Nintendo Entertainment System General Maintenance

NES control deck PCB

The Nintendo Entertainment System is now four decades old, and most of what goes wrong with one is mechanical or contact-related rather than electronic. The cartridge connector, the power module and the solder joints around the connectors account for the overwhelming majority of faults. This guide covers preventive care and periodic checks for the front-loading NES-001 and the top-loading NES-101.

Identify your machine

NES console types

Model	Cartridge interface	Video out	Lockout
NES-001 (front loader)	72-pin zero-insertion-force connector fed by a spring-loaded caddy	RF and composite A/V	10NES / CIC fitted (U10)
NES-101 (top loader)	72-pin edge connector, non-ZIF; only 68 of the 72 positions are populated (1–17, 20–53, 56–72)	RF only on all but a small number of factory A/V units	No CIC
Famicom (Japan)	60-pin slot	RF, plus composite on the later AV Famicom	No CIC

The board revision is printed on the PCB silkscreen, not on the case label. NES-001 boards are marked NES-CPU-xx; top-loader boards are marked NESN-CPU-01, NESN-CPU-JI0-01 or NESN-CPU-AV-01.

Chip map

Knowing which package is which saves a great deal of probing. Console5's maps, compiled from boards, are the most complete published lists.

Integrated circuits by position

Position	NES-001	NESN-CPU-01 (top loader)
U1	WRAM (2K×8 SRAM: LH5216AD-10L, MN4216-20 or CXK5816SPS-15L)	RAM: LH5216AD-10L
U2	74LS373	HD74LS373P
U3	74LS139	HD74LS139P
U4	VRAM (2K×8 SRAM, same types as U1)	not fitted
U5	PPU: RP2C02	PPU: RP2C02G-0
U6	CPU: RP2A03	CPU: RP2A03G
U7, U8	74368 (74HC368N or PC74HC368P)	SN74HC368N
U9	74HCU04	+5 V regulator: 7805
U10	CIC (10NES lockout): 3193A	not fitted
Regulator	AN7805, inside the power/RF module	U9, on the mainboard with a heatsink

The relocation of the regulator matters for maintenance. On the NES-001 the 7805 and its heatsink are sealed inside the power/RF modulator can; Nintendo's own description of the board is that the "Power Supply Section" is "contained in the RF Modulator" and "provides all of the operating voltages for the CPU PCB, gamepak and accessory devices". On the NES-101 the regulator is out in the open at U9 with its own heatsink, which is why a top-loader case has to be reassembled carefully: the heatsink is close enough to the rear shell that added A/V jacks can foul it.

The power supply

NES-002 AC adapter

Both the NES-001 and the NES-101 take the NES-002 adapter, which outputs 9 V AC — not DC. The rectifier, reservoir and regulator are all inside the console. This is the opposite of the Super Famicom and the SNES, which take a DC supply, and it is why an SNES or Famicom adapter must never be substituted.

Reading the adapter with a meter

Because the NES-002 puts out AC, there is no regulator in it — a linear regulator cannot act on an alternating output. It is a mains transformer and nothing else, so its terminal voltage falls as the load increases and only reaches the 9 V on the plate at something near the rated 1.3 A. An unloaded reading well above 9 V is what a transformer does.

This has a practical consequence: a high reading on an unloaded NES-002 is not a fault and is not a reason to replace it. Set your meter to AC volts, confirm the adapter produces AC at all, and measure it under load if you want a meaningful figure. Discarding a working adapter because it reads high off-load is a self-inflicted problem — and fitting a "regulated 9 V DC" replacement in its place will not work, because the console expects AC.

The +5 V rail

After the bridge and the reservoir, a 7805-class regulator produces the single +5 V logic rail. Everything on the control deck PCB runs from it. On the NES-001 the regulator is an AN7805 inside the power/RF module; on the NES-101 it is on the mainboard, bolted to a heat sink. A rail measurably outside the 7805 family's specified output window means the regulator, the reservoir capacitor or the bridge needs attention; see Nintendo Entertainment System Capacitor Replacement Guide for the per-module capacitor lists.

The 72-pin cartridge connector

This is the single most common NES fault and the single most misunderstood one.

Nintendo designed the NES-001 with a zero-insertion-force connector. That makes cartridges easy to push in, but it removes the wiping action that a conventional edge connector gets every time a cartridge is inserted — the small scrub that keeps a normal cartridge slot clean. Without it, contamination accumulates and never gets cleared.

Dirty is not the same as worn

• A dirty connector is the usual case and is fixable. Both the connector and the cartridge edge need cleaning — the cartridge as much as the console. Remove the connector from the mainboard and clean its PCB contacts as well as its pins. Isopropyl alcohol on a lint-free swab is adequate for most boards; a dedicated electronics contact cleaner works better on a badly contaminated one.
• A damaged connector has bent, dislocated or missing pins. No amount of cleaning fixes that. Inspect inside the connector before you start cleaning.
• A worn-out connector — pins permanently fatigued by insertion cycles — is much rarer than the folklore suggests. Console5's position, after examining used connectors, is that if the pins were going to take a set they would have done so in the first year, and that NES units left with a cartridge locked in for a decade still work on their original connector. Bending the pins inward does make an NES work again, but what it actually does is raise the contact force; that accelerates plating wear and gives back some of the ZIF behaviour the design depends on.
• Gold that looks like rust usually is not. The plating on OEM connectors can take on a rust-like appearance. A clean paper swipe shows real rust as brown or orange marks and ordinary dirt as black.

