Bugs in the [Tape] Machine
Simple Mods to Keep Your Timecode DAT Happy, by Eddie Ciletti

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Despite an almost endless variety of higher-performance disk and tape formats available in the market, the ubiquitous 16-bit, 44.1/48kHz DAT format is far from dead. Yet at the same time, I feel pretty safe predicting that no new DAT recorders will be presented at any trade shows, this year or ever.

Historically, attempts to expand DAT’s horizons have been limited to 24 bits at the “standard” sample rates (as with the Tascam DA-45) or doubled (88/96kHz) rates at the “standard” bit depth, with the Pioneer/HHB 9601 being the sole example. Unfortunately, we can’t have our high-sample-rate cake and eat those tasty wide dynamic-range bits at the same time, due to the lack of real estate on tape. Increasing tape speed to four-times normal reduces a 120-minute DAT to a half hour. Four-hour (120 meter) data DAT tapes are in the wings ready to save the day, but few tape transports can reliably move such thin tape. Such a Full-Monty format change would probably require extensive mechanical redesigns as well...

Since we’ll be dealing with the DATs for some time, I’d like to offer my flu shot for some of the bugs that plague two popular timecode DAT machines, the Tascam DA-60 and the Sony PCM-7030, with a little trickle-down to some of the more “common” recorders. In this and future articles, some of the projects will be DIY—I very much encourage users to pop the cover and get familiar with what is “normal.” Other “nonuser-serviceable” tips are FYI. It’s better to know what’s coming than get caught with your pants down, or skirt up, eh? (You can always pass along these tips to your favorite technician.)

ECONOMY OF SCALE

Figure 1A: Slant-block component parts

With the exception of the Fostex D-15, timecode DAT decks are priced well above their “timeless” cousins. Ironically, timecode-equipped 8-channel MDMs—such as Tascam’s DA-98 and the Alesis M20—are more affordable than many timecode DAT machines. Why? The selling price is directly related to demand and the economy of scale. More people need sync-able multitrack recorders than need TC DAT decks, hence the price disparity. And considering their higher price tag, TC DAT recorders are typically less user-friendly (lots of video-related features) and not necessarily more reliable, an annoyance that is compounded if the device in question sees infrequent use.

Figure 1B: A "rebuilt" Sony slant block

According to Brian Falatovich of Chicago’s Midwest Digital, “The best DAT machine ever made (not considering its digital converters) was the now-discontinued Sony PCM-7030.” This can be confirmed by taking a peek under the hood, especially if you are familiar with other DAT decks.

While most pro DATs are genetically modified consumer models, nearly everything about the PCM-7030 suggests that knowledgeable pros put great thought into developing its extensive feature set. Compared to the PCM-2500, the PCM-7030 is quite serviceable even with its many circuit boards. Notice I said “nearly,” because this machine shares a few common flaws with many of Sony’s less-endowed children.

ONE OLD WIVES’ TALE
It is a widespread rumor that Sony and Panasonic DAT decks are not aligned to the same standard. Both companies specify their own test tapes; they differ in terms of approach, yet yield the same end result. Panasonic’s consumer-based transports hold an alignment better than most of Sony’s more professional mechanisms, a disparity that is easily remedied by a skilled technician. The illustration Fig. 1A and photo Fig. 1B detail the “slant block,” the assembly that unloads the tape from the shell and precisely guides it around the head-drum. This part is responsible for the compatibility myth.

The slant block is so called because it consists of an angled post that positions the tape to match the head assembly, which is not “square” with the deck plate. Other key components include a precision stainless-steel tape guide with a roller made from ceramic, stainless steel or plastic. The guide and its mate—a machined brass insert—have extremely fine threads to allow precise vertical alignment.

Figure 2: Top photo shows the hidden side of the DA-60's junction PCB. The white arrow points to the switch in question while the di-rection of travel is indicated in yellow. The photo at right shows the visible side of the right junction PCB where interconnectins are made. The "beginning of tape" (B.O.T.) sensitivity adjustment (VR1) is also located here (click either image for larger view).

Once in the ballpark, less than one-eighth of a turn in either direction will make a major difference in the output of the RF envelope (the signal from tape) when using the test tape. In reality, the system is very tolerant of typical machine-to-machine variations. Once the tape path is optimized, a locking screw secures the adjustment.

The fly in the ointment is that the brass insert is merely press-fit into the base of the slant block. After repeated load and unload cycles, the two parts no longer behave “as one,” causing the alignment to change every time a new tape is inserted. Sony’s new parts are no better than the originals. Remarrying the old parts with epoxy (after degreasing) has so far yielded a 100% success rate.

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