Apr 1, 2001 12:00 PM, EDDIE CILETTI

For any product, environment can affect the MTBF (Mean Time Before Failure). This is especially true for tape machines, where air quality can contribute to MTBF as much as the type of use or abuse. The Tascam DA-88, for example, has a fan that draws air through the tape slot, among other spaces, potentially accelerating mechanical component degradation when the air quality is in the negative.

Note how the following three environmental examples can affect the MTBF: a post house that relies heavily on tape is likely to have the machine turned on 24/7, a power user; workstation users may need access to tape but mostly work “offline,” constituting intermediate use; weekend warriors — who always wish they had more time — would be classified as inconsistent users, hot and heavy some weekends, possibly dormant for many weeks.

In each of these cases, understanding the environment can help predict, prevent or minimize downtime during periods of critical need. If the post house's DA-88 was installed in a proper video machine room — where air quality control is an art — then it could be “on” 24/7, yet stay relatively clean and likely go for longer intervals before requiring major service. In a more typical control room environment, turning a “stock” machine off when not in use will minimize the collection of accumulated dust. High humidity will shorten the MTBF of any videotape-based, helical scan recorder. This is especially true for the earliest DA-88 version — now eight years old — because its nonanodized reel table clutches have been redesigned three times since and are now stable (for a wear-item part).

Note: The information in this column could be dangerous in the wrong hands. Trying to help two diverse groups — users and technicians — could potentially compromise the material. All tape machines should get a routine inspection every 250 to 500 hours, especially if you aren't the type to pop the hood and at least take a look-see. Also, establish a relationship with a service company: There's a reasonable chance life can be extended and maintenance costs reduced via scheduled maintenance.

NICE HEAD

Like a used car, tape machine usage is judged by head hours, not just the “on” hours. Yet being “on” and in a negative environment, for a stock DA-88, invites foreign matter into the transport area. No counter keeps track of that. The fan reversal and filter modification detailed at www.tangible-technology.com is very effective at trapping airborne contaminants before they can muck up to the transport. There are plenty of digital tape machine tips on my Website. Your feedback is welcome.

In light of California's power deregulation mistake — don't get me started — I wouldn't suggest that you counteract high humidity by leaving the machine on (even with the added filter). Note that DA-88s were shipped with a silica gel pack. For humid environs, mount the machine in its own rackcase with the silica gel pack tucked inside. Put the front and rear covers on when not in use. With luck, the heat generated when the machine is on will “reset” the pack for the next storage period. If not, then the food dehydrator specified on my analog tape restoration page is a good choice.

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.

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?

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.

DA-88 UPGRADE LIST

You might be surprised at how many DA-88s still have their original heads. This is not a comment on the machine's genetic disposition, just an indication that some older, low-mileage machines come in for service needing many of the factory upgrades. I still get them. DA-88s before serial number 100000 were made around 1993-ish, while serial number 320000, for example, is circa 1995. The fifth- and sixth-place digits are lot numbers. In these examples, lot 10 and lot 32, respectively. If you own or encounter one of the older, low-mileage decks, then it should be overhauled sooner rather than later.

Here's a partial list of DA-88 updates (with an error message in parenthesis when applicable).

1. self-cleaning kit: This two-part kit consists of a sharp Ruby “scraper” that is highly effective at removing loose debris from the tape before it gets on the heads. A second rather annoying sub-assembly periodically dabs the heads in an attempt to clean them.

2. slide cam: This simple piece of plastic engages the brakes and tightens the clutches. The difference between old and new parts is a smoother transition to a notch that sets up the Fast Wind mode.

3. slide cam lever/actuator: (S-err-31/41) There are three possible Fast Wind failure modes: bad solenoid, bad solenoid circuit (cold solder joints) and a damaged lever arm, the latter caused by a fracture at the bend in the metal work. The crack is hard to see, even when you know it is there. See Fig. 3.

4. reel tables: Redesigned three times, and one of the primary annoyances is the interaction of the black optical encoder disc with the lower portion of the reel table clutch. Over time, the black shaft compresses and creates friction. The new reel table design seems to have solved this problem.

5. cam and sector gear: (S-err-11) A limited run of DA-88s — with serial numbers between (approximately) 240000 and 360000 — suffer from this “stuck loading guide” error message. Under-spec pin length causes damage to a plastic cam.

6. back tension coil spring: When replacing the back tension felt, a “silver” coil spring on the underside of the assembly ensures positive contact with the tension lever position adjustment mentioned above and detailed in Fig. 2. This spring was modified both to facilitate reassembly and to ensure positive engagement.

7. back-up battery: The three VDC lithium backup batteries should be periodically checked to make certain it measures 2.6 volts or higher. Even more important, inspect for leakage before circuit board damage is beyond repair. I use a different battery than recommended, because its vapor seal minimizes “battery exhaust” that can eat copper circuit traces.

8. slant blocks: The slant block is the lower portion of the loading guides. When there isn't a tape in the machine, they are loose to the point of seeming to be broken. This is normal. They become “precise” once fully extended and pressure fit against the “V” guides. Slant blocks are perhaps the most mysterious parts in all digital audio recorders, because they are difficult to measure and there are no adjustments except for guide height. There should be a front-to-back tilt adjustment to minimize curling, especially in the exit guide.

9. lubricant: For all helical tape transports — DAT, ADAT and DTRS — any lubricant applied to the loading guide paths should be wiped clean, leaving only a molecular layer. Tape that accidentally becomes slack and makes contact with the lube will become very attractive to a head spinning at 2,000 rpm!

Happy Motoring! Next month: under the hood of a Tascam DA-78HR.

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For more information about maintaining your gear in top shape, visit Eddie Ciletti's Website at www.tangible-technology.com .

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