Do not blow on the cartridge

Nintendo's own consumer maintenance sheet instructs: "DO NOT touch or blow on the metal game pak edge connector." Console5 gives the mechanism: blowing does briefly improve conductivity, but the moisture in your breath makes the contamination worse over time. This is the one piece of received NES wisdom that is actively harmful.

Aftermarket connectors

Replacement connectors are readily available and are usually the wrong answer. They grip much harder than the original, which makes cartridges difficult to remove and abrades the plating off the pins fast — Console5's examination of failed aftermarket pins found the outer plating already gone, with oxides forming that will then transfer to cartridges. A genuine Nintendo connector carries a moulded number and the Hoseiden logo in the housing, and its fastening holes are flush with the body; aftermarket housings show machining marks around the holes.

Clean the original first. Replace it only when it is physically damaged.

Connector alignment

If the console has ever been apart, the alignment between the 72-pin connector and the spring-loaded cartridge caddy may have shifted. A few degrees of misalignment is enough to make contact intermittent. Loosen the six screws holding the connector and caddy, seat a cartridge to let the assembly find its own position, then retighten. Do this every time the console is stripped for other work.

The blinking light

A flashing power LED with a blank or flashing screen is the 10NES lockout chip resetting the console in a loop. The 10NES in the console talks to a matching chip in the cartridge; if that conversation fails, it holds the machine in reset. The Famicom has no such chip, which is why Famicoms do not do this.

The important point is that a bad connection produces exactly the same symptom as an unlicensed cartridge. Nintendo's own troubleshooting sheet lists the causes of a blinking screen and a blinking power light in order as: game pak not inserted properly, then dirty connectors. Work through the connector first.

Where the connector is sound and the blinking persists, the lockout can be disabled permanently by cutting pin 4 of the IC marked CIC on the board. The console then runs without the lockout doing anything.

Nintendo's fault table

Reproduced from the Control Deck section of Nintendo's technical training workbook. The causes are listed in Nintendo's order of priority.

NES-001 control deck — malfunctions and probable causes

Malfunction	Probable causes
No control deck operation	Defective RF modulator; defective power/reset switch assembly; defective PCB
Excessive current draw or 60 Hz buzz	Defective RF modulator; defective IC(s); crushed capacitors; defective PCB
No reset, display otherwise normal	Defective CIC; defective power/reset switch assembly; defective C8 or C9 (replace C8/C9); defective PCB soldering
Continually resetting display	Defective CIC; defective 7404; defective 4 MHz oscillator; defective power/reset switch assembly; defective PCB soldering
No power light	Defective 7404; defective CIC; defective 4 MHz oscillator; defective power/reset switch assembly; defective PCB soldering
Input port error (with the port test cartridge)	Defective 40368(s); defective CPU; defective DA1–DA4; defective RA1 or RA2; defective PCB soldering
Normal display, no audio / low audio / distorted audio	Defective CPU; defective 7404; defective RF modulator; defective PCB soldering
Games lock up or jump to other parts of the program	Defective CPU; defective WRAM; defective 74139; defective RA1 or RA2; defective PPU
Solid colour or blank display, no sound	Defective 21 MHz oscillator; defective CPU; defective WRAM; defective 74139; defective RA1 or RA2; defective PPU; defective VRAM
Solid colour or blank screen with sound	Defective PPU; defective VRAM; defective 74373; defective CPU; defective Q2; defective RF modulator
Scrambled video images	Defective PPU; defective VRAM; defective 7404; defective 74373; defective CPU

Two entries deserve emphasis. "Crushed capacitors" under excessive current draw is Nintendo telling servicers that a physically damaged capacitor — squashed during a previous repair or by the shield — is a realistic cause of a console that draws too much and hums. And "defective C8 or C9" is a specific, named pair in the reset circuit around U9.

Board sections

Nintendo's own division of the control deck, useful when a fault points at a subsystem rather than a part.

NES-001 control deck sections

Section	Components
Power supply	Contained in the RF modulator. Supplies all operating voltages to the CPU PCB, game pak and accessories, and puts 10 V DC on the RF output to switch the RF switch
Reset	U10 (CIC), 4/6 of U9 (7404) and X2. Blocks unauthorised software; supplies reset to CPU and PPU
Input	U7 and U8 (40368), DA1–DA4, capacitors C26–C37, BC1, BC2, RA1, RA2. Tri-state buffering of controller and Zapper data, plus noise and spike protection
System timing	Q2 and X1. Supplies the 21 MHz clock to CPU and PPU
Display	U5 (PPU), U2 (74373), U4 (VRAM), 1/6 of U9 (7404) and part of the RF modulator
Program	U6 (CPU), U1 (WRAM), U3 (74139) and 1/6 of U9 (7404)

Clocks and test points

The NTSC master crystal X1 runs at 21.47727 MHz. Nintendo's own section breakdown describes the system timing section as Q2 and X1 supplying "a 21 MHz clock signal to the CPU and the P[PU]". Official PAL PPUs expect a 26.601712 MHz master clock instead — the NES generates colour from a master clock six times the colour subcarrier, and the PAL subcarrier is about 24 % higher.

