4164/41256 RAM Modification
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Williams MPUs shipped with 24 × Mostek 4116 DRAM (16,384 × 1-bit), introduced in 1977 — only a few years before Defender.
These chips need three voltages (triple-rail):
- +5 V — TTL input/output buffers
- +12 V — primary power
- −5 V — substrate bias / shield
If −5 V drops while +5 and +12 are still present, the chip overheats and fails quickly.
That triple-rail requirement made the 4116 a weak link on these boards, especially as the linear supplies aged and the rails got noisy.
4164 First Successor
In the early 1980s the direct successor appeared: the 4164 (65,536 × 1-bit).
An internal bias generator supplies the −5 V, so the part runs from a single +5 V rail. Pins that used to carry the extra supplies became another address bit, which is how capacity went up. 4164s generally run cooler and tolerate supply variation better.
41256 Last 16-Pin Successor
By the late 1980s density had increased again. Using the last unused supply pin as another address line gave the 41256 (262,144 × 1-bit).
Pinout Comparison
Here is a comparison in pinout:
Modify the MPU
What we need to do: For the 4164/41256 to work, we need to accomplish a few things.
- Remove the -5v from Pin 1
- Remove the +12v from Pin 8
- Substitute +5v on Pins 1 and 8 (If we left these unconnected/floating, the address value would randomly change)
- The goal is to have +5 on Pins: 1, 8 and 9. Ground stays on Pin 16
- This mod does NOT damage the PCB; no cutting traces, etc. Everything can easily be undone/reverted!
- No change to wiring harness or power supply needed - board will work in all cabinets
Original MPU Power Connector Pinout
Step 1 — Cut pins 5 (+12 Regulated) and 9 (−5 V). Keep one of the cut-off pins.
- Note — The polarizing key is on Pin 4. With the mod, if the connector is reversed/flipped it can't be plugged in.
- Note — Pin 1 is on the right and Pin 9 is on the left.
Step 2 — Modify the solder side of the MPU
