Tascam Tips
originally appeared in the April'96 EQ
(References to other articles will eventually be linked)
BACK-UP BATTERY ALERT

A pair of DA-88s and DA-60s (the latter is Tascamís timecode DAT deck) came to my shop with unusual problems.  On power-up, one DA-88 lit up like a Christmas tree but never initialized, the completion of which occurs after the word "TASCAM" (or the message of your choice) scrolls across the meter display.  The DA-60 would accept and load a tape, but go no further.  It would not eject the tape.

At first I thought the problems were due to the careless installation of new firmware, but that was not the case.  After close inspection ó and somewhat by accident ó it was discovered that all the machines had a bit of crusty green schmutz around the backup battery.  One battery was surrounded by wisps of crystalline hairs ó way too alien for my taste!

I immediately got on the line to TASCAM hoping to score a few points for tipping them off, but no one seemed aware of this problem.  For at least three of these machines, I knew for sure that their environment was a bit hot under the collar.  (See Tay Hoyleís article on air conditioning.)  It may very well be that the battery behaves under "normal" thermal conditions.  If your machine is over two years old and/or occasionally seems a taco short of a combo platter, itís worth your while to inspect the battery just to be sure.

Access the System Control board (SYSCON PCB) either from the rear of the machine or by removing the top cover.  In the same way the SY-88 sync card is installed, remove the black and chrome rear panel that covers all of the circuit boards.  A special tool is required to facilitate card puling.

Or, for a simple inspection, remove both the top cover as well as the copper chassis plate underneath (optional on some machines).  Look for a  shiny, coin-like object centered on the left side of the SYSCON PCB (the top most circuit board).  If anything seems "foreign" in this area, clean it up with an alcohol-soaked cotton swab (or two).  Be sure to wait for the alcohol to dry before powering back up.

Iíve been adding a little "insurance" to this procedure by installing a piece of "Fish Paper" (MCM part number 21-1125) under the battery plus a wire tie to hold it in place.  This will delay damage to the circuit board from alien substances that are capable of eating the copper right off the board...  Yes, if the traces get eaten the board will have to be repaired and the battery must be replaced.

RCA CONNECTORS

For another pair of machines, the complaint was simple: one of the balanced inputs on the DB25 multi-pin connector was dead.  The same channel accepted signal at its RCA jack, so I checked the schematic to learn that the output of the balanced input amplifier is routed through the RCA jack.  The input connectors have gold-plated contacts, but the "normalled" connection was suffering from diminished contact pressure (see Glen Colemanís reference to RST ó Relaxed Spring Tension ) which is possibly due to one or more of the following:

· An RCA plug was inserted for a long period of time exposing the contacts to airborne contamination.
· An oversized plug may have bent the normalled connection so that it could not return to make contact.
· The spring material and/or the mechanical design of the jack is flawed

The repair options are easy.  The only obstacle are the number of screws required to access the Analog Input A-to-D card.  After extracting the card, I used a very fine lapping film to burnish the normalled contacts.  (A "coarse" business card, cut into small strips and briefly soaked in alcohol, will also do the job.)   This made only a slight improvement, so I inserted an RCA plug and applied pressure to the normal, bending it toward the "closed" position.  While this did the trick, I chose instead to replace all four of the connectors. (TASCAM Part Number 5330515400).  Following is the step-by-step procedure of that replacement.  To view, open the graphic file "DA_88_RCA."  Each of the following numbers corresponds to that composite picture.
 

1. On the rear of the machine is a black metal plate that covers all of the circuit cards.  Once it is removed, the "A/D" card can be extracted  (Use the tool that came with the machine).
2. For each jack, only two of the three pins are accessible.  Remove the screws that secure the plate through which the RCA jacks protrude and desolder the three connections that secure each jack.
3. To desolder, use either a desoldering braid or desoldering tools (as shown).  The former should be one-tenth inch wide and is available in 5.5, 10, 50 and 100 foot lengths. (Ten feet of Chem-Wick Lite, MCM part number 21-2190, is $6.)

 Note: The thin copper shield between the back panel and the jacks, creates a ground plane.  Any ground potential problems (from the external wiring) are routed directly to the chassis (rather than through the PCB wiring) so that the internal ground will not be contaminated.

4. When soldering the new jacks, be sure to apply heat to one side of the connector terminal while applying solder to the opposite side.  The terminal must be hot enough to melt solder on its own to guarantee a good connection.

5. When the job has been completed, stand the board up on end and brush denatured or anhydrous alcohol on the newly soldered area to remove the flux.  Inspect your connections with a magnifier.

6. A rear-view close-up of the RCA connector assembly.  The jack on the left shows a "closed" normal (no plug inserted) while the jack on the right shows an "open" normal.  Professional patch bays have far more contact pressure than these RCA jacks, but even so, it is a good idea to not leave patch cords in a bay for extended periods of time.

DIGITAL COPIES

Looking for a more affordable alternative to Tascamís Digital Interface Cable (Model PW-88D, Part Number 5350515400)?  While Tascamís price is justified IF the harness is fabricated with low-capacitance cable, with individual shields for each signal.  PLEASE NOTE: Digital equipment generates ó and can be the recipient of ó interference (hence the FCC label on most gear regarding compliance with same).  Below, Figures Two through Four  indicate the use of a generic flat ribbon cable which has been fine for my own applications up to 25 feet. However, you might also consider these "more sophisticated" versions from DigiKey (800-DIGI-KEY): A shielded, 25 conductor flat version (Part Number: MC25R-X-ND) or an unshielded 2 conductor twisted pair version (Part Number: MC26F-X-ND).  Both are available from , as well as the connector (Part Number CMM25G-ND).  Approximate material cost: $25 to $35.


 
Hereís the step-by-step procedure

STEP-1: The IDC-style DB-25 connector uses a technique known as "insulation displacement" to make solderless connections to a ribbon cable.  Note the ridges across the top to keep the cable aligned.


FIGURE-1: An Insulation-Displacement type "D" connector.


STEP-2: There are special tools for mating ribbon cable with conectors, but in this case, I chose a standard vise (one of my many...) with cable from Radio Shack.  The stripe marks every fifth conductor. Keeping track of Pin 1 is most important.


FIGURE-2: Using a vise to crimp the ribbon cable to the DB-25 connector.


THE SPECIAL TWIST

STEP-3: The image immediately below shows the "special twist" required to make a functional cable.  On the left is the schematic and on the right, the "business-end" of a DB-25 connector. 

NOTE: It is no coincidence that Pin-13 (left) and Pin-1 (right) are directly across from each other.   Since inputs and outputs are on the same connector, simply flipping the ribbon at one end (before crimping) makes is possible to cross-connect four digital signals (each carries a L/R, odd/even pair), L/R clock (LRCK), emphasis (EMPH), and two sample rate (Fs) data lines.  You have just witnessed a digital handshake.  On the schematic, you'll see that "Pin-1 = Dout-1" and "Pin-13 = Din-1." 



FIGURE-3: Tascam's TDIF port schematic and a DB-25 connector.


 
 

My thanks to Professor Paul Kozel at the City College of New York (CCNY) for beta-testing the cable under "real world conditions."
 
INTERFACING TO ANNA LOG

To the left is the balanced analog input and output ( I/O ) connector.  Like the TDIF, it is also a female DB-25 connector which can be crimped to a ribbon cable in the same way as the TDIF connector above. Sure, you can hand-wire using traditional soldering methods.  There is also a connector with individual pins that can be crimped in the same way as a DL / ELCO or EDAC connector.

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