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NAB 35 Reparación de un Mesa Boogie Triaxis
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Guitarists and audio technicians interested in vintage amplifier repair and troubleshooting.
TL;DR
This video details the repair of a Mesa Boogie Triaxis, focusing on troubleshooting a faulty optocoupler (LDR) responsible for volume attenuation. The process involves detailed circuit analysis, resistance measurements, and identification of a transistor array integrated circuit.
Key Takeaways
- The Mesa Boogie Triaxis offers programmable amplifier parameters, allowing users to save custom settings for instant recall.
- Internal shielding separates digital and analog sections to minimize high-frequency interference, crucial for maintaining signal integrity.
- The Triaxis utilizes photocouplers (optocouplers) for volume control, which can degrade over time and cause signal issues.
- Troubleshooting involves checking specific LDRs (Light Dependent Resistors) and their associated transistors within the switching matrix.
- Schematics may not always be perfectly up-to-date, with component layouts and designations sometimes differing from older diagrams.
- The control logic for the photocouplers appears functional, suggesting the issue lies within the photocoupler components themselves or their immediate circuitry.
- Capacitors with high resistance values are used, and their condition (shorts or leakage) can significantly impact the performance of the switching system.
In This Video
- 00:00Mesa Boogie Triaxis Repair Introduction
This video details the repair of a Mesa Boogie Triaxis preamp. It begins by exploring its unique features and internal components.
- 02:14Internal Components and Shielding
The Triaxis features a toroidal power supply and a well-shielded digital section. Analog controls manage volume optocouplers.
- 02:50TX4 Card and Components
The TX4 card houses the core circuitry. It uses typical Bugera resistors and large tantalum capacitors, characteristic of their sound.
- 03:24Switching and EQ Issues
A problem is identified in the switching for the red channel's EQ. The presence control also malfunctions on this channel.
- 04:11Optocoupler and Attenuator Analysis
Analysis of the switching matrix reveals LDR1 and LDR6 are exclusive to the red channel. LDR8 acts as a giant attenuator.
- 06:09Optocoupler Resistance Measurements
Resistance measurements of optocouplers in different modes are taken. Some show expected open circuits, while others indicate issues.
- 07:38Identifying the Faulty Optocoupler
A discrepancy in the schematic leads to identifying a faulty optocoupler (LDR7 or LDR8) causing attenuation issues.
- 10:15Transistor Array and Integrated Circuit
The control transistor is part of an integrated circuit, not a discrete Q7. This integrated circuit is suspected to be faulty.
Questions & Answers
What is the main problem with the Mesa Boogie Triaxis?
The main problem identified is with the switching matrix, specifically with LDR7 and LDR8, which are optocouplers that seem to be failing or not functioning correctly in certain channels.
How does the Triaxis switch between modes?
The Triaxis uses optocouplers (LDRs) that are controlled by transistors. When a transistor conducts, the optocoupler activates, switching different parts of the circuit.
What is the function of the LDRs in the Triaxis?
LDRs, or optocouplers, act as electronic switches. They are used to control signal paths and activate different circuit components based on the selected mode or channel.
What was the initial suspicion about the Triaxis problem?
The initial suspicion was that an optocoupler, possibly LDR8, was faulty, causing signal attenuation and issues with the presence circuit in certain channels.
What is the role of the TX4 card in the Triaxis?
The TX4 card houses the optocouplers (LDRs) and associated circuitry responsible for switching and controlling the amplifier's functions.
Why is the schematic information not entirely accurate for this Triaxis?
The schematic appears to be for an older version. Some components, like the transistor array, have been replaced by an integrated circuit in this unit, making the diagram outdated.
Key Terms
- Optocoupler (LDR) — An electronic component that transfers electrical signals using light, acting as a switch.
- Switching Matrix — A circuit responsible for selecting and routing signals between different modes or channels in the amplifier.
- Toroidal Power Supply — A type of power supply transformer with a donut-shaped core, known for efficiency and low noise.
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