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.
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.