October 23, 2014

The driving forces behind the two major improvements to the BMX-III Control Room module were the increasing costs of the P&G rotary faders, and long term reliability of the 12 station selector. The P&G rotary fader is an excellent product but our cost for these stereo faders has risen to $187.00 each. The pot we selected along with high quality VCA’s from THAT Corporation,made possible a retrofit to be added to this very important console module.

The 10k rotary pot we chose is made by Vishay/Spectrol. It’s really a servo pot and has no mechanical stops. We solved the stops issue by modifying the Control knob and faceplate-extrusion assembly. If you look closely at the bottom of the Control Room knob skirt and the Headphone knob skirt, you’ll see our modifications. The Knobs get a cotter pin and the extrusion gets a 4-40 set screw. An 11,000,000 rotation life is specified for this pot. It is used for precise positioning of industrial manufacturing equipment.

The cue and talkback pots are 10k linear off the shelf parts with conductive plastic elements.

We have modified the front panel extrusion assembly with our Poly Carbonet inlay. The extrusion has been modified to include knob stops for the Control Room and Headphone servo pots.

Compare the J numbers for the various audio sections using the PR&E Control Room monitor module schematic with the VCA color code sheet. For example VCA Left in, the violet wire connects to J6-3. This is the audio that fed the top of the old stereo pot. The left output of the VCA, violet/black connects to the wiper at J6-2. The VCA simply substitutes for the original pot connections. Audio common has (1) connection to the main PC board audio common edge connector. +16 volts (Red) is tack soldered to the LM-317 regulator output. The -16 (Black) is tack soldered to the LM-337 regulator output.

We have included (2) photos showing the pot wiring and the audio wiring. Schematics are also included for the typical VCA section and the 12 station selector schematic. This information should be kept with the BMX-III/AMX manual.

We pre-test the VCA assembly here at MOORETRONIX to verify the wiring and audio attenuation.

The Monitor Selector PC board assembly is now discussed.

We have been using a surface mount relay for high reliability switching applications to replace the A/B input switch and the PGM/AUD/UTL bus assignment switches on input modules. These relays are rated for long and reliable service. This relay was designed for industrial tel-com use. It made sense to use these relays to replace the 12 station mechanical selector for long term, noise free switching. To replace the wink-eye lens caps we chose the ITW LED/Switch used in the PR&E RadioMixer console.It’s used for cue, timer control and telco selection functions. This switch has proven itself over time for reliable operation.

We wired audio through the relays in the same way it is done with the PR&E 12 station switch. If you look at the schematic for this assembly, you’ll note that the audio bus routes through all 12 stations and terminates into a short circuit. This was done to reduce cross talk of the high level audio sources running +4dB levels and higher, to a minimum. Only the selected audio source input is terminated into the left and right audio buses. This method is used on all PR&E audio selectors. As we mount the relays from position #1 to 12, we test for this short to be sure all surface mount connections are soldered.

The CMOS logic to drive the relays uses a CD4514 selector fed by a simple switch/diode matrix. The selected BCD code causes the designated relay pair and LED position indicator to operate. Timing functions are completed by the CD4538 chip. This simple logic circuitry produces a highly reliable switcher. No more switch latching problems, or lens caps popping off the switch post and the end to intermittent audio switching.

Please call us with any questions you may have about the VCA board or the 12 station selector module.

Thanks

Bob Moore

MOORETRONIX

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