LOCATING PARTS, IMPROVING POWER AND GROUND DISTRIBUTION

By Eddie Ciletti
This is the third part in a series on analog equipment maintenance for geeks in training. Part One hit the ground running, diving straight into troubleshooting capacitors (the weakest link in aged gear) by using a square-wave generator and an oscilloscope. (The traditional method of measuring frequency response requires a sine wave oscillator and an AC voltmeter. A 1kHz square wave provides an overall snapshot of both low and high-frequency response that is good enough for troubleshooting, and sometimes more.) The same tools were used in Part Two to evaluate amplifier performance, particularly to upgrade early IC op amps, from ’70s-era products to just about anything modern. No matter whether attempting to repair or upgrade, the constant theme of troubleshooting is comparative analysis—take two examples of the same device and compare them; if they don’t measure or sound the same, then one of them is defective. This is relatively easy in the case of a recording console or tape machine, because having one good channel to compare with the problem channel is a major timesaver.

Beyond repairs, the topic that most interests my e-mail correspondents is “how to make things better.” This is a complicated issue. The best advice is to find a technician who is willing to do the geek stuff and show you how to do the dirty work. After changing caps on 24 modules, there’s no way your soldering skill won’t improve. But upgrades require a combination of skills, tools, time and money.

Before attempting any enhancements, you need to establish a solid foundation. Start with the obvious troublemakers—switches, pots and faders. Older American and British-made gear (from the ’50s through ’70s) can be easier to work on than their modern counterparts. Older stuff has more real estate (more room to work), while connections to pots and switches are hand-wired, making “off-the-shelf” replacement fun and easy. (Well, almost!) All you need are a few catalogs from digikey.com, mouser.com, newark.com, Antique Electronic Supply (tubesandmore.com) and New Sensor Corporation (sovtek.com), to name a few.

Why go to the trouble of upgrading an older console when newer digital gear is so cheap? In this analog-to-digital transition era, many “affordable” analog consoles—new and used—have found a niche as “monitor mixers” and are increasingly used for the relatively simple task of multitrack playback. In fact, analog mixers can simply resolve the digital latency issue—an analog mixer can provide a direct signal path from the mic/direct box to the musicians, free of unnecessary A-to-D and D-to-A steps. And, though no one denies the power of mixing in the digital domain, many miss the simplicity and ease of “playing” even the cheapest analog mixer. Dedicated knobs and switches make for tangible fun without the worry of clocks, masters or slaves.

Upgrades become more problematical on mass-produced equipment that has gone through an automated assembly process. The physical characteristics of pots mounted directly to circuit boards are often so specific that replacements can only be purchased from the original manufacturer, if still alive and well. Assuming you’ll want to use the same knobs as on the original, finding the same shaft dimensions alone can be an ordeal akin to a wire-brush enema. An entire article could be written on the topic of finding replacement Clarostat pots for an MCI JH-600 Series console—nearly all the pots have custom dimensions and tapers. And even quality pots succumb to the additional physical stress of being mounted to both the front panel and the PCB. Any flexing will eventually break the connections inside the pot. Trident Series 65 and 80 consoles are prime examples of cheap pots plus poor mechanics (and some of the worst caps ever).

It’s easy to point the finger at poor component selection in vintage gear, but new designs are not immune from design error. After installing a new 32-input mixer, a customer found the effects returns to be unacceptably noisy. Though the problem was omnipresent, the extra gain in the aux summing amps compared to the stereo mix bus resulted in the hum being processed by chorus and flanger effects into a swirling 3-D buzz.


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Reprinted with permission from Magazine, December, 2000
2000, Intertec Publishing, A Primedia Company All Rights Reserved



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