Electronics dunce needs help repairing crossover / LPF


2007-09-21 11:42 pm
I have come into a pair of early-90s biampable bookshelves from a Reputable Maker that need crossover-board repair. One speaker will not pass signal to the woofer through the binding posts / LPF network, although the woofer is working fine. I cannot find any obvious dead or broken solder joints in examination or testing, so I assume we must have a bad component somewhere.

Here is the schematic for the LPF board that the Reputable Maker was nice enough to send over to me (and no, they no longer have the parts or boards for sale):


I have had one pair of these speaks for almost twenty years as my studio monitors, so it is important to me to maintain the exact same sonic signature, or as close to it as possible, as I had on my other pair. Certainly I also want these bad boys to match each other when my repairs are done, so I assume I will have to do to one what I do to the other.

Now the questions begin:

- In case it turns out to be more practical to go active / biamp than to mess with this: anyone able to tell, within reason (and without knowing exact driver specs, as these are unknown), what frequency this goes over at, and how deep the dropoff is meant to be? I can find lots of info on network types and cookbook schematics for two-way crossovers, but not for LPF-only sorts of contraptions like this.

- I have no idea how to start testing components on the board to find the bad one. I have a multimeter, a soldering iron, and no real idea what to go looking for (i.e., I know how to resolder stuff that falls off, but I know almost no useful electronics theory as it tends to break my brain whenever I try to learn in earnest). What should I do? Am I right in guessing that if any of the three parts has gone bad, the inductor is probably the issue?

- If it *is* the inductor... where on earth do I *find* a new inductor to replace this thing? I have checked a number of big usual suspect suppliers and they don't have anything even close to this; I can get the value, but not the same wire gauge, and I don't know how that will impact the performance. What specs matter the most for finding a drop-in replacement, and which ones matter the least?

Thanks in advance for your help on these (I know, unfortunately very broad) questions.
I suspect inductor gone open circuit. Obtain a pair of new ones with the same inductance and DC resistance as what is on the drawing and replace on both speakers. When looking for inductors also look if wire gauge or current rating is specified. What power are the speakers rated at? You probably want something that can handle 3 amps or more, and has ferrite core not iron - if you can take a picture of the original one we may be able to identify the core material.
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2007-09-21 11:42 pm
Thanks richie-- as shown in the schematic above, the wire gauge is specified at 22awg, which is where I'm worried. I cannot find a 1.25mH inductor at anything less than 18awg (and man, they aren't cheap either) and not knowing much about this circuit or how it actually works, I don't know how this will affect crossover frequency or anything else if I have to go up to a higher gauge.

The speaks are rated at around 60-80wpc, if I recall; I usually use them nearfield and feed them with TA2020- or TA2024-based amps, so they don't need to handle even that much, frankly.

Here's a pic of the boards; sorry, they are just what I have handy (pics I took a few weeks ago) as I'm at the office right now and don't have them handy or disassembled. The board that has the issues is to the bottom / left.



2007-09-21 11:42 pm
Thanks for the info; I didn't think wire thickness would impact much (aside from board fit) but I couldn't be sure. (Also, I do have a bad habit of calling thicker wire "higher gauge", even though it's not, uh, "quantitatively" higher; if you go *up* in diameter and cost... anyway).

I'm still having an issue sourcing anything like the original spec even if I ignore the wire gauge. The current closest I can come is a 1.2mH with .8 ohms DCR (in a 20-gauge wire).

I also still wonder what frequency and slope this thing is operating at, so that if I need to throw in some other solution altogether, I can do it. Is there any way to figure out the freq / slope information from what little info we currently have?
Do check the wiring carefully as it may be a bad connection. Unsolder both ends of the coil - Use the meter set on Ohms at a full scale of @ 200 ohms - and measure the coil
Overrange - the coil has had it - Replace
Madisound Air Core Inductor have some

a low ohm measurement, the coil is OK but its connections to the rest of the world must be faulty, use the meter to measure resistance from the point the coil wire connects to the board to where the cable comes in from the amp, look for a break somewhere. try and trace the path of the connection looking for bad joints.


2007-09-21 11:42 pm
Thanks to both Spiny and VictoriaGuy. The speakers were in fact damaged as a result of a horrible, horrible packing job and subsequent shipment; I am quite sure that's what busted up the board, but there's nothing visible. I will be giving the coil a test per Spiny's very helpful instructions, but I still can't find a new replacement (on Madisound or elsewhere) with the same mH and DCR values or even close. I've written a couple places to see what it would cost to get some made.
I am quite sure that's what busted up the board, but there's nothing visible.
My advice:
Get the coil unsoldered and test it, before ordering a replacement. It's hard to imagine bad packing causing a coil to go open.
Broken PCB traces and cold solder joints are often hard to detect.
You don't need a PCB for that crossover circuit - just assemble it on a piece of wood if necessary. It looks like the crossover is hung from the speaker jacks (?) - move the crossover to the bottom/side of the box and run wires to the jacks.