Identifying Power Transformer Windings

It isn't unusual for anyone who restores vintage tube equipment to have several power transformers that have either been removed from some old radio or other piece of equipment laying around. Due to the expense and availability of new power transformers, it would be nice to identify the windings and voltages of these spare units so that they could be re-used.
If the color-coding on the leads is still readable then you only have to refer to the chart below to determine the windings. But often the color coding on the leads of these transformers has faded to the point it is difficult or impossible to read the color code and know which winding is which.
By using the procedures outlined here, you should be able to identify the windings so the next time you discover the transformer in the set you are about to restore is defective, perhaps one of those spare transformers will qualify as a replacement unit.
Below is a diagram of a typical power transformer. This one has a primary winding, three center-tapped filament windings , and a center-tapped high-voltage winding . As you can see there are thirteen leads coming out of this transformer. The diagram gives the color code of the leads for each winding.

[Xfmr Diagram]

Identifying The Various Windings

On a transformer where the color code has faded to the point as to be unreadable, here is a method to identify the various windings. The ohm-meter is the test instrument we use to accomplish this. As can be seen from the diagram, each winding will have continuity from one lead of the winding to the other, but there should be no continuity between windings. Also, each set of windings will have a resistance value determined by the number of turns and size of the wire. So we can use the ohm-meter to determine which leads have continuity and use the resistance reading to determine which winding is primary, filament and high-voltage, ect.
To begin, set the ohm-meter to the scale that gives the lowest ohms reading, and calibrate the meter so when the test leads are shorted the meter reads zero ohms. The lowest reading scale is use as the windings will have very low resistance with exception of the high-voltage winding. Now pick one of the transformer leads and clip one test lead of the meter to that lead. With the other meter lead, begin checking the other transformer winding leads.
When you find the other lead of the winding under test, the ohm-meter will show continuity. Make a note of the resistance reading. With the first lead still attached, check all other leads to see if any others also have continuity with the first lead. If so, make a note of that reading also. If three leads have continuity , then this winding has a center-tap. Tag these leads as being checked, and proceed to check the other leads in the same manner, noting the resistance readings. After all leads have been checked and tagged, it can be determined which windings are which from these readings. Since the high-voltage winding has the most turns of wire, it will have the highest reading; filament windings have only a few turns of larger wire and will have very low readings, with the primary winding having a resistance somewhere in between. A lead that has no continuity with any other lead indicates an open winding. A center-tapped winding will give a resistance reading between the center-tap and the outer windings equal to approximately one-half of the resistance of the two outer windings, however, on filament windings the resistance may be so low that it may be hard to tell the center-tap from the outer leads. We will be able to determine which is the center-tap when we take voltage readings as outlined later.
The chart below shows typical readings for a transformer with a primary, high-voltage, 5-volt rectifier, and 6.3-volt filament windings.

Primary4 - 8 ohms
High-voltage200-400 ohms
H-V center tapHalf the resistance
of the two outer leads
5-volt rectifierless than 1 ohm
6.3 volt filamentless than 1 ohm

Note:These resistance's are approximate. Readings may vary by several ohms from one transformer to the next. The idea is not to be concerned with the exact resistance of the windings, but to identify the various windings by the difference in the resistance of the windings. (Refer to the resistance readings on the schematic diagram above for resistance's of an actual transformer )

Determining the Secondary Voltages

Caution must be used in the following procedure as dangerous voltages are present on the transformer windings when input voltage is applied.

