May 9, 2015
Instructions for VCA installation into BMX-III Control Room Monitor module
The MOORETRONIX VCA PC board assembly consists of four sections. Section 1 is for Mono or Stereo Cue depending upon Control Room module application. Section 2 is the Stereo Headphone VCA. Section 3 is the Stereo Monitor VCA. Section 4 is dedicated to mono Talkback VCA control.
A THAT Corporation 2180 VCA in combination with a 5532 or LF353 IC forms the VCA control circuit. Two 2180’s are required for a Stereo VCA. The use of a Vishay rotary control for Headphone and Monitor yields 11,000,000 rotations before failure. Good luck on trying to wear them out. We use military grade, single turn, trim pots for Cue and Talkback control. A wiring harness with the level control pots is pre-wired. We have marked the pot names for easy location and installation to the front panel. The harness also contains the inputs and outputs for the 4 control sections.
The original audio pots had a ground wire, ccw terminal. The cw terminal was supplied audio to be attenuated. The wiper or center terminal provided the variable level control. The VCA inputs for each section are wired to the audio to be attenuated. The VCA outputs replace the wiper terminals of the old audio pots.
It will be necessary to remove the old pots from the front panel. Also remove the pot wiring from the Control Room module circuit board. The holes must be open at the various J connection locations. Use solder wick or a de-soldering tool to clear these holes.
A 3 wire cable is provided to supply power to the VCA PC board. The cable has a 3 pin connector at one end that mates with the connector on the VCA board. The other end of the cable is cut to length and solder tinned. These wires are tack soldered to locations on the Control Room module and described below.
The photo shows the location of controls and audio signals
The far left pot is for TALK, the next pot is CUE, then HEADPHONE pot and MONITOR pot.
The audio groups are Headphone ins and out’s. Next is left and right CUE audio. Talkback in and out is above the CUE pot. The Monitor left and right inputs and outputs are between the Headphone and Monitor pots. The table below shows wire color from the VCA harness and the J numbers are located on the Control Room module.
The pin-outs are as follows:
Left in Left out Right in Right out
Headphone audio Red J8-3 Red/Blk J8-2 Org/Blk J8-6 Pink J8-5
Cue audio Green J7-3 Lt/Grn J7-2 Blue J7-5 Lt/Blu J7-4
Talkback audio Brown J13-3 Orange J13-2
Monitor audio Yellow J6-3 Violet J6-2 Gray J6-6 White J6-5
The VCA PC board has (4) IC’s in sockets. The Cue and Talkback IC’s are type LF-353. The Headphone and Monitor IC’s are type NE5532. The 22pF compensation capacitors are mounted on the bottom of the PC board next to the IC socket locations. These are very tiny parts that can be seen using a magnifying glass.
The 3 pin power cable to power the VCA must be installed to locations on the Control Room module PC board.
The Green wire is inserted into the small hole in the edge connector pin as shown. This pin is below the regulator section. The two wires to the left of RN200 are the wires that power our 12 station monitor selector board.
Two traces are between the LM317 regulator and the K7 IC relay. It will be necessary to scrap the solder mask about 1/16” from the trace where the Black and Red wires will be tack soldered. The wires are just above and to the right of the LM317 regulator.
The Black wire is tack soldered to the trace closest to the K7 IC.
The Red wire is tack soldered to the trace closest to the LM317 regulator.
Use the ty-wrap supplied to secure the cable to inductor L7.
The mounting holes shown are counter sunk for the 4-40 flat head screws. The lower hole is located ½” from the bottom and .15” from the extrusion edge.
The upper hole is 3.15” from the lower hole and .15” from the extrusion edge.
This photo shows the VCA board mounted to the extrusion.
The new pots mount to the front panel using the hardware supplied. The bottom nut stays in place for proper spacing. The top nut secures each pot in place.
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