Apr 1, 2004 12:00 PM, By Eddie Ciletti

DAT machines are still a repairable item in 2004. The shift from absolute to timecode (TC) repairs reveals a trend among two groups: those who can afford to “wade out” a new technology wave and those who must jump in and ride the wave to shore. Either way, people are keeping their TC portables in shape for both primary and backup duty. We're slowly reaching the end of DAT's lifecycle; in a few years, the technology will be passed from the recording domain to that of archivists.

Compared to the consumer marketplace, pro audio is still small potatoes, and within it, location and post-production make up the latest in-transition micro-niche in an already specialized industry. I only state the obvious because DAT is the best example of trickle-up technology, “prosumer”-ized by adding features like TC and XLR connectors. The next wave is borrowing heavily from affordable computer technology combined with some very customized electronics. Consider DAT to be the last tangible “format” — everything in the future will be virtual. But that's for a future article.

I'm not sure how many users will invest $5,000 to $15,000 twice for a pair of DAT replacements, but that will happen over time. Just as the 4mm format successfully challenged the ¼-inch Nagra, analog is still alive after all these years. At press time, dozens of industry professionals braved record-breaking January temps, meeting at New York's Silvercup Studios to discuss the matter. Check out www.gothamsound.com for pix and videos and the rec.arts.movies.production.sound newsgroup for feedback. Consumer gear may be disposable, but pro gear is still an investment.

PUBLIC ENEMY

Keeping DAT alive requires just more patience than before. Failures were initially mechanical, but now, electronics contribute to the workload. Capacitors have always played a key role in the aging process, going all the way back to the vacuum tube era. Miniaturization via large-scale — and application-specific integrated circuits (LSICs and ASICs, respectively) increased the feature set, reducing many common failure modes while creating a potential killer. Heat is enemy number one, reducing the life span of capacitors and batteries in particular; more concentrated heat (especially in portables) also affects everything from component life to solder connections.

Of all capacitor types, electrolytic caps age the least gracefully. Not only do values change, but caps can also leak acidic chemicals onto the printed circuit board (PCB), which occasionally occurs in the Fostex PD-2. The same is true for some coin-style batteries, particularly in Tascam's DA-88. Time and repeated temperature changes affect solder connections, too. For surface-mount devices (SMDs), a lower melting-point solder is used, which is softer than the 60/40 lead/tin combo of the vacuum tube era. The best advice is to send your machine in for routine service rather than tolerate intermittent problems. My motivation is the hope of catching a little problem before it creates a big problem.

RATTLE AND HUM

No matter what the product is, a rattle is not okay. A loose screw, spring or piece of plastic may be the cause, and while the latter is relatively benign, any metal bits can potentially short circuits with unpleasant consequences. Simply pop the cover and shake the unit upside down over a large white cloth to confirm success. (Springs and plastic bits are most likely to come from cassette shells, so be gentle when handling the,.) If nothing comes out, then it might be time to visit a specialist.

FLUX FOR YOU

A while ago, I demonstrated a technique to extract and replace SMDs by using conventional soldering equipment with a kit that included liquid flux and a low melting-point solder. The kit is distributed by ChipQuick and is available from many electronic suppliers. The flux is awesome and applicable to other difficult-to-solder applications. I learned the mechanics of the process from a Sony instructional video that was Peter Florance of Audio Services loaned me.

I didn't initially realize that the low melting-point solder was for de-soldering. Extremely brittle when cool, the trick is to apply it to all of the legs on a chip. The solder stays molten long enough — even on a 100-pin chip — to allow the legs to lift off without damaging the “pads.” The chip may also be glued into place, so be prepared for a sudden pop that can send molten solder all over the board and in your face.

POSTAGE STAMP REPAIRS

The most challenging TC portables are the Fostex PD-2 and the HHB PDR-1000. The Fostex is a rather heavy unit. Inside, all of the critical PCBs are rigidly mounted. The HHB is smaller and lighter, but by being much more compact, more disassembly is required and as such, the thinner PCBs are subject to flexing that can loosen solder connections. Built as a single-box tape machine about the size of Tascam's DA-P1, an optional TC module is mounted directly below so that the two become one.

One PDR-1000 that smoked before being brought in for service was caused by overly tight cable ties on a wire harness in the TC unit. Because normal procedure is to remove the TC module for transport maintenance, the cause was not immediately apparent. Common wear items for the PDR-1000 are mechanical: sticky brake shoes and stretched treading belt. Changing shoes requires transport extraction, including disconnecting a half-dozen subminiature connectors, forcing the tech to closely inspect all for potentially cold solder joints. This requires a jeweler's magnifying loupe and a very small screwdriver.

TROUBLESHOOTING 101

I wish my job was as easy as inspecting wire harnesses and as simple as replacing worn items! Once this unit was mechanically functional, the first evaluation tape played five or 10 minutes before being interrupted by an alarming distortion. After touching up more cold solder joints and checking all of the various power supply voltages, a few caps were found to be outside their typical 20-percent tolerance and replaced. So far, all of the extra work turned out to be preventive, as the unit is still intermittent.

By testing the AES output, one of two DSP chips was confirmed to be good, pointing to the D/A converter as driven by the I/O switching circuitry. Bingo! The “switch” was thermally unstable. The application of cold and heat can sometimes help localize a problem. For example, I recently located a bad surface-mount cap in an Alesis M20 head amplifier in this way, but the HHB only responded to a power cycle and not to the cold shoulder or the hot flash.

The next obstacle occurred during an attempt at record alignment, but there was no confidence output (the ability to monitor the signal from tape via the extra pair of heads for which there is a separate preamp and DSP). When the RF signal was confirmed, the next logical suspect was the slave DSP, a postage-sized Sony ASIC pin around its perimeter. Changing the chip was the trick!

Regarding the use of cable ties, many installers use a tool that automatically tightens the bundle and snaps the excess plastic. For the HHB's TC unit, high tension pinched the insulation to the point where it was too thin to be useful. I've never used that tool because it leaves a sharp edge that cuts like a razor. Diagonal cutters make a clean, safe and flush cut. Snake-oil cable fanatics take note: Over-tightened cable ties can also reduce capacitance and increase reflections in signal critical wiring.

THE SOFTER WRAP

Service techs see the world from the inside out, which is a bizarre enough perspective on its own. With the exception of vacuum tube and discrete transistor gear (modern or otherwise), just about everything else has shrunk to the point of being almost unserviceable. But contrary to belief, miniaturization and the highly specialized tools required to make such repairs are not an obstacle, as long as the item in question has maintained its value.

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Eddie requires special looking glasses and is partial to bunny rabbits and smiling cats. Visit him at www.tangible-technology.com.

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