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 analysistake two examples
of the same device and compare them; if they dont 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, theres no way your soldering skill wont 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 troublemakersswitches, 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 consolesnew and usedhave 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 issuean 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 youll 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 consolenearly 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).
Its 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.