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A20112 Power Supply | Schematic

typically refers to a 14-watt guitar amplifier power supply circuit

, most notably used in certain boutique or smaller tube amplifiers like those from or similar custom builds.

While a complete factory-released schematic for the "A20112" board itself is not often published as a single standalone document, the circuit is largely based on classic Class A tube power supply architectures. Circuit Breakdown & Common Features

Based on technical discussions and common repairs for this unit, the power supply generally includes the following sections: Input Stage

: Uses a standard IEC inlet with a main fuse (often 1A or 2A depending on the region). Power Transformer

: Typically a custom-wound transformer providing high-voltage AC (B+) for the tubes and 6.3V AC for the heaters. Rectification : Often utilizes a 5AR4/GZ34 rectifier tube or a solid-state bridge rectifier to convert AC to DC. Filtering (B+ Rail)

: Uses a series of high-voltage electrolytic capacitors (e.g., 22uF to 47uF rated at 450V+) and often a choke or large power resistors to smooth the DC voltage. Switching Options : Some versions include a 7W/14W power switch

, which works by dropping the voltage to the screens of the power tubes (Pentode/Triode switching). Common Components in A20112 Supplies Component Type Common Value/Part Power Tubes 2x 6V6 or EL84 Main power section 5AR4 or Diodes Converts AC to DC Screen and grid resistors Capacitors 22uF - 50uF (450V) Filtering Ripple Troubleshooting Tip a20112 power supply schematic

If your unit has stopped working or is cutting out, users frequently report issues with the output transformer blown screen resistors

resistors on the tube pins). Check these first if the main fuse keeps blowing. Class A tube amp schematic that matches this 14W power profile for comparison?

Fixing noisy eico hf-12 amplifier with tube issues? - Facebook

2. The Switching Stage

This is the heart of the unit.

How to Read the Schematic

If you have a schematic for the A20112, it will usually be divided into four distinct "blocks." Here is how to navigate them:

3. Connector Pinout & Wiring

The A20112 connects to the Namco I/O board via a custom connector block (usually a friction-lock header).

| Pin Number | Voltage | Wire Color (Typical) | Function | | :--- | :--- | :--- | :--- | | 1 | +12V | Yellow | Drive / Fan Power | | 2 | +12V | Yellow | Drive / Fan Power | | 3 | GND | Black | Common Ground | | 4 | GND | Black | Common Ground | | 5 | +5V | Red | Logic Power (CPU/GPU) | | 6 | +5V | Red | Logic Power | | 7 | +3.3V | Orange | I/O & Logic | | 8 | +3.3V | Orange | I/O & Logic | | 9 | GND | Black | Common Ground | | 10 | GND | Black | Common Ground | typically refers to a 14-watt guitar amplifier power

(Note: Wire colors may vary based on revision. Always verify with a multimeter.)

Key sections

  1. Purpose and specs (assumed)

    • Purpose: Regulated DC output for electronics.
    • Typical specs to pick: Input voltage 7–24 V, output 5 V or 12 V, max current 1–3 A, ripple <50 mV, efficiency target (switching) >85%.

  2. Block-level schematic (conceptual)

    • Input connector and reverse-polarity protection (diode or MOSFET).
    • EMI input filter: C_in (electrolytic) + L (ferrite bead) + C_decoupling (ceramic).
    • Regulator stage:
      • Linear: pass transistor (NPN/P-MOSFET) + series resistor + error amplifier + reference.
      • Switching (preferred for >0.5 A): buck converter: switching MOSFET, inductor, diode or synchronous MOSFET, output capacitor, feedback network to controller IC.
    • Output filter: L and C or C_lowESR for ripple suppression.
    • Feedback and compensation network to stabilize loop.
    • Protection: current limiting (foldback/OC), thermal shutdown, soft-start.
    • Indicators and connectors: Power LED, sense pins, enable/shutdown.

  3. Typical component choices

    • Input cap: 100–470 µF electrolytic + 0.1 µF ceramic.
    • Output cap: 100–470 µF low-ESR (or polymer) + 1 µF ceramic.
    • Inductor (buck): value tuned for ripple—e.g., for 5 V, 2 A, switching 500 kHz, L ≈ 10–22 µH.
    • Schottky diode for non-synchronous: low V_f, rated >I_out.
    • MOSFET: R_DS(on) low enough for conduction losses < acceptable; Vds margin 1.5× Vin_max.
    • Controller IC: choose by topology (e.g., buck controller with internal MOSFET for compactness).

  4. Example: simple 5 V buck schematic (conceptual)

    • VIN → input cap → switching MOSFET (driven by controller) → inductor → VOUT.
    • Schottky or synchronous MOSFET from switch node to ground.
    • VOUT → output cap → load.
    • Feedback divider from VOUT to controller FB pin.
    • Compensation network (R/C) between error amp pins.
    • Soft-start/cap to limit inrush on startup.
    • Place current-sense resistor or use high-side MOSFET Rds(on) sense for overcurrent detection.

  5. PCB layout tips (practical, high impact)

    • Keep high-current loops (switch node, input cap, diode, MOSFET, inductor) as short and compact as possible.
    • Put input and output capacitors close to the IC and power pins.
    • Route the feedback trace away from noisy switch node; use a single-point ground near the controller.
    • Thermal relief: expose MOSFET/diode copper for heat sinking or add thermal vias.
    • Place the sensing resistor near controller sense pins; minimize parasitic resistance in ground return.

  6. Stability & debugging checklist

    • Verify feedback divider yields correct output voltage with controller reference.
    • Check compensation network per controller datasheet; simulate loop or use frequency sweep.
    • Measure switching waveform: clean edges, no excessive ringing; add snubber if needed.
    • Inspect thermal behavior at rated load; ensure current limit works.
    • Measure ripple and transient response to step changes in load.

  7. Safety and protection details

    • Add input TVS for surge protection if exposed to harsh environments.
    • Fuse on VIN for catastrophic failures.
    • Ensure creepage/clearance if board may see high voltage.
    • Add enable/shutdown pin to allow controlled startup/shutdown.

  8. Example improvements and variants

    • Synchronous buck: higher efficiency for battery-powered designs.
    • Spread-spectrum switching or input EMI filtering for sensitive RF environments.
    • Post-regulation LDO for ultra-low noise analog supplies.
    • Multi-output rails using SEPIC or coupled-inductor topologies if input can be above/below outputs.

  9. Quick troubleshooting flow

    • No output: check VIN present, enable pin state, fuses, thermal shutdown.
    • Low output: verify feedback resistors, sense resistor, controller operation.
    • High ripple: bad/output caps, loose ground, poor layout.
    • Overheating: excessive switching losses, wrong MOSFET orientation, high ESR caps.

  10. Resources for deeper design

If you want, I can:

Which would you like?

Note: The designation "A20112" is most commonly associated with proprietary power supply boards used in arcade hardware (specifically Namco System 246/256 platforms used for games like Tekken 4, Time Crisis 3, and Soul Calibur II). Schematics for these specific AC/DC converter units are rarely released as public service manuals, as they are treated as "replaceable units" (FRUs) by the manufacturer. Components: Power MOSFETs (or a PWM controller IC)

The following report details the functional architecture, component analysis, and troubleshooting guidelines based on the known design topology of this unit.

3. Design Choices and Topology Selection

Design trade-offs: