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Nintendo Entertainment System CIC Lockout Disable

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NES-001 CIC U10 pin 4 (NES-CPU-07)
The lockout chip U10 (3193A) on an NTSC NES-CPU-07 board, with pin 4 circled. The 72-pin connector is towards the bottom of the picture (crop of a photo by Evan-Amos, public domain)

The front-loading NES-001 Nintendo Entertainment System has a lockout chip, the CIC, at U10 on its mainboard. An identical chip sits in every licensed cartridge. The console's chip holds the CPU and PPU in reset until the two chips agree, and if they ever disagree it resets the console repeatedly.[1] The repeated reset runs at 1 Hz, which is why the picture and the power LED blink together.[2]

Because the console's chip occasionally fails to authenticate legal cartridges, a common modification is to disable it by cutting pin 4.[2] Pin 4 sets whether a chip acts as the lock or the key: it is at +5 V in the console and at 0 V in the cartridge. With pin 4 of the console's chip disconnected or grounded, both chips are keys and neither does anything, so the console is never reset.[1] Kevin Horton records that the unlicensed publisher AVE gave its customers the same instruction, cutting pin 4, and calls it "quite effective".[3]

The top-loading NES-101 has no lockout chip and does not need this modification.[2]

Symptoms

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  • The screen and power LED blink about once a second with a game inserted, and cleaning the game and the connector does not stop it.[2]
  • Unlicensed games, or games from another region, blink on a console that runs its own region's licensed games. Knibbs found that NES-CPU-11 consoles would not run some unlicensed games, such as Action 52 and Crystal Mines, until the chip was disabled.[1]

Dirty connectors cause the same blinking, and Nintendo's leaflet lists a game pak not pushed fully in and dirty connectors as the causes.[4] Clean the connector and the games first: see Nintendo Entertainment System 72-Pin Connector Replacement. Disabling the chip stops the reset loop only; a poor contact still gives the solid-colour and scrambled screens that the leaflet puts down to dirty connectors.[4] For blinking with a type A power/reset switch, see Nintendo Entertainment System Power and Reset Switch Replacement, and for the rest of Nintendo's fault table see the Nintendo Entertainment System Troubleshooting Guide.

Safety

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  • Unplug the AC adapter before opening the console.
  • The metal shields are very sharp; Nintendo recommends gloves.[5]
  • Cutting pin 4 is a permanent change. Knibbs suggests desoldering the whole chip and bending pin 4 out as a neater alternative, but notes it is hard without an IC desoldering tool.[1]

Tools and parts

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From Knibbs's list:[1]

  • A Phillips (crosshead) screwdriver for the case and the screws inside.
  • A very sharp, very small knife or a small pair of scissors to cut the pin. A two-sided cutter such as scissors is better than a knife.
  • A soldering iron and solder, and optionally desoldering braid.
  • About 2 cm of thin insulated wire with stripped ends, if you are going to ground pin 4.

Procedure

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Disassembly

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  1. Open the console and lift out the mainboard as described in steps 1 to 8 of Nintendo Entertainment System 72-Pin Connector Replacement#Disassembly, but leave the 72-pin connector on the board.[1][5]
  2. Lay the board component side up, with the black cartridge connector towards you.[1]
  3. Note the board revision printed in white near the centre (for example NES-CPU-11) and the region sticker (for example NTSC or PAL-MTL).[1]

Disabling the chip

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NES-001 CIC circuit (Schenk redraw)
The CIC circuit from a 2020 redraw of the NES-001 schematic: pin 4 (SEL) of U10 is tied to +5 V, and pins 11 to 15 go to ground
  1. Find the lockout chip. The silkscreen above it reads "U10 CIC". It has 16 pins and is marked 3193A on US consoles; Knibbs also lists 3195A (European), 3196A (Hong Kong) and 3197A (UK).[1]
  2. Find pin 4: on the row of pins nearest to you, the fourth from the left.[1] In the photograph at the top of the page the chip's notch is at the left.
  3. If the capacitors beside the chip are in the way, bend them gently aside. On the NES-CPU-07 board in the photograph, C9 and C8 sit just below the pins.[1]
  4. Optionally, remove the solder from pin 4 with desoldering braid first.[1]
  5. Cut pin 4 as close to the board as you can, so that a decent length stays on the chip, and bend it up and away from the board. If the whole pin breaks off, Knibbs found that the console still works.[1]
  6. Optionally, solder the length of wire from pin 4 to ground. Pins 11, 12, 13, 14 and 15 of the chip are grounded; pin 15 is on the far row, second from the left. Knibbs found the console works with pin 4 left unconnected.[1][6]

