PLA Replacement Options

The original MOS 906114-01 Programmable Logic Array (PLA) in early C64 board revisions (ASSY 326298 → 250466) is _the_ single highest-failure-rate part in the machine. It runs hot (≈ 65 °C surface on an un-ventilated bread-bin), consumes ≈ 120 mA, and any internal electrical overstress manifests as:

• Black screen / no border on power-up.
• Colour-flashing “garbage” that crashes after warm-up.
• Selective cartridge or bank-switch errors (EXROM/GAME decoding faults).

Because original spare parts are scarce and almost forty years old, several replacement families have appeared. They differ in technology, power draw, compatibility and cost.

Selection criteria

Criterion	Why it matters
Timing accuracy	PLA outputs must meet a < 60 ns access window to avoid VIC-II bus contention; marginal parts work in BASIC but fail with fast cartridges.
Voltage tolerance & heat	Lower idle current (≤ 20 mA) keeps internal case temperature down and avoids over-loading the 5 V rail. PLAnkton, for example, dissipates < 0.2 W compared with ≈ 0.6 W for the ceramic MOS part.
Board compatibility	250469 “short-board” machines integrate the PLA inside the 251715-01 gate-array; external plug-ins are not required there.
Cartridge support	Fast loaders (Epyx FastLoad, KFF, EasyFlash) depend on tight /CASRAM and /ROML asserts — earlier EPROM-based substitutes can break these.

Replacement families

1. Salvaged original MOS ceramic PLAs

• Perfect electrical fit if tested good, but still run hot and may fail again.
• Short supply & rising price (US $40–80 on auction sites).

2. EPROM + TTL glue (first-generation DIY)

• EPROM (27C512/27C256) programmed with PLA truth-table, gated by 74LS04/74LS86.
• Cheap but slow output edges and ~65 mA current → will over-heat inside a closed case.
• Timing marginal with some Ocean/AR II cartridges.

3. GAL / PAL-based (PLA20V8, ThED PLA20V8, “128 PLA in C64”)

• Single GAL20V8 or ATF22V10C, pin-compatible.
• Draw ≈ 20 mA, emit little heat.
• Open-source JEDEC; hobbyists can burn their own chips.
• Slightly higher propagation delay than CPLD, but passes timing on > 99 % of boards in field reports.

4. CPLD / FPGA-based (current gold standard)

Replacement	Logic core	Idle current	Notable points
SuperPLA V3	Xilinx XC9536XL	≈ 15 mA	Earliest “plug-and-play” commercial part, wide PAL/NTSC & cartridge compatibility. 20 € PCB kit.
U17 PLAnkton	Altera MAX CPLD	< 10 mA	Eight-layer µPCB nestled inside a plastic SIL socket; runs cool & supports C16/+4 PLAs via jumper.
RealPLA / “Ultimate PLA”	Lattice ispMACH 4064	12 mA	Shipped with _Ultimate-64_ boards; cartridge timing verified with Kung-Fu Flash & Epyx.
SaRuMan-64	iCE40LP384 FPGA	~ 8 mA	Adds a tiny linear-drop regulator to isolate noisy 5 V rails; user-upgradable bit-stream.

5. Multi-chip TTL recreations (educational)

• 27 × 74LS/AHC gates that implement the PLA equations.
• Excellent edge-rate but enormous current draw; avoided except as teaching tools.

Installation

1. Power off & unplug PSU; discharge by pressing the power switch several times.
2. Gently remove the original PLA from socket U17 using a PLCC puller or flat tool.
3. Visually inspect the socket for oxidation or lifted pads.
4. Align the notch or pin-1 indicator on the replacement with the board silk.
5. Firmly press straight down — do not rock side-to-side.
6. Power on and test:
   1. BASIC screen appears → `PRINT FRE(0)` returns 38911.
   2. Run a fast-loader cartridge or *Dead Test* to exercise all decode paths.

Comparison summary

Solution	Cost	Heat	Cartridge compat.	Availability
Salvaged MOS	High	🔥🔥🔥	✅	declining
EPROM TTL	Low	🔥🔥	⚠️ mixed	DIY only
GAL20V8	Low-mid	🔥	✅	open-source
CPLD/FPGA (e.g. PLAnkton)	Mid (15–25 €)	❄️	✅✅✅	multiple stores
TTL array	High (parts)	🔥🔥🔥	✅	niche

Further reading

• Raymond Carlsen, “PLA Failures and Substitutes”