Now that we have the various windings identified, what voltages does our transformer produce? To identify the voltages we must apply voltage to the primary and measure the secondary voltages. We could apply 120 vac to the primary and start measuring the secondary windings, however, our previous resistance readings tell us that the windings have continuity, but it is difficult to determine from these readings if a winding has shorted turns. Applying full input voltage may spell disaster in the form of a smoking transformer. The best way is to use a Variac (variable auto-transformer) which will allow us to vary the input voltage to the transformer, and bring the voltage up slowly (see note below).
Connect a line cord to the primary winding of the transformer (be sure to insulate the connections to avoid an accidental shock). Insulate the exposed ends of all but one of the secondary windings so they will not short together or come in contact with you. Electrician's wire-nuts are good for this, or use electrical tape. Attach an ac voltmeter to the leads of the secondary to be tested (for safety, use test leads with insulated clips instead of probes; this allows your hands to remain free of the leads). If the secondary being measure is a filament winding, set the voltmeter on a scale that will read 10 to 12 volts AC. Plug the primary of the transformer into the Variac (make sure the voltage is set to zero), and then adjust the Variac to slowly increase the primary voltage (some Variacs have a built-in voltmeter to monitor the output voltage, if yours does not, hook up a second voltmeter to do this). Note the reading on the secondary winding. If it doesn't begin to rise as the input voltage is increased, the transformer may have some shorted windings and should not be used. If the meter reading does increase, bring the voltage up to the normal input voltage (115-120 vac) and note the secondary reading. A filament winding should produce one of the typical filament voltages (1.5, 2.5, 5.0, 6.3 ect). On a center-tapped filament winding it will be difficult to tell which is the center tap lead from the resistance readings, as these values are very low. If you are checking a filament winding with a center-tap you can tell the two outer windings as they will give the highest voltage reading, with the center-tap giving half the voltage reading of the two outer windings.
Example: A 5 volt, center-tapped winding will show 5 volts ac between outer windings, and 2.5 volts from center-tap to each outer winding (since the transformer we are testing is not under load, the voltage readings may be slightly higher than if the transformer were under actual operating conditions).
After taking this reading, check the other secondary windings in the same manner taking care to reduce the input voltage to zero before each measurement. Make sure all the bare ends of the windings, except the one being measured, are insulated. Be very cautions when measuring the h-v winding as voltages may be as high as 800 vac. It is suggested that you take measurements between the center-tap and each outer winding. By doing this you will only be measuring one-half of the high voltage. Be sure and set the voltmeter on the proper scale to read this higher voltage. Both measurements from the center-tap to each outer winding should yield the same reading within a few volts. If the voltages are quite different, it may mean that one side of the h-v winding has some shorted turns.
Now that you have the windings identified, label each one and place the transformer in stock for that next set that needs a new one.
Refer to the transformer schematic above for typical voltage readings.

Note: An auto-transformer does not offer isolation from the ac line. For safety reasons, it is recommended that a 1:1 isolation transformer be used in conjunction with the auto-transformer.

Replacement Size

When replacing a power transformer, consideration must be given to the power handling capabilities of the replacement unit. One rule of thumb is, if the replacement units physical core size is as large or larger than the original, it will probably work. The true test however, will be if the secondary voltages are up to specifications and the unit does not run to warm when under operating conditions. A transformer should not be so warm that you cannot hold your hand on it after it has been operating for 20-30 minutes.
RMA (Radio Manufacturers Association)
Standard Power Transformer Lead Color Code
WindingColor Code
PrimaryBoth Black if not tapped,
if tapped other winding may be Red
Primary Tap (optional)Black/Yellow
High-Voltage SecondaryRed
High-Voltage Center-tapRed/Yellow
No. 1 Filament Secondary (5 volt rectifier)Yellow
No. 1 Filament Center-tapYellow/Black (see note)
No. 2 Filament SecondaryGreen
No. 2 Filament Secondary Center-tapGreen/Yellow
No. 3 Filament SecondaryBrown
No. 3 Filament Secondary Center-tapBrown/Yellow
No. 4 Filament SecondarySlate
No. 4 Filament Secondary Center-tapSlate/Yellow

Note: The number of filament windings can vary, and may or may not be center-tapped. Windings that are center-tapped, the center-tap will carry the same color as the winding plus a yellow tracer. The exception is the five-volt rectifier winding where the center-tap will carry some other color tracer such as black or blue.

RCA Power Transformer Lead Color Codes
Old and New
WindingOld RCA
Color Code
Color Code
Tapped primary
Start - redblack
Tap - red & blackblack & yellow
Finish - black & red tracerblack & red
Rectifier filamentgreen & red traceryellow
High Voltagebrownred
High Voltage CTbrown & blackred & yellow