Tech's Files: Servicing Vintage Audio Gear

Sep 9, 2010 2:18 PM, By Eddie Ciletti

WHEN IT'S NOT THE TUBE, TRANSISTOR OR OP AMP

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This two-stage tube preamp schematic highlights potentially problematic components. The high-value resistors are noise suspects, while bad coupling and bypass caps can alter frequency response.

This two-stage tube preamp schematic highlights potentially problematic components. The high-value resistors are noise suspects, while bad coupling and bypass caps can alter frequency response.

When tube gear is unhappy, the easiest thing to do is swap tubes—not necessarily because they are the problem, but they’re often accessible and (except for subminiatures) are always socketed. The July 2010 column detailed the key components around the tube, but not the schematic. Before picking up that thread, let’s take a few steps back.

Despite the hazardous voltages of tube circuitry, part of vintage gear’s charm is simple design and generous real estate—units are easier to work on and learn from. Vacuum tubes, transistors and op amps can all be configured as amplifiers—gain stages that are often plagued by similar problems, such as noisy resistors and capacitors. (See Fig. 1.)

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SHRINK TO FIT
The short-term goal is not to be intimidated by technology, but to dive in—tinker, geek out and be merry. Familiarity with the simple stuff makes it easier to migrate from signal flow and block diagrams to schematics. Organization and troubleshooting skills can be used to eliminate the “dumb stuff,” after which we can dig deeper at the component level.

An analog console is the perfect example for comparative analysis. Interrogating each channel with an oscillator and a patch cord (plus lots of repetition) is a huge help toward understanding signal flow (and signalis interruptus) when you combine your best troubleshooting skills with known good spares, such as easily swappable plug-in items like tubes, op amps, modules and cables.

From tweed-era instrument amps to USB/FireWire converters, the obvious trend of putting more technology in less space has resulted in decreasing serviceability. This is why I encourage readers to hunt, gather and save schematics, and study and repeatedly draw them until they’re etched into your brain. Familiarity with schematics allows for comparisons; as with music, there are many variations on a theme. Know these potential variations and you can amaze your friends by using “The Force” to diagnose basic problems. Even when a schematic is not available, you’ll know, for example, that between two gain stages will likely be a capacitor, which, if bad, will act like a highpass filter.

CRAMPED QUARTERS
In the ’60s and ’70s, transistors and op amps were socketed, but as confidence in circuit design and component performance improved, the sockets went away. By the ’80s, smaller component parts and large-scale integrated circuits (LSICs) further increased component count, density and heat. Up until this time period, the majority of these parts had “legs,” or “leads” as they’re known in component parlance. Such parts can be used in everything from point-to-point vintage tube gear to early digital equipment using through-hole–style PCBs.

The past three decades brought major advances in miniaturization and automation. A “pick and place” machine can quickly “stuff” a PCB with surface-mount components (SMCs), of which the Application Specific Integrated Circuit (ASIC) can be added to the list. Unlike through-hole components that are soldered on the opposite side of the PCB to which they are mounted, SMC parts (with minimal to no legs) are soldered on the same side to circuit board “pads.”

In that same 30-year time span, the use of plastic, PCB-mounted connectors has repeatedly been the Achilles heel of most electronic devices. From the Walkman to the iPod, personal multitrack to laptop computers, a little pressure on external connection plugs—such as eighth-inch headphones or DC inputs—can damage the jack, its soldered connection or the PCB trace(s). Back in the day, if a guitar amp fell over with the instrument cable connected, the plug would get bent or break.






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