copyright 1997 by Eddie Ciletti
WHAT’S THE POINT?
Everyone knows what a funky switch sounds like — crunch! But when buried deep inside a digital tape machine, an unhappy Mode switch can stop your session cold. The point of this article is to determine if a switch is the cause of error messages and eaten tapes. If yes, then stop using the machine. Too often, I learn that even after a funky machine eats one tape, ab-users continue to attempt work. Don't act surprised or curse the machine when it does this a second or third time. The culprits -- Mode and Load switches -- are not user-serviceable items. They last from one to five years depending on make, model and use. One listen might cure anyone of the habit.
All modern tape transports are "computer" controlled. The "brain" is a dedicated microprocessor whose "eyes" and "ears" are optical and electro-mechanical sensors. Within five seconds of inserting a tape, the machine knows the type, the location (heads, tails or elsewhere) and whether a recording can be made. An instant after pressing Play, tachometers precisely detect head, capstan and reel motion. This mission-critical information is fed to the support circuitry and then compared to "what was sent" until the entire system is servo-locked. If the signal on tape can be decoded, you’ll hear it in mere seconds.
A la Mode
A rotary or slide switch can be mechanically coupled to a cam or a lever whose position indicates the main transport functions (fast wind, stop, play and shuttle). When the "brain" sends a command to the transport, it expects a quick response, typically in less than a second. When a dirty or worn switch transmits indecipherable gibberish, the microprocessor "winces." Users have seen the results, aka those ubiquitous error messages and frozen tapes.
SCRATCH AND SNIFF
To interrogate a switch without test equipment you’ll need a probing device. This can be a shielded cable with clip leads, voltmeter test leads or an oscilloscope probe. Use a .1mF capacitor in between the "hot" lead of the probe and the Line input of an amp. (Use a non-critical speaker because the switched DC will make a racket even when blocked by the cap.)
Note: Your mission is simple, but please be careful. Don’t probe any other points except those specified in this article.
PICTURES AND AN EXPEDITION
The original Tascam DA-30 is the first example. Photo 1 shows the "Junction PCB" that is mounted on the right side of the transport assembly. The DA-30 has a Mode switch (see the insert in Figure One) and a Load Switch (the latter is connected to CN 7). The Mode switch reports transport functions such as Wind and Search. The Load switch senses when Thread, Play and Search are activated.
PROBING FOR CLUES
Connect the Ground lead of the probe to the chassis and carefully place the probe tip on the exposed metal of the connector. In Figure One, the Mode switch leads are the Black, Brown, Red and Orange wires of CN-8 (the legend is obscured by the probe). The Load switch leads are Yellow, Blue, Brown and Black. Cycle the transport by starting with no tape, then cycle through all of its mechanical states (Play, Fast Wind, Search, etc). Loud clear "pops" are good. Grunge is bad.
Photo 2 shows the lower right corner of the main circuit board of an "original formula" black-faced Adat. On the far left of connector J-11 (labeled "short") are four wires — Black, Brown, Red and Orange — that connect to the Mode switch. The most sensitive state is the transition from unload to load — just as the tape is lowered. Listen to the orange lead first and work your way left.
In both machines, there will be one wire on each switch that connects to ground. For the DA-30 there shouldn’t be any noise on the Blue terminal of CN-7 or the Red terminal of CN-8. On the Adat, the Red wire (third from the left) should also be silent.
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