Constructing the Amadore64: An Exhaustive 12-Step Analysis

The Amadore64 motherboard initiative represents a monumental undertaking in the field of retro-computing restoration. Designed with the dual objectives of faithfully recreating the legendary Commodore 64 while introducing thoughtful enhancements, this intricate process explores a vast array of engineering, diagnostic, and assembly techniques. By chronicling each step in detail, this document serves as both a technical reference and an intellectual exploration into the intricacies of replicating a beloved computing classic with modern methodologies.

Amadore64


Step 1: Establishing the Power Rails

A robust power supply system is indispensable for ensuring stability across all subsequent stages of motherboard functionality. The external power supply delivers two inputs: 5VDC and 9VAC, which are systematically transformed into several critical voltage rails on the Amadore64 board. These include:

  • 5VDC: The primary supply rail, rigorously controlled to remain below 5.5V to avoid potential damage to sensitive components.
  • 3.3VDC: Derived from 5VDC using an INA219 DC current meter, allowing precise monitoring of voltage and power consumption.
  • CAN +5V: Generated via full-wave rectification of the 9VAC supply and regulated by a 7805 chip, specifically allocated to power the VIC-II video chipset.
  • 12VDC: Similarly derived from the 9VAC input to accommodate additional video and peripheral components.
  • +5VA Rail: Dedicated to powering the ATtiny85 microcontroller and the ACS712 current sensor for monitoring AC loads.

To enhance real-time diagnostics, an OLED display and microcontroller were integrated into the power monitoring system. These additions allowed for visualization of voltage levels and current consumption, proving invaluable during early testing phases. Initially, the +5VA rail exhibited significant noise, compromising the ACS712’s precision. The issue was resolved by incorporating additional capacitors, which improved signal fidelity by over 100-fold.

Key Insights

This phase highlights the paramount importance of iterative construction, enabling the identification of design flaws early in the development process. Ensuring robust and noise-free power delivery forms the cornerstone of reliable motherboard functionality.

Powerrails of amadore64


Step 2: Clock Signal Generation and Validation

The precise generation and validation of timing signals is a prerequisite for the proper operation of integrated circuits. As legacy components became unavailable, the clock generation circuit underwent a comprehensive redesign utilizing the custom MOS X701 chip. This sophisticated circuit generates two key frequencies:

  • PHI COLOR: The primary clock signal, delivered at the standard operating frequency.
  • PHI DOT: A derivative of PHI COLOR, reduced via frequency division to synchronize auxiliary components.

The circuit’s stability was ensured by routing the signals through a finely tuned LC network, incorporating capacitors and inductors for filtering. This configuration guarantees consistent oscillation amplitudes and reduces phase jitter.

Observations

Errors in clock frequency or timing stability often necessitate significant design revisions, including manual trace cuts and retrofitting with zero-ohm resistors. By isolating and validating this subsystem during an early stage, such complications were mitigated, streamlining the overall development timeline.


Step 3: Reset Signal Architecture

The reset signal plays an integral role in system initialization, ensuring all subsystems stabilize before normal operation commences. Utilizing a 556 dual-timer IC, the reset circuit holds the system in a reset state during power-up, allowing voltage levels to normalize.

However, the timer’s output signal was inverted relative to the desired polarity, necessitating the inclusion of a 7406N open-collector inverter. To ensure the inverter’s proper function, a pull-up resistor was added to stabilize the high output signal.

Results

This subsystem exhibited impeccable performance during initial testing, enabling seamless progression to subsequent assembly phases without requiring modifications.


Step 4: Implementation of Video Output Systems

Following the successful assembly of foundational systems, the focus shifted to video output circuitry. The Video Interface Chip (VIC) and Sound Interface Device (SID) were the primary components installed, serving as the core drivers of video and audio signal generation.

Subsystem Design

  • VIC (U19):
    Handles video signal generation, synthesizing luminance and chrominance outputs for display rendering.
  • SID (U18):
    Renowned for its capability to generate complex audio waveforms, the SID underpins the distinctive sound profile of the Commodore 64.

