Tech's Files: Restoring Vintage Gear, The Quest Continues

Dec 8, 2010 7:04 PM, By Eddie Ciletti

TUBE MIXERS, TUBE MICS, DATS, A CLASSIC VCA PROCESSOR

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The thoroughly modernized Altec 1567A tube mixer

The thoroughly modernized Altec 1567A tube mixer

Due to circuit simplicity and component size, vintage gear is more easily serviced than most newer equipment, which is one reason I’m constantly revisiting the topic. In the process of increasing functionality and complexity, miniaturization has made visual analysis—and heat dissipation—of audio equipment more challenging. Yet some of the same basic problems plague all gear regardless of its place in the evolutionary time line.

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I recently wrapped up work on an Altec 1567A (a 4-input vacuum-tube mixer), an Eventide Omnipressor (VCA compressor/expander), a pair of Sony PCM-7030 DAT recorders and a Neumann SM 69 tube mic (a stereo version of the M 269, which is essentially the European version of a U67). This odd audio collection spans some 50 years’ worth of electro-mechanical evolution. As viewed from the tech’s perspective, the internal real estate ranges from luxuriously spacious—Altec and Eventide—to tediously cramped, as in the case of the SM 69 and the Sony DAT.

GETTING SMALL
Such restoration/repair tasks necessitate a steady hand, a good light and assisted vision, like reading glasses. Working near mic capsules and DAT heads requires a hyper-awareness of their vulnerability, so I’ve made protective sleeves and covers to keep capsules and heads safe. A speck of hot solder or even flux landing on a capsule diaphragm will leave its mark as a crater or a hole. An accidental screwdriver slip can puncture a diaphragm, scratch a head drum or, worse, break off the head chip. Such potentially costly mistakes emphasize the need for preventive measures.

Inside the SM 69, most of the components are soldered to two circular circuit boards, including the vacuum tubes. For that reason, only known, good tubes should be installed because the circuit boards won’t tolerate repeated tinkering. Many early circuits boards are similarly intolerant. I recommend Chem-wik Lite de-soldering braid (Chemtronics Part #10-100L, MCM Order #21-325).

COMPARATIVE ANALYSIS
Once the PCM-7030 transports were rebuilt, I relied heavily on comparative analysis by swapping transport and circuit boards, and checking system default parameters until both units behaved identically. The last anomaly to be resolved was the length of time each PCM-7030 required to power up; a nearly 30-second difference implied that a future failure was right around the corner. The problem was localized to the 5-volt power supply used by the logic circuitry (various microprocessors that deal with transport control, front panel display and user presets).

Of the two styles of power supplies, the linear type was found in conventional vintage gear like guitar amps, recording consoles and, with the exception of the Stephens, most analog tape machines. Switch-mode supplies, which have always been a key component in computers and cell phones, have been migrating into linear’s territory. Switch-mode supplies are not only more efficient at converting AC to DC, but are also smaller and lighter. An Apple laptop’s external supply/charger delivers 18 VDC at 4 amps if the line voltage is anywhere in between 100 and 240 volts AC. This is called regulation. The equivalent linear supply would be at least the size of a U87 storage case, be much heavier and require the user to set jumpers or switches to accommodate various line voltages.

The PCM-7030 has multiple linear supplies and one switch-mode supply for Logic that conveniently hangs off the rear of the unit and for which no documentation is provided. There was no obvious heat fatigue. However, on a whim—and a hunch based on a similar switch-mode problem found in an Alesis ADAT PSU—I swapped out a few low-voltage caps around the regulator and—voila!—not only did the two units now power up the same, but a third dead supply was resurrected. (Google “uc2844 application notes” for a typical schematic.)






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