The Elusive Monitor 12

westmassguy
westmassguy Posts: 6,850
in DIY, Mods & Tweaks
These are certainly rare, and an odd bird with respect to the crossovers. There are two versions, the original and revised. This set of crossovers were the revised version. They added a current sensing contour network to protect the tweeter. I had always thought Polyswitches were eliminated by 1990, however the revised crossovers added a Polyswitch shunted with a 4.2uf Capacitor. I had intended to replace the Polyswitch with a .5 ohm Resistor, which is what we normally do, however this would nullify the protection aspect, and add yet another Contour Network downstream from the first. I contacted Ken, who explained its purpose. We both agreed, instead of replacing the Polyswitch with a fixed Resistor, just eliminate the network altogether. The Crossover would then be identical to the original Monitor 12.
This particular pair used the new, square circuit board, so mounting components would be more challenging.
Clarity ESAs were chosen for the Hi and Lo-Pass, Mills Resistors, and Cardas Binding Posts rounded out the remaining upgrades. All the original inductors were retained. Clarity doesn't make a 16.4uf Cap, so two 8.2ufs in parallel were used for the Lo-Pas Shunt. The Mills Resistors were relocated to the bottom of the board. New heavier lead wires were installed along with the Cardas Binding Posts. It still amazes me that Polk used #18 gauge lead wires from the Binding Posts to the Circuit Boards on all the old Monitors.
All new components were secured with 3M VHB Tape, Nylon Wire ties, and Cardas Quad-Eutectic Solder was used throughout.

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Home Theater/2 Channel:
Front: SDA-2ATL forum.polkaudio.com/discussion/143984/my-2as-finally-finished-almost/p1
Center: Custom Built forum.polkaudio.com/discussion/150760/my-center-channel-project/p1
Surrounds & Rears: Custom Built forum.polkaudio.com/discussion/151647/my-surround-project/p1
Sonicaps, Mills, RDO-194s-198s, Dynamat, Hurricane Nuts, Blackhole5
Pioneer Elite VSX-72TXV, Carver PM-600, SVS PB2-Plus Subwoofer

dhsspeakerservice.com/
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