This document outlines the electrical checkout procedures for the Micro-Tech 1200 Amplifier, a device designed for audio amplification. The procedures are intended to verify the amplifier's operation, assist with alignment, and aid in troubleshooting. For optimal results, these tests should be performed in the specified order.
The amplifier's primary function is to amplify audio signals. Its operation is verified through a series of tests that assess various electrical characteristics, ensuring it meets performance standards. The tests cover aspects such as DC offset, output bias, ODEP voltage adjustment, AC power draw, common mode rejection, voltage gain, phase response, level control functionality, current limiting, slew rate, square wave response, crosstalk, output power, reactive load handling, ODEP limiting, LF protection, signal-to-noise ratio, turn-on transients, turn-off transients, intermodulation distortion, and clipping behavior.
Usage Features:
The amplifier is designed for use in a stereo configuration, with a Stereo/Mono switch allowing for selection between stereo and mono operation. It features level controls for adjusting the output gain, which are typically set to a fully clockwise position for most tests to ensure full gain. A sensitivity switch allows for selection between different fixed gain positions, such as 26 dB, 0.775V, and 1.4V, to accommodate various input signal levels.
Input signals are connected to the amplifier's input channels, and the amplified output is delivered through output connectors on the rear panel. The amplifier is designed to operate from a regulated 120 VAC source.
During normal operation, the amplifier is expected to handle various types of loads, including resistive, capacitive, and inductive loads, without oscillation or damage. It incorporates protective features such as ODEP (Output Device Emulation Protection) limiting, which prevents damage to the output stage under high stress conditions, and LF (Low Frequency) protection, which mutes the amplifier when low-frequency signals could cause damage.
The amplifier's current limiting feature ensures that the output current does not exceed a safe threshold, protecting both the amplifier and connected speakers. The slew rate and square wave response tests verify the amplifier's ability to accurately reproduce fast-changing audio signals without distortion.
Maintenance Features:
The checkout procedures serve as a crucial part of the amplifier's maintenance. They allow technicians to identify and address potential issues, ensuring the amplifier performs reliably over time.
For instance, the DC offset test checks for unwanted DC voltage at the output, which could indicate a malfunction if it exceeds the specified tolerance. While there is no direct adjustment for output offset, a measurement outside the spec suggests an electrical fault, possibly related to specific integrated circuits (U104/U204).
The output bias adjustment involves measuring and adjusting DC voltages on the output module across specific resistors (R02, R21) using potentiometers (R26, R23). This ensures the output transistors are properly biased for optimal performance and longevity.
The ODEP voltage adjustment is a critical maintenance step that ensures the ODEP circuit functions correctly. This adjustment, performed within a few minutes of turning on the amplifier from a cold state, involves measuring voltages at specific pins of integrated circuits (U100, U200, U103, U203) and adjusting corresponding potentiometers (R121, R221, R132, R232) to match a temperature-dependent bias chart. This ensures the ODEP system accurately emulates the thermal behavior of the output devices, providing effective protection.
Common mode rejection (CMR) can be calibrated by adjusting potentiometers (N100, N200, R921, R1021) on the main modules, particularly for older board revisions. This ensures the amplifier effectively rejects common-mode noise on its inputs.
The AC power draw test helps identify excessive power consumption, which could indicate a fault such as high bias voltage or other electrical malfunctions.
The ODEP limiting test involves intentionally obstructing the fan to simulate overheating and observing the ODEP limiting waveform. This verifies that the protection circuit engages correctly under stress, preventing damage to the amplifier.
After all tests are completed and if the amplifier meets all specifications, the controls are returned to factory default settings or customer-specified positions. These settings typically include level controls at a specific clock position, sensitivity switch at a default voltage, stereo/mono switch in stereo, ground lift in the "lift" position, and power off. This ensures the amplifier is ready for use or storage in a known, safe configuration.
The procedures emphasize the use of specific test equipment, including an oscilloscope, DMM (Digital Multimeter), signal generator, loads, and IMD/THD (Intermodulation Distortion/Total Harmonic Distortion) noise test equipment, highlighting the precision required for proper maintenance and verification.
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