Reassembly

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  1. Before closing the case, test the board: put it back in the lower housing, connect the power and controller leads, slide the game pak mech into position with its lip below the board, insert a game and switch on.[1]
  2. Reassemble the console as described in Nintendo Entertainment System 72-Pin Connector Replacement#Reassembly.

How the circuit works

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In the redrawn NES-001 schematic, U10's pin 4 (SEL) is tied straight to +5 V, which is what makes the console's chip the lock. Its clock comes from the 4 MHz oscillator X2 through sections of the 74HC04 at U9, pins 1 and 2 are its data output and input to the cartridge, pin 7 takes the reset button line and pin 9 drives the CPU reset.[6] The same reset line also drives the power LED through U9D and R5, which is why the LED blinks while the chip is resetting the console.[6]

Testing

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  • Switch on with no cartridge inserted. With the chip disabled the screen is blank and does not flash.[1]
  • Run a licensed game from the console's own region. It should start normally.[1]
  • Games made for the other television standard may still fail with the chip disabled. Knibbs lists High Speed and Pin Bot, which hang the console, and Time Lord, Digger T. Rock and other games developed by Rare as incompatible between PAL and US consoles.[1]

Revision notes

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  • Knibbs carried out the modification on a UK NES-CPU-11 board and had it reported working on a US NES-CPU-11.[1]
  • Kevin Horton records that Nintendo added 1 kΩ series resistors and protection diodes to later consoles to stop the voltage-spike lockout defeaters used in some unlicensed cartridges. Cutting pin 4 does not depend on those lines.[3]
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References

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  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 Mark Knibbs, Disabling the NES "Lockout Chip", rev. 0.4, 22 September 1997 (copy hosted by heinpragt.nl; the author permits unmodified copies). How the lock and key chips work and the role of pin 4; the modification and why it works; tools; steps 1 to 14; chip markings 3193A, 3195A, 3196A and 3197A; boards tested (NES-CPU-11); game compatibility after the modification.
  2. ↑ 2.0 2.1 2.2 2.3 Wikipedia, "CIC (Nintendo)" (retrieved 2026-10-25). The 1 Hz reset, the occasional failure to authenticate legal cartridges, cutting pin 4 as a common modification, and the absence of a lockout chip in the top-loading NES.
  3. ↑ 3.0 3.1 Kevin Horton, "The Infamous Lockout Chip", kevtris.org (retrieved 2026-10-25). AVE's instructions to cut pin 4 (master/slave); Nintendo's later 1 kΩ series resistors and diodes on the lines used by voltage-spike lockout defeaters.
  4. ↑ 4.0 4.1 Nintendo of America, Product Maintenance, Troubleshooting and Service Guide (World Class Service leaflet), troubleshooting table, "Blinking/flashing screen" and "Blinking power light"; "Solid color screens" and "Scrambled screen" also put down to dirty connectors. Hosted as File:Nintendo Entertainment System Product Maintenance Troubleshooting and Service Guide7.pdf.
  5. ↑ 5.0 5.1 Nintendo of America, Authorized Service Center Manual, revision June 2002, chapter 4, disassembly procedure, pp. 26-28. Hosted as File:Nintendo Authorized Service Center Manual June 2002.pdf.
  6. ↑ 6.0 6.1 6.2 N. Schenk (Schenktronics), NES Schematics, file Original_NES.sch, rev. 1.0, 2020-06-05, CIC block: U10 3193A pin 4 (SEL) to +5 V, pins 11 to 15 to ground, pin 16 to +5 V, pin 8 to ground. Hosted as File:NES-001.pdf; see NES-001 Schematic.
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