Tape Recorder Diagnostics
Magnetic recording is a process that involves several simultaneous physical effects that combine to create the final playback of a recording. The complex nature of the combination of the magnetic, mechanical, and electronic components of the recorder create a very difficult environment for troubleshooting whenever the process fails. The basic diagnostic technique for a complex process is to break the large problem into smaller, simpler independent components. In the case of the magnetic recorder, these components include:
The magnetic storage medium (tape),
The magnetic interface between the tape and head,
The record/playback electronics,
The mechanical transport, and
The control logic for the transport and electronics.
Typical tests of the magnetic storage medium include sensitivity, distortion, noise, dropouts, and amplitude modulation. The first three of these tests require a general-purpose audio signal analyzer; measurements of dropouts and amplitude modulation require specialized devices.
The magnetic interface between the tape and head has several aspects. The transport has primary responsibility for maintaining good tape-to-head contact at the head gap by controlling the tension and guiding of the tape. The face of the head must also present an appropriate curved profile that converts the tape tension into the correct contact pressure of the tape against the head gap. The design and degree of wear of the head determine how well the head converts the magnetic patterns recorded on the tape into electrical signals during playback or electrical signals into magnetic patterns during recording. Diagnostic devices include standard alignment tapes and flux loops.
The playback and record electronic signals are processed by linear amplifiers and equalizers that are easily measured with an audio signal analyzer. Typical tests include frequency response, distortion, clipping and noise.
Nonlinear circuits used in some recorders to compensate for tape saturation require special attention. These "linearizers" stretch the crests of waveforms recorded at high levels to compensate for crest flattening due to magnetic saturation of the tape. The amount of predistortion that must be added to the record signal depends upon the saturation characteristics of the type of tape being utilized. Proper linearizer adjustment requires amplitude and frequency sweeps in record/play mode to achieve optimum distortion reduction.
Several diagnostic aids are required to test the tape transport. Tape speed accuracy is checked with a strobe wheel or speed test tape. Tape speed uniformity requires a flutter meter. Tape tension measurements require a tension gauge; tape tracking is best viewed with a magnetic viewer that makes visible the magnetic tracks on a recorded tape. The mechanical alignment of heads, guides, tension arms, and other components in the tape path requires standard machinist's tools for measuring height and perpendicularity.
If the transport and electronics control logic is fairly simple, only an oscilloscope and voltmeter are required to trace signals and detect faults. More complicated microprocessor-based systems require extensive diagnostic capabilities or board swapping if the problem is not confined to the peripheral circuits related to switches, lights, actuators, and sensors.