After many years of collecting dust, I have finally completed the restoration and modification of my JBL L212 system. It was originally purchased by me in the late '70s. After many hours of sonic experimentation, I added time alignment plates, fabricated from solid walnut planking to match the wood finish on the originals. This was done soon after I purchased them, and significantly improved the depth of the soundstage and imaging.
One of the weak points of this system, in my mind, was the cross over network. I had always felt the quality level of the components was never quite up to the quality of the drivers. Originally, I had tossed the stock crossovers for a tri-amped active crossover system, but never got around to building and purchasing all the electronics required. So the system sat in limbo and failure of the surrounds on the 121A sub bass driver and the two 112A bass drivers stopped things for quite a while. When I decided I wanted to get these back in action, I had new surrounds put on the drivers. Luckily, I was able to keep the original cones and spiders.
I constructed new crossovers from two stacked 2mm thick epoxy glass turret boards. The inductors I used where Jantzen and Erse air core 18 ga. To simulate the low Q versions JBL originally used, I added non-inductive resistors of the appropriate value in series with the inductors. The capacitors were upgraded from the mylar film types JBL originally used to polypropylene film types. I used the OEM schematic for the crossover network, including the original Zobel (driver impedance correction) networks. I wired all the drivers with 14 ga (which is the largest wire size that will fit the spring terminals on the drivers).
I relocated the LPads from the front to the rear and placed them on a panel mounted internally. The time alignment plates had blocked access to the mid-range adjustment L-Pad on the original positioning.
Here are some photos of the finished project: