Bogen MXM Modifications
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Aside from the dust layer, which may indicate that the equipment was stored unused for a long time, the MXM appeared in fair condition on quick inspection (Figure 2, top). The interlocking top and bottom perforated steel covers easily came off (Figures 3 and Figure 4) after removing two aluminum thumb screws on either side of the MXM. All five input channels had Bogen T155 transformers for 150- to 200-ohm balanced inputs. All input jacks were male XLRs (females are the norm today). A Bogen T165B plug-in transformer ("broadcast quality," according to the manual) occupied the output transformer socket. The pentodes and the 7025s were from Telefunken, and the other tubes were Bogen-branded USA-made units.
A look beneath the chassis plane at the point-to-point wiring quickly revealed the unit's most serious restoration issue: the insulation on all of the solid-core hookup wiring was shiny--too shiny. It was coated with a sticky, oily liquid, indicating advanced decomposition. Because of this, I would have to nearly completely re-wire this MXM. The only exceptions were the shielded cables (running between rear-panel jacks, audio transformers, and points on the front panel) and the power transformer's leads. These were free of visible (and tactile) decay, having been manufactured with insulation of differing formulation. I don't know what environmental factors may have contributed to the hookup wiring's degradation. The same problem afflicted the Bogen MO30 power amp (discussed separately in an upcomming link).
Although I later found a manufacturer's schematic, I traced my own schematic (Figure 5) from the MXM before its disassembly. I also powered it up, albeit slowly using a variac. All vacuum-state devices lit up except for one of the two #47 pilot lamps. I verified that the power transformer was working, but didn't spend much time testing audio functions at this point. Square waves at the channel inputs came out integrated, probably indicating coupling capacitor failure. Further diagnostics were pointless since I planned to strip the unit down and rebuild it anyway.
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