Bogen MXM Modifications
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Disassembly, cleaning, and new holes. Confident in the accuracy of my schematic (Figure 5, which turned out to be in good agreement with Bogen's published schematic), I systematically desoldered all connections involving the MXM's decomposing hookup wire and discarded this wire. I also removed and discarded all capacitors except for the ceramic disc units. Other components permanently removed included the AC line cord, the AC output receptacle, the remote volume control jack and all RCA-style connectors on the rear panel, the five Cannon male jacks that served as mic inputs, and the 3P3T rotary switch associated with Channel 1 on the front panel. While the instrument was stripped down in this manner (Figure 3, middle), I did the following: (1) Tested all carbon composition resistors, and discarded those whose values fell outside their tolerance limits. (2) Washed the whole chassis with 90% isopropyl alcohol using a toothbrush, cotton swabs, and paper towels (done outdoors). (3) Thoroughly cleaned all potentiometers and slide switches with ECG RX500 aerosol cleaner/lubricant. (4) Planned and executed the layout for all new holes (or alterations of old ones) including provisions for mounting: (a) an additional power transformer for the solid-state buffers; (b) additional terminal strips for the new power supply circuitry and the replacement filter capacitors (to be mounted underneath the chassis); (c) an IEC-style power input jack; (d) two toggle switches on the rear panel for power and ground lift; (e) a toggle switch on the front panel for the Channel 1 input selector (enlarging the hole that had mounted the original 3P3T rotary switch); (f) six new Neutrik 1/4-inch jacks to serve as direct outputs and a line input; (g) a perf-board carrying the solid-state output buffers and their voltage regulators.
Power supply. Notably, the MXM's stock power supply was already designed for low-noise performance in a high-gain vacuum tube amplifier. Two independent secondary windings are used for two distinct heater circuits (see Figure 5): One supplies 6V AC to the twin #47 pilot lamps (which I replaced with fresh units) and the heaters of the 6X4 rectifier, 6BF6 V.U. metering tube, and 6CG7 audio output driver tube. The other winding couples to a full-wave selenium rectifier and an associated 2000 microfarad filter capacitor (I increased this to 4700 microfarad upon rebuilding); this latter circuit runs the heaters of all four EF86 pentodes and both 7025 dual triodes (in the input channels, summing amplifier, and tone controls) on 5.4V DC. Use of DC helps minimize hum induction in the high-gain stages. Additionally, the lowered voltage greatly increases tube life span for this industrial design; the Telefunken EF86s and 7025s may be the original factory-installed tubes, thanks to this sapient design feature.
Figure 6 is a schematic of the modified Bogen MXM power supply. My power supply modifications mainly involved updating the primary circuit of the original power transformer and adding a new transformer-isolated supply for the solid-state direct output circuitry. I also replaced the original electrolytic filter capacitors, which were aluminum-clad multiple units mounted above the chassis plane, with modern discrete units soldered to terminal strips below the chassis plane. For plate circuit filter capacitors, I used the same capacitance values but increased the voltage ratings as compared to the original units.
In the 120-V primary circuit, I kept the main fuse at the same value (0.5 A) and location, but eliminated the auxiliary AC output jack and in its hole (after enlargement) added a standard IEC-style power jack. To help isolate the 60-Hz line circuitry from the audio signal path, I mounted a toggle switch in the rear of the unit (where the original line cord entered the chassis) to serve as the main switch, and left the original main switch on the back of the master gain pot unused. I added a rear panel-mounted ground-lift toggle switch between the IEC jack's ground pin and the modified MXM's star grounding point. It should normally be kept closed.
To power the op amp-based direct output buffers, I specified high-quality +/- 15V DC. I used a trusted power supply design based on a 9V AC/500 mA transformer followed by a voltage multiplier/rectifier and filters/voltage regulators (Figure 6). The transformer needed to fit in limited space beneath the chassis plane. As a cost-effective way to meet this requirement, I chose a transformer removed from the plastic housing of a "wall-wart." I soldered insulated leads to the primary and secondary windings, and ruggedized the assembly by potting these connections in generous dollops of epoxy cement. After insulating its core from the steel chassis plane with a few layers of PVC electrical tape, I secured the transformer to the chassis with robust nylon cable ties. As noted in the schematic (Figure 6), I located the voltage multiplying rectifier circuitry and initial filter capacitors beneath the chassis plane (adjacent to the hacked 9V transformer); however, the +15V and -15V three-terminal voltage regulators and associated filtering/bypass circuitry reside on the perf-board assembly containing the operational amplifier chips (i.e., at the point of load; see Figure 4). Yellow and green LEDs mounted on this assembly indicate operation of the positive and negative supply rails, respectively.
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