Replacing 1.3 mH inductor...Help Pls

[baco99]

I have to replace 2 inductors in the TF-1000s. One is a 1.3 mH metal core inductor in 20 ga. wire. The other is a 3.5 mH metal core inductor in 22 ga wire.

L1 and L2 in this schematic:

My question is, if I buy a 1.3 mH and 3.5 mH inductor to replace these, can they be air core? If they are sold as air core, can I use the metal slug from the old one and insert it into the new one or will that change the inductance? Lastly, how important is the gauge wire? I know it will affect the DCR of the inductor, but is 18 ga instead of 20 ga close enough?

Thanks!

 

[elnaldo]

In those large values, air core inductors will become very large and expensive. You can use them, air core are better than iron core for certain applications. If in a budget, I'd use air core in L1, what is in the direct signal path.

You can´t place the iron core in an air core inductor, it will change the inductance. Iron cores are used so you need less copper winds to get the same inductance. So iron core inductors are cheaper.

Wire gauge affects the resistance of the inductor, so read the specs of an iron core inductor with the original gauge (most vendors will detail the resistance in ohms) and compare it with the replacement you want to buy.

BTW, what's wrong with the original inductors? They usually last forever....

 

[baco99]

In those large values, air core inductors will become very large and expensive. You can use them, air core are better than iron core for certain applications. If in a budget, I'd use air core in L1, what is in the direct signal path.

You can´t place the iron core in an air core inductor, it will change the inductance. Iron cores are used so you need less copper winds to get the same inductance. So iron core inductors are cheaper.

Wire gauge affects the resistance of the inductor, so read the specs of an iron core inductor with the original gauge (most vendors will detail the resistance in ohms) and compare it with the replacement you want to buy.

BTW, what's wrong with the original inductors? They usually last forever....

The inductors blew out. Melted together.

So, if I got new air core inductors in the same values as the originals, i should be OK, except the DCR of the coil might be slightly different.

From experience the DCR doesn't vary all that much from one AWG from another unless you drop or raise the AWG by a few steps at a time. Also, since these are metal core, they probably have fewer windings than air core, which means a lower DCR. So going up AWG with air core might result in the same. I'll measure the originals on the other speaker and see what the DCR is for both coils.

I also assume I won't need the alum ring if I use air core.

 

[elnaldo]

That aluminum ring seems to be a shield for the inductor. If you don't use it, place the inductor further from the other components, specially far from the other inductors.

I'd stay with iron core in the 3.5mH and keep the money for a recap....

Try to choose the awg trying to match the resistance.

 

[baco99]

That aluminum ring seems to be a shield for the inductor. If you don't use it, place the inductor further from the other components, specially far from the other inductors.

I'd stay with iron core in the 3.5mH and keep the money for a recap....

Try to choose the awg trying to match the resistance.

Yes, new Mundorf caps have already been ordered.

I have decided, since these were originally hand wound, and since I cannot find an exact replacement, I am going to hand wind my own.

The 3.5 mH coil had 330 turns. I have a pile of coated copper wire from a prior DIY project that I found in the trash 10 years ago.

This is 128 turns in. 1/3 of the way there.
The wire thickness is slightly off. I think this is 18 or 19 ga. But I wasn't going to get it perfect anyway.

If the real deal pops up, I might switch these up again in the future.

 

[wweigle]

I have seen a couple of reasonably intuitive web sites that will help you calculate winding DYI air inductors. Basically, if I remember correctly you input the wire size and your stated inductance, and it will tell you your spool size, and the number of windings. I think one site I found allowed you to solve for any of the variables with at lease one variables and your goal inductance. I found these sites when I was thinking of trying to wind some DYI inductors also, but the project never came to fruition. Unfortunately, an air core will require a lot of windings for the values you need so your strategy is probably the best way to go.

 

[baco99]

At the urging of someone on the AVS Forum who used to work at DCM and helped design these speakers, I ordered the proper magnet wire for rewinding the coils.

Will take some shots of progress once the wire arrives.

 

[elnaldo]

I think this will work. I was to suggest you to unwind the coil, re-varnish the wire, and wind it again.

Apart of the number of turns, I'd measure the wire lenght. I've found a relationship between wire lenght and inductance. If you take the on-line calculators, different coil sizes, with the same wire lenght, end in a very similar inductance value.

 

[baco99]

I think this will work. I was to suggest you to unwind the coil, re-varnish the wire, and wind it again.

Apart of the number of turns, I'd measure the wire lenght. I've found a relationship between wire lenght and inductance. If you take the on-line calculators, different coil sizes, with the same wire lenght, end in a very similar inductance value.

understood. if the gauge is the same and the # of turns are the same, the length should be the same as well, or close enough.

 

[KingBubba]

I'm surprised that DCM gave up on the protection circuit using a bulb or a bulb and a relay/circuit breaker like on the TF-600. My guess would be the son just cranked yours too far with an under powered amp and got the mess you pictured here. What a shame.

 

[baco99]

I'm surprised that DCM gave up on the protection circuit using a bulb or a bulb and a relay/circuit breaker like on the TF-600. My guess would be the son just cranked yours too far with an under powered amp and got the mess you pictured here. What a shame.

I'm not sure. I think the 2 ohm resistor failed first, which caused the coils to be overdriven.

The speakers actually still worked. It looks like the signal was just being passed to the woofer full-range.

The relay/breaker was used to protect the tweeter. But I agree, interesting to not see it here. Maybe the complex timing circuit acted as a choke for signal spikes that could damage the tweeter.