The CPU pin assignments below come from the hosted schematic. They are worth stating explicitly because they are commonly misquoted: pin 1 is an audio output, not reset.

RP2A03 (U6) — pins you will probe

Pin	Signal
1	AD1 (audio output 1)
2	AD2 (audio output 2)
3	RESET
29	CLK (master clock input)
31	M2
32	IRQ
33	NMI
34	R/W
35, 36	OE2, OE1
37, 38, 39	OUT2, OUT1, OUT0 (controller strobe and outputs)
20, 30	GND (pin 30 is TST, tied to ground)
40	+5 V

Test-point waveforms from Nintendo's technical training workbook

Nintendo's workbook carries a plate of oscilloscope traces for eighteen numbered test points, with the timebase and volts-per-division marked on each. The traces were captured running Super Mario Bros. and Duck Hunt, and Nintendo notes that what you see depends on the game and where it is in the program — they are representative, not reference waveforms. The test-point map below matches Console5's transcription of the same list.

Test points

Test point	Component	Pin(s)
TP1	CPU	29
TP2	U9 (7404)	2
TP3	U3 (74139)	14
TP4	U3 (74139)	9
TP5	PPU	21
TP6	U3 (74139)	5
TP7	U7 (40368)	1
TP8	U7 (40368)	2
TP9	PPU	25–30
TP10	CPU	2
TP11	CPU	1
TP12	U9 (7404)	10
TP13	PPU	1
TP14	PPU	23
TP15	PPU	10
TP16	CPU	39
TP17	U3 (74139)	2
TP18	U3 (74139)	4

Things that are not faults

• An unloaded NES-002 reading well above 9 V AC. It is a bare transformer. This is normal.
• Visible vertical lines on a top loader. The NES-101's RF output shows them as designed. They are a known characteristic of the machine, not a failure, and they are why an A/V modification to a top loader involves lifting PPU pin 21.
• A snowy picture with the console off. On an NES-001 with the original RF switch, the aerial feed passes straight through when the console is off. That is the switch working.
• Certain games producing no sound until play starts. Nintendo lists this explicitly as normal behaviour.
• A gold connector that looks rusty. See above.

Cleaning

Case and controllers

• Wipe the ABS shell with a damp microfibre cloth and mild detergent. Avoid abrasives.
• Yellowed plastic can be treated — see Retrobrite — but watch surface temperature and exposure; over-treated ABS becomes brittle.
• Open controllers with a Phillips driver, wash the button caps and D-pad in warm soapy water and dry thoroughly. Clean the PCB contacts with isopropyl alcohol; replace the conductive pads if buttons stay unresponsive.

Cartridge slot and ports

• Unplug the console first. Clean the 72-pin connector and the cartridge edges as described above.
• Clean the controller ports and the A/V sockets with isopropyl alcohol and a soft brush.

Inside

• Blow dust out with compressed air and use an anti-static brush on stubborn deposits.
• Inspect the solder joints at the DC jack, the A/V sockets, the controller ports and the power/reset switch assembly. Nintendo's fault table names the power/reset switch assembly as a probable cause of four separate symptoms, and cracked joints at the connectors cause intermittents that look like chip failures.
• Do not use water on internal components.

Tools

• Phillips #2 driver. The NES-001 shell is held by six ordinary Phillips screws — no security bit is needed to open the console, and an unmodified machine should contain nothing but Phillips fasteners.
• 3.8 mm Gamebit driver — for opening NES cartridges, not the console
• Digital multimeter with an AC volts range
• ESD wrist strap and anti-static brush
• Isopropyl alcohol (99 %) and lint-free swabs
• Soldering iron, flux and desoldering braid for connector and capacitor work

See Recommended Tools for the general list.

Preventive maintenance

1. Check the adapter produces AC before connecting it to a console you have not used in years.
2. Clean the cartridge connector and your cartridge edges when games start needing a second insertion — not after they have failed completely.
3. Reset the connector-to-caddy alignment any time the console has been apart.
4. Inspect for capacitor leakage on the power/RF module annually. The mainboard has only three electrolytics and they rarely leak; the module is where the large parts are.
5. Reflow the solder at the power jack, A/V sockets and controller ports if intermittents appear.
6. Store the console with the cartridge door closed — Nintendo's first maintenance instruction, and it still works.

Related pages

• Nintendo Entertainment System
• Nintendo Entertainment System Capacitor Replacement Guide
• Nintendo Entertainment System Troubleshooting Guide
• NES-001 Schematic
• NOA Independent Servicer Technical Training Workbook
• Capacitor Failure Symptoms