Bogen MXM Modifications
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Since my friend wanted his Bogen MXM to be a useful studio tool rather than just a restored antique, he agreed to let me drill any holes and make any alterations I deemed important towards that goal. (This type of agreement has been dubbed "Clark blanche." It works best when backed up with a substantial budget.) Here are the modifications I made, in rough order of decreasing severity (compare the "before" and "after" block diagrams in Figure 1):
1. Addition of post-fader direct channel outputs. In the multitrack recording studio, the MXM's five mic preamps will be most useful when their outputs can be accessed independently of the unit's master section. I employed op-amp-based output buffers for this function. These buffers put a negligible load of 1 M-ohm on the wipers of the rotary channel faders and provide low-impedance, ground-referenced ("unbalanced") outputs at 1/4-inch phone jacks on the rear panel. A sixth op-amp buffers the master output signal and drives the 1/4-inch ground-referenced mix output jack in the modified MXM. To avoid putting any additional load on the MXM's original power transformer, I installed a separate transformer-isolated power supply for these buffers.
2. Channel 1 simplified. The twin-triode-based versatile input channel (Channel 1) had provided three input options: auxiliary, mic, and phono. I simplified this to just two options (mic and "line"). This eliminates numerous signal-path transits between mechanical contacts of the original 3P3T rotary switch, and eliminates all (old-style) phono EQ from the channel. The new 1/4-inch "line" input has a high impedance (1 M-ohm), suitable for magnetic pickups, harmonica microphones, and most other monaural ground-referenced sources. Throwing the new toggle switch on the front panel simply switches the preamp's input to the channel's mic transformer for the mic option.
3. 120-VAC primary circuit updated. I replaced the original non-polarized line cord with a standard IEC power jack, eliminated the built-in AC outlet, and located a main-power toggle switch on the rear panel (instead of the original switch co-axial with the master volume control). I also added a ground-lift toggle switch to the rear panel, which normally links the IEC jack's ground pin to the modified MXM's star ground (see #6, below).
4. "Remote volume control" jack eliminated. A remote volume control is neither available nor desirable. I permanently eliminated this 9-pin socket and its associated wiring to the four pentode channels.
5. Gender interchange for mic inputs jacks. Although aesthetically pleasing with their big round flanges, five original Cannon-manufactured male XLR jacks on the MXM's rear panel were for mic inputs. I replaced these with Neutrik female XLRs to accept today's standard balanced mic cables. I left the original male balanced output jack in place.
6. Better grounding and capacitor coupling. Since I had to remove all of the original hookup wiring, I re-wired the MXM with more stringent grounding rules than the original. With a few exceptions, I ran all ground connections individually to a single terminal, which is bolted to the chassis at one of the power transformer's mounting screws (i.e., a "star ground"). Notable exceptions included audio transformer shields, whose corresponding socket terminals were left soldered to the chassis plane adjacent to their socket. I replaced all signal-coupling capacitors with new high-quality polypropylene units.
Next: Mechanical Alterations and Power Supply Modifications ==>>
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