V-TWIN RACKMOUNT PREAMP
The MESA BOOGIE© V-TWIN PREAMPLIFIER was available in a Rack-Version and Pedal-Version. (I have both types, but, here is the info on the Rack-mount unit...)
The Rack-mount version has 2-Channels with 6-Modes… any of those channels/ modes are available and selectable separately via front-panel toggle switches or via six ¼” phone plug tip-to-ground contact closure connectors on the rear-panel. Those rear-panel connectors also allow you to mix Channel 1 modes with Channel 2 modes to get different sounds using an external control switcher and activating two of the Tip-to-ground contact closure connectors.
External Control Switching Panel
This rear-panel Tip-to-ground connector panel was an option from MESA©, my second-hand preamp did not have the connector panel, just a blank cover plate, meaning that all channel switching had to be done manually from the front panel or with the included 2 button footswitch.
I fabricated a custom printed circuit board (PCB) for the six ¼” phone plugs and a 5 pin midi connector for installation on the blank panel. I had the stock blank cover plate Laser engraved by AD Engraving and drilled it for installation of the custom PCB 1/4" connectors. I purchased an 8 inch 0.100” 8-pin header cable for connection of the custom switching panel to the V-TWIN circuit board. All of the components for the switching circuitry were installed on the PCB from the factory, so, no PCB mods or component additions were required.
Overheating of Current Limiting Resistors
After inspecting the V-Twin circuit board, I noticed some burn marks on the PCB around some 3 Watt resistors near the power supply. To increase the reliability, I removed those 3 Watt resistors, cleaned the PCB and replaced all of the power-resistors with 5 Watt Versions. I didn't want to further load the existing V-Twin transformer/ power supply with the new MIDI Control Board, so, I installed a separate circuit board with a torroidal transformer, rectifier & regulator to power the MIDI control Circuitry separetely.
Those power resistors still got hot so I ventilated the chassis by cutting vertical slots in the chassis and added a small 5 VDC brushless fan to blow cool air over the resistors... much cooler now.
MIDI Control Board
MIDI capability was added to the V-TWIN preamp to allow any single mode selection or a mixed mode selection of any 2 modes from the 2 channels.This was facilitated by the installation of a Highly Liquid© MSA-R MIDI Control Board and connection to a rear-panel MIDI connector and Internal 9VDC Power Supply.