Excellent questions! It's very smart to be thinking about preventative maintenance on your Black Knight 2000. You're touching on some of the most important and well-known reliability issues in the Williams System 11 family.
Let's break down your questions.
1. The Unfused Bridge Rectifier Fire Hazard
You are correct that early System 11 games (like High Speed, F-14 Tomcat, etc.) had a serious design flaw. The AC lines from the transformer's secondary windings ran directly to bridge rectifiers, which were often mounted in the backbox. If a bridge rectifier shorted, it would draw unlimited current from the transformer, causing the transformer to overheat, smoke, and in some reported cases, catch fire.
The good news is that Williams fixed this issue with the System 11B revision.
Your Black Knight 2000 is a System 11B game. The key architectural change from System 11A to 11B was the introduction of the
Auxiliary Power Supply Board (C-11998).
- How System 11B Fixed It: On your machine, the AC power from the transformer goes to the Auxiliary Power Supply board first. On this board, the AC lines pass through fuses (F1, F2, F3, and F4) before they reach the bridge rectifiers (BR1, BR2, BR3, BR4), which are also mounted on that same board.
Conclusion: Yes, your System 11B machine has fuses in place to protect the transformer from a shorted bridge rectifier. The cluster of fuses you see on the auxiliary power supply board is providing exactly that protection. You do not need to add any new fuses.
2. Replacing the Bridge Rectifiers
Even though the fire hazard is addressed, your second question is still extremely relevant.
Regardless of the fuses, is it worth replacing the bridge rectifiers for new ones with heat sinks?
Answer: Yes, absolutely. This is one of the best and most highly recommended preventative maintenance tasks you can perform on a System 11B/11C machine.
Here's why:
- Age and Heat Stress: The original bridge rectifiers are now over 30 years old. They are under constant thermal stress every time the machine is on, especially the ones driving the main logic power and the lamps. Electronic components degrade over time with heat, and bridge rectifiers are a common failure point.
- Improved Reliability: A failing bridge rectifier doesn't always die spectacularly by shorting out. More often, it will start to fail "softly." Diodes within the bridge can open, leading to half-wave rectification. This results in weak or noisy DC voltage, which can cause all sorts of bizarre and hard-to-diagnose problems, such as:
- Game resets
- Humming or buzzing from the speakers
- Flickering lights (especially GI)
- Weak flippers or coils
- Inadequate Original Cooling: The original design simply bolted the bridge rectifiers to the circuit board, using the board's ground plane as a minimal heat sink. This is adequate, but not great. Heat is the enemy of longevity for these components.
The Recommended Upgrade Path:
When you replace the bridge rectifiers, you have an opportunity to make a significant upgrade for very little cost.
- What to Buy:
- New Bridge Rectifiers: Get new, modern replacements with a higher rating than the originals. A KBPC3504 (35 Amp, 400 Volt) or similar is a perfect, robust replacement. They are very inexpensive. You will need four of them for the Auxiliary Power Supply board.
- Heat Sinks: Buy four small, individual heatsinks that can be bolted onto the new bridge rectifiers. Search for "KBPC heatsink" or "bridge rectifier heatsink."
- Thermal Compound: A small tube of thermal paste (like the kind used for computer CPUs) is essential. This ensures efficient heat transfer from the rectifier to the heatsink.
- The Process:
- Power off the machine and unplug it.
- Remove the Auxiliary Power Supply board.
- Carefully desolder and remove the four old bridge rectifiers.
- Apply a thin layer of thermal paste to the back of each new rectifier.
- Bolt the new rectifier to its new heatsink.
- Solder the new rectifier/heatsink assemblies onto the board. Ensure the polarity markings (+, -, AC) on the new bridge align with the markings on the PCB.
- Reinstall the board.
The result will be a rock-solid, reliable power supply that runs cooler and will last for decades, eliminating a major source of future problems.
Other Key Preventative Maintenance for BK2000
While you have the soldering iron out and you're in the mood for reliability mods, here are a few other highly recommended items for a System 11B machine:
- Replace Connectors: The #1 cause of problems in games of this era is bad connectors. Inspect all the power connectors, especially the GI (General Illumination) connectors on the interconnect board. If they look browned or burnt, replace the pins in the connector housing and the header pins on the board. At a minimum, reseat them all firmly.
- Replace Capacitors: The large filter capacitors on the power supply boards dry out with age.
- Auxiliary Power Board: Replace the large axial capacitor (C2, typically 15,000uF at 25V).
- CPU Board: Replace the main 5V filter capacitor (C30, typically 100uF at 25V). This is a very common cause of game resets.
- Batteries / NVRAM: Check the CPU board for any battery leakage from the original AA batteries. If there is any green/white corrosion, it needs to be neutralized and cleaned. A great upgrade is to install a remote battery holder (to move the batteries off the board) or, even better, replace the memory chip (6116 RAM) with an NVRAM chip. This eliminates the need for batteries entirely.
Enjoy your Black Knight 2000! It's a fantastic game, and putting in this work now will ensure it runs reliably for many years to come.
