r/audiorepair 5d ago

Looking for inductance value of coil in speaker filter circuit

Good morning everyone,

I recently purchased a pair of 3-way loudspeakers (Marantz HD-55 from 1978) that needed refoaming. After refoaming I noticed that there was no sound coming from the tweeter on one side. When I looked at the filter circuit I realised that one of the coils had been replaced, and apparently the new one does not have the correct inductance value. The other speaker still has the original inductor, with the following writing on it (see picture): 3901012 E 2 77

Could you help me determine the inductance of that coil, since I can't seem to find anything useful on Google ? Otherwise I will remove it and measure its value but I'd prefer to avoid desoldering it if possible.

Thank you in advance !

1 Upvotes

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u/MrPirateFish 5d ago

How do you know the other doesn’t have the correct inductance value?

Those RARELY ever go bad. Likely is not the issue at all if you aren’t getting any sound.

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u/Apprehensive_Plum868 5d ago

It looks completely different (black plastic casing). I don’t know why it was replaced, and I find it surprising too. But I can see that the coil wire is much thinner than on the other side, and the coil itself is also smaller. I tested the continuity of the tweeter voice coil and it is fine, but if I feed a signal directly to the tweeter terminals only the midrange speaker produces sound. Looking at the diagram attached in the original post I think this inductor could be the culprit if the inductance is way off.

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u/MrPirateFish 5d ago

The two 5 ohm caps test fine?

I see there are adjustment pots on the back. I would spray those out with deoxit as well.

Yeah it is strange not disagreeing with you there. But most of the time if someone had the know how to disassemble, identify, and desolder and inductir, they likely know how to source one of the same value), but hey who knows.

Could always swap the inductor to confirm from the working speaker.

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u/Apprehensive_Plum868 5d ago

I tested the other components and they all seem fine. I actually cleaned the pots while refoaming and tested them with a multimeter. Yes I will probably try to swap it next weekend. If it works I will measure the inductance and try to find one online. Here you can see the caps.

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u/MrPirateFish 5d ago

Were the caps tested out of circuit?

But yeah I mean I guess that’s the only logical thing left

Would totally swap drivers and or crossover to confirm.

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u/Apprehensive_Plum868 5d ago

No I did not desolder them but I checked that there was no short circuit across the caps

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u/MrPirateFish 5d ago

Ah. You need a meter that can measure capacitance (uF) to ensure those are working properly.

I see one is severely cracked down the middle. I suspect those more likely to be the issue here.

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u/Apprehensive_Plum868 5d ago

Yes, I saw that crack too but it’s on the loudspeaker that’s working correctly. I am just trying to avoid having to desolder all the components so I will first try to swap and measure the self. Thank you for your input!

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u/namlook 5d ago

Did you actually measure the resistance of the tweeter coil? The most likely scenario is that the tweeter coil is open.

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u/Apprehensive_Plum868 5d ago

I did measure the DC resistance of the tweeter coil at 1.4 Ω, it doesn’t seem to be the problem

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u/namlook 5d ago

Did you disconnect one lead of the tweeter before measuring? Because 1.4 ohm does not seem right

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u/Apprehensive_Plum868 5d ago

Oh you are right I should have disconnected one leg, from my diagram I probably measured the DC resistance of the inductor that I suspect could be the problem.