Despite initial success in generating video signals, a design oversight involving incorrect transistor footprints introduced complications. The issue was resolved by manually reorienting the transistors and adding wire connections to correct the pinout, thereby restoring full functionality. Verification tests revealed a pristine video signal upon completion.

wrong footprint fix


Step 5: Cassette Port Integration

The cassette port subsystem was designed to interface with legacy peripherals, facilitating data loading and storage. This stage encountered challenges due to an error in the transistor footprint, which reversed the base and emitter connections of the switching transistors.

The cassette circuitry consists of a three-transistor amplification and switching stage, culminating in a zener-diode-stabilized voltage output. By rotating the affected transistors and soldering additional wire jumpers, the circuit was restored to full functionality without compromising reliability.


Step 6: Surface-Mount Device Installation

With core subsystems operational, attention turned to the installation of surface-mount devices (SMDs). These components included resistors, capacitors, ferrite beads, and logic ICs, forming the backbone of signal conditioning and power regulation.

Meticulous soldering techniques were employed to ensure proper placement and electrical contact, minimizing the risk of cold joints or misalignments. The installation of these components set the stage for subsequent integration of higher-level components, such as the processor and memory modules.

Outlook

Step 7 will introduce the central processing unit (CPU) and character ROM, enabling the first functional tests of the Amadore64’s computational capabilities.

SMD components mounted


Step 7: Processor and ROM Initialization

The processor, PLA, and character ROM were carefully installed into their respective ZIF sockets. Despite initial successes in powering and clocking the board, the system failed to boot, prompting extensive diagnostic investigations.

Diagnosis and Resolution

The issue was traced to the absence of a pull-up resistor on the NMI (Non-Maskable Interrupt) line, which left the signal floating and susceptible to undefined behavior. After soldering a pull-up resistor directly to the PCB, the system displayed partial functionality, with the processor beginning to execute rudimentary operations.

amadore64 nmi error


Step 8: RAM Module Troubleshooting

The RAM subsystem, comprised of 4164 DRAM chips, initially exhibited unreliable performance. Precision ZIF sockets were installed to ensure stable electrical contacts and facilitate easy chip replacement during diagnostics.

Using a C64 Dead Test Cartridge, the RAM subsystem was rigorously tested, with all banks verified as operational. This marked a pivotal milestone in system stability and functionality.


Step 9: Mitigating Power-Cycling Instabilities

Persistent startup inconsistencies were attributed to voltage instability in the 5V rail. This was caused by insufficient drive current to the IRFR5305 P-channel MOSFET, responsible for regulating the rail.

Corrective Action

The base resistor (R77) for the MOSFET’s driver transistor was replaced with a lower-value resistor, increasing the drive current and enabling full MOSFET activation. This modification resolved the power-cycling issue, ensuring consistent startup behavior.

motherboard testprogram


Step 10: Validation of TOD Signal

The Time of Day (TOD) signal, critical for CIA chip operation, was validated using a multimeter and oscilloscope. This signal, derived from the 9VAC input, was verified to oscillate at the required 50Hz frequency and remain within safe voltage limits.

TOD Signal for CIA


Step 11: Final Assembly and Diagnostic Verification

With all major components installed, the system was powered on, successfully booting to a customized kernal ROM startup screen. Comprehensive diagnostics, facilitated by the 586220++ test suite, confirmed the operational integrity of all subsystems.

amadore64 test harness


Step 12: Thermal Optimization and Design Refinement

The final phase focused on thermal management and design enhancements. Key actions included:

  • Refining PCB footprints to improve assembly precision.
  • Addressing thermal hotspots, particularly in the VIC and power regulation components.
  • Planning the replacement of linear voltage regulators with high-efficiency switching regulators to reduce heat generation.

Thermal Picture of amadore64


Concluding Remarks

The Amadore64 project epitomizes the intersection of retro-computing preservation and modern engineering innovation. By navigating complex assembly challenges and iterative design refinements, the project achieved a functional and meticulously optimized recreation of the Commodore 64. This endeavor stands as a testament to the enduring appeal of vintage computing and the ingenuity required to revive it for a new era.