Tascam DA-P20
A portable DAT deck whose service manual shows how hard it is to wring 16 bits out of a spinning cassette.
Overview
The DA-P20 is a Digital Audio Tape Deck, and the documentation that survives for it is the service literature. That tells you most of what you need to know. Tascam's own service instructions open with a warning: the unit cannot be disassembled without lifting the cassette cover, and a thin rod has to be inserted into a small hole and pushed before anything comes apart. That is not a quirk. It shows how tightly the mechanism and the electronics are packed together. The deck is built around a portable transport, and the manual says so. The DAT mechanism is described as designed for portable type use, composed of tape loading, tape transport poles, reel driving, and cassette holder mechanisms.
The manual spells out a lot of the conversion and servo work, which helps anyone servicing vintage digital gear. The analog playback path is documented step by step. The head output is demodulated into a 16-bit parallel signal across a chain of LSIs, then converted to analog by a PCM55 D/A converter. The left and right time-sharing signals are separated through a sample-and-hold circuit. The sample-and-hold frequency is 48kHz during analog recording and 44.1kHz when playing a DAT music tape. A 20kHz low-pass filter used for recording is switched in for playback as well, and a rear aperture compensating circuit lifts the high end to flatten the response. The manual specifies 3dB up at 20kHz.
The digital side of the spec sheet is where it gets good. The signal processing LSI handles 8-10 modulation and demodulation, with double Reed-Solomon error correction and generation. C1 correction goes up to double, C2 correction up to sextuplex, and the state of error correction can be monitored. Sub-code processing is included, and the manual notes that ID and Pack data remain reproducible even during search. The system supports three sampling-frequency configurations: 48kHz/16-bit/2-channel, and 44.1kHz/16-bit/2-channel for playback only. A single 28.224MHz crystal puts the whole system into operation. Sub-code after-recording is supported.
Tracking is where the DA-P20 gets demanding to service. The manual is blunt about why. DAT recording uses a high-density system at 114 MBPl², with a track pitch of 13.591µm, so tape speed, cylinder rotary frequency, and tracking all have to be held with real precision. Tracking is handled by a servo system called ATF - Automatic Tracking Finding - implemented as PWM control driven by digital signals. Playback signal runs through a sync detector and pilot detector via the head amp. The sync filter detects signals f2 and f3 and passes them to CPU B as signal PBS. The pilot filter detects f1 and passes it as PBP, and its output feeds the ATF sample-and-hold circuit, where crossing f1 signals are sampled and their voltage levels compared. The capstan and reel servo circuit then controls the capstan motor's rotation frequency so the head traces the tracks correctly. The manual also documents a C2 error check for service: track No. 13 of test tape TY-7551, a frequency counter reading under 96Hz, and an oscilloscope check that no C2 error occurs at muted areas on the tape. It adds that a poor test tape will throw frequent C2 errors, so condition must be confirmed before adjustment.
The mechanism is documented in plain terms. Tape loading - extracting tape from the cassette and wrapping it around the cylindrical head drum at a precisely defined angle - is performed by one DC motor, controlled by a Mode Sensor Switch. The main CPU pair, HD62004A and HD62014A, splits the work: both DAT signal processing and servo control are carried out by combining the two units. The microcomputer interface can exchange system control signals, sub-code data, the maximum PCM value inside one frame (used for the level meter), general-purpose port I/O values, and measured reel pulse values over a serial interface. A µPD75312 microcomputer handles DA-P20 system control, LCD display control, console switch scanning, cassette hole detection, and start/end position detection. The manual also lists an AD/DA interface capable of both digital-to-analog and analog-to-digital operation via a high-speed D/A converter and comparator using a sequential comparator circuit, with a serial interface allowing even serial-type AD and DA configurations.
The service documentation is unusually complete for a deck of this kind. It has exploded views, pin tables, voltage regulator notes (S81250HG), and the full servo theory, which helps anyone keeping one running. The mechanism's dependence on a single DC loading motor and a Mode Sensor Switch, and the ATF servo's sensitivity to head and tape condition, are the areas the manual itself points to as service-critical. The C2 error warning about tape condition is worth taking seriously, since these decks are only as good as the tape you feed them. The manual is silent on production years, original pricing, and market values, and this archive will not guess at them.
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