Electrodynamic

This page describes how to test an Electrodynamic speaker. The Electrodynamic speaker has a field coil to generate a constant magnetic field that the voice coil opposes. These are basic Go No-go tests and not the extensive frequency and impedance response tests. For extensive speaker tests find a Speaker Cabinet building book. Radio Shack and Sam's (of the Sam's Photofact line) sold good beginner volumes. Two other books are pictured below.

These tests are typically done in the cabinet. But a proper self respecting restorer will pull the speaker to clean the dust out.

First, clean-up and physical movement check.

This speaker is from a Philco 39-30.	The Voice coil gap is exposed. No dust cover.	Knock off the dust being careful of the dry brittle delicate paper cone.	. Carefully clean the dust from around the voce coil gap. Use low pressure air or gently blow out the dust from the gap.
Clean the dust from the back side.	Clean. No debris to buzz and rattle	Gently move the cone in and out. Listen for any scratching.	I have used rubber cement and Tea bag paper to patch tears. The added mass will lower the resonant frequency of the speaker. But you will not notice. Google speaker repairs for more ideas.

Figure out where the wires go.

The Field coil is independent of the other two coils and has two dedicated contacts on this particular speaker. The Voice Coil is in series with the Hum Bucker Coil. The red wire connects the Voice Coil and the Hum Bucker. This pair will be tested together with an ohm meter.

This speaker has three coils: Field Coil Voice Coil Hum bucker coil	The red wire that goes from the coil to the contact on the speaker's metal frame comes from the Hum Bucker Coil.
The arrow points to the Voice Coil. The Hum Bucker is below in series. Colors are indicative of the cable coming out of the chassis to the speaker connector and not the colors of the wires used on the speaker its self. To se the whole schematic click here.
The field coil #44, is connected in series with the B+ high voltage circuit. The current flow through the coil to the rest of the circuit creates the magnetic field the voice coil's varying field opposes. The hum bucker cancels out any induced 60/120hz ripple hum. To se the whole schematic click here.
This clip is from a Philco 38-4. It has the DC resistance of the field coil marked on the schematic. The colors are for the wire going from the chassis to the speaker jack.

Test the Voice Coil and Hum Bucker Coil with a digital meter.

Green - Voice & Hum Bucker

This Voice & Hum bucker coil reads 4.9 ohms and is good. Below 10 ohms is good. This is NOT the AC impedance (specs).

Bench speaker. Voice Coil only is 2.4 ohms.

If the Voice Coil/Hum bucker combination is open:

Check the individual coil separately.
If the voice coil is open try re-soldering all connections. If that does not work and the voice coil is open then the speaker needs re-coned.
If the Hum Bucker is open try to pull off some of the tap/insulation and look for a bad wire near the surface.

Voice and Hum Bucker with an analog meter (VOM).

Put the meter on the times one (X1) scale.

Short the leads. See this meter does NOT read zero (0) ohms.

Correct this with the zero control. If the needle does not reach 0 then the battery is weak.

About 4.5 ohms. Good!

Field Coil test with a digital meter.

Red wires - Field Coil	This Field Coil is bad. It should read about 700 to 2000 ohms. Most of the time the DC resistance is marked on the schematic above. +/- %20 tolerance is good.
This is my test bench Electrodynamic. It has a good field coil reading 1,145 ohms (1.14 k ohm).

If the Field coil is open or shorted:

Try re-soldering all connections (open).
Pull off some of the tap/insulation and look for a bad or a burned wire near the surface.
If that does not work and the field coil frame is welded toss it in the garbage. There is not much you can do to fix it.

A bad Electrodynamic can be replaced with a permanent magnet speaker and the equivalent wire wound 10 watt high power resistor in place of the field coil. Use Ohm's Law formulas to determine exact wattage and add at least 20%. Place it where the heat will not damage anything.

One caveat: If you plan to use a PM speaker put larger capacitors in the circuit. For example, capacitor # 43 in the above schematic clip, is 5uf should be replaced with a 10 or 12 uf (the next standard value above 5uf). You will probably will replace them with larger values any how.

Field Coil test with an Analog meter (VOM)

Switching to the x 100 scale the needle swings off scale to the stop (battery is good).	Zero the meter.	Reads infinity on the original speaker. BAD.	Increase the scale to the x10k, short leads to zero and test. 32 times 10k is 320k. Way too high. BAD.
Lets test the bench speaker to and see a good field coil resistance.	Reset the meter to x10 and set the Zero.	11.5 times 100 is 1150 or 1.15 k ohms. The bench speaker is good.

Lastly, check for shorts to the metal frame.

Establish a good connection to the metal frame. Read zero ohms (some meters read the lead resistance at less than one ohm).

Test each point to the frame. Should read infinity ohms.

Click the Philco Repair Bench web site to see typical speaker specs.

A Homebrew Universal Shop Speaker and Dummy Field Coil by:

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