watts, if that.

Pull all the vacuum tubes before trying this.  As usual, make sure there are no shorted capacitors, cracked power cord, or anything like that.  Then test the transformer's power draw with a plug-in watt meter.

The dim bulb test should be first, just to make sure there's no dead-short condition.  Some watt meters may lack an internal fuse, so really I'd only use it to check for shorted windings that are not catastrophic.  Another method is to use a variable autotransformer and a watt meter, bringing up the AC line voltage slowly.  The watt meter has to be on the WALL side of the autotransformer;  running one at too low an AC voltage can ruin the meter.  Make sure to read How To Use an Autotransformer With a Watt Meter.

Watt meters are very low-cost and they're quite useful.  Get one now

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This was a look at five ways to test power transformers.  Even if they can't always identify every bad one, they sure help.

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Another Article You Might Like:  Fixing The Solar #1002 Battery Charger... Sort Of

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