DA-88 Maintenance, Upgrades and Tweaks
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Figure 1a: DA-88 head assembly detailing the “rabbet.”
(click image for larger view)

TENSION IN THE CASSETTE SHELL
Since the dawn of the ADAT, it is often recommended (and I concur) that users fast-wind new tapes before recording or formatting—primarily to redistribute the tape pack. New tapes may have higher tension than rewound tapes, so this is one source of potential wear or instability during that first important recording. Depending on whether the tape is wound in the shell or through the transport, some shedding may occur, but contrary to myth, tapes that are shed-prone don’t just “fix” themselves. Loose bits of oxide have to go somewhere.

After the earliest DA-88s rolled off the assembly line, a “self-cleaning” mechanism was added, and a retrofit kit is still available. (The part and not the labor, which is minimal, is covered under warranty.) The kit comes in two pieces; the most essential being a sharp Ruby “scraper” that removes surface oxide before it can contaminate the heads. Keep in mind that tape edges also contribute to the shed factor and are not addressed by the scraper.

Look at the bottom of the stationary guide just to the left of the capstan shaft. If you see black, then the “rabbet,” the bottom ledge along the circumference of the head drum, is also likely to be contaminated. Figure 1a zooms in on a DA-88 head assembly—the rabbet is the ledge around the circumference of the head drum. Figure 1b shows how a clogged rabbet affects RF output, especially at the left side of the head where the tape enters. This condition, which can happen to any DTRS model, will compromise the machine’s ability to read timecode, as well as tracks 1 and 2. Cleaning the rabbet is a technician’s job. Don’t try this at home, kids.

Figure 1b: Not a pretty bunny; the RF Envelope (the signal from tape) is compromised when the rabbet is clogged, as indicated by the arrows. The scope image should be “square.”
(click image for larger view)

EC’s OBSESSION
I have two obsessions regarding all tape recorders: The mechanism should be gentle to the tape, and the tape, via tension tweaks, should be gentle to the heads.

The most significant issue for a digital tape recorder is the efficiency that data can be exchanged with the tape. When all parts are new, there is a significant amount of “data headroom.” A new head needs less tension than an “old” head. As parts wear, this headroom is diminished until the errors can no longer be concealed. The best preventive maintenance will be repeated ad nauseum here: Check the error rate and learn how to manually clean the heads.

Head life is most significantly affected by tension, not just in play, but in reverse play, when the former supply reel temporarily becomes a take-up reel. It is easy to overlook this measurement, and when the supply reel clutch is defective, the tension across the heads in Reverse Play mode can be double the specified range.

“BACK” TENSION?

Figure 2: Drawing of DTRS transport indicating adjustment locations for tension arm position and back tension.
(click image for larger view)

This term describes the supply reel tension applied to the head assembly when in Play mode. On all DTRS transports, back tension is adjustable via black coiled spring as detailed in the January edition of “The Tech’s Files” (as a picture) and this month as a drawing. But it’s not as simple as that, because there is also a tension arm position adjustment, and the two “tweaks” interact with each other.

Figure 2, from the DTRS manual, indicates the optimum position of the tension arm. This is one adjustment that I feel could be more precisely detailed. I start with the tension spring set to minimum, playing a fully rewound 113-minute tape to set the tension arm position. Only afterward should the tension be measured. The “spec” is 10 to 12 gram-centimeters (g-cm); it is often 14 to 16 g-cm on older machines, and, in my opinion, 10 g-cm should be the maximum value.

The point of optimizing the tension arm position is so that the machine will not be fussy at the head of longer tapes. The addition of the self-cleaning mechanism narrowed the usable range of the adjustment.

Reprinted with permission from

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