Editing Amstrad GX4000 Troubleshooting Guide

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[[File:Amstrad GX4000 rear connectors.jpg|thumb|right|360px|GX4000 rear panel: 5&nbsp;V and 11&nbsp;V DC inputs, monitor DIN, Peritel socket, sound switch and UHF socket]]
Amstrad's service manual for the [[Amstrad GX4000]] has no fault-finding charts. A note to engineers on its second page refers them to the RP11 diagnostic ROM cartridge and its own manual, which are not hosted here.<ref name="smgx">Amstrad plc, ''464 PLUS Home Computer / 6128 PLUS Home Computer / GX4000 Games Console / MM 12 Monochrome Monitor / CM 14 Colour Monitor Service Manual'', GX4000 section. Hosted as [[:File:Amstrad GX4000 Service Manual.pdf]], described at [[Amstrad GX4000 Service Manual]]. Note to engineers p. 2; specification and electrical adjustment p. 23; main PCB top view p. 24; power regulator schematic p. 29; RGB converter schematic p. 30; power regulator schematic (France) p. 34; electrical parts list p. 36.</ref> The checks below come from Amstrad's user instructions, the GX4000 schematics and parts list, and the shared Plus range hardware. Procedures are in the [[Amstrad GX4000 Maintenance Guide]], and the [[Amstrad 464 Plus Troubleshooting Guide]] covers the same chip set in the Plus computers.

== Owner checks ==

The user instructions give these checks before anything is opened.<ref name="uigx">Amstrad plc, ''GX4000 User Instructions'' (1990). Hosted as [[:File:Amstrad GX4000 User Instructions.pdf]], described at [[Amstrad GX4000 User Instructions]]. English section: looking after the console p. 1, features p. 2, TV connection p. 3, operation p. 5, problem solving p. 6, connector pinouts p. 7.</ref>

{| class="wikitable styled-table" style="width:70%; text-align:left;"
|+'''Amstrad's problem-solving table (user instructions p. 6)'''
! Symptom !! Probable cause !! Action
|-
| No picture, screen lit || Console off, no power, wrong channel, cartridge not seated || Check the power/interlock switch is on, the adaptor is plugged in and switched on, the TV lead is in the aerial socket, the cartridge is fully inserted, and the TV is tuned to channel 36
|-
| Picture rolls or has bars or lines || TV or monitor hold controls || Adjust the vertical and horizontal hold
|-
| Poor picture || Loose leads, tuning, console too close to the TV || Check the leads, tune manually with automatic fine tuning off, move the console away from the TV
|-
| No game sound || Sound switch, TV tuning || Change the sound switch on the console; adjust the TV volume and fine tuning
|-
| TV programmes not received || Console still connected || Switch the console off and disconnect the TV lead; check the aerial
|}

== Power ==

The console has two DC inputs, drawn on the power regulator schematic (p. 29).<ref name="smgx" /> The 11&nbsp;V adaptor input J102 feeds the L7805 regulator IC181, whose 5&nbsp;V output joins the 5&nbsp;V monitor input J101 after the filter L1. That line passes through the power switch SW1 to become the console's +5&nbsp;V. On the UK board the 11&nbsp;V input also feeds a µPC78L05 (IC17) for the RF modulator supply, and a diode, capacitor, resistor and zener (D181, C182, R181, D180) for a +10&nbsp;V line. The power LED is D1.<ref name="smgx" />

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Supply points (power regulator schematic, p. 29)'''
! Point !! Labelled on the schematic !! Fed from
|-
| J102 || 11&nbsp;V || AC adaptor (UK adaptor rated 11&nbsp;V DC 500&nbsp;mA)
|-
| J101 || 5&nbsp;V || MM12 or CM14 monitor
|-
| IC181 output (L7805, 5&nbsp;V 1&nbsp;A) || 5&nbsp;V || J102
|-
| After SW1 || +5&nbsp;V || IC181 or J101
|-
| IC17 output (µPC78L05, 5&nbsp;V 0.1&nbsp;A), UK board || +5&nbsp;V MOD || J102
|-
| After R181, across D180 (GZB12B) || +10&nbsp;V || J102 through D181 and C182
|}

The adaptor rating is from the label on the UK unit.<ref name="psuphoto">Evan-Amos, [https://commons.wikimedia.org/wiki/File:Amstrad-GX4000-Power-Supply.jpg "Amstrad-GX4000-Power-Supply.jpg"], Wikimedia Commons, 2017 (public domain). Retrieved 2026-10-02. Rating label (GX4000 PSU/UK, 240 V 50 Hz in, 11 V DC 500 mA out) and connection warning on the UK adaptor.</ref> The France regulator drawing (p. 34) has no IC17; there the modulator supply comes from the +5&nbsp;V line through link LK7.<ref name="smgx" />

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Power faults'''
! Symptom !! Probable cause !! Action
|-
| No power LED, running from the adaptor || Adaptor, lead, J102 || Measure the adaptor output at its plug, then at J102 with the plug in place. Try running the console from a monitor through J101.
|-
| No power LED, running from a monitor || Monitor 5&nbsp;V output, lead, J101 || Measure 5&nbsp;V at the monitor's DC plug and at J101. The monitor's 5&nbsp;V setting is on the [[Amstrad 464 Plus Troubleshooting Guide]].
|-
| 11&nbsp;V at J102, no 5&nbsp;V after SW1 || IC181, SW1, L1 || Measure the input and output of IC181, then each side of SW1
|-
| Works from a monitor, not from the adaptor || Adaptor, J102, IC181 || The two inputs share everything after IC181, so check the adaptor path
|-
| Picture on an Amstrad monitor, no RF picture when running from the adaptor (UK board) || IC17 or the modulator || Measure the +5&nbsp;V MOD output of IC17
|}

Amstrad says to connect the DC plug to the console before plugging the adaptor into the mains, and to use only the supplied adaptor or a monitor's power lead.<ref name="uigx" /><ref name="psuphoto" />

== Picture ==

The monitor DIN socket carries separate red, green and blue, sync and luminance (pins 1 to 5).<ref name="uigx" /> For a television, the RGB encoder IC101 (Sony CXA1145P) produces the signals for the Peritel socket JD102 and composite video for the RF modulator IC102 (UM1234E36).<ref name="smgx" /> A good picture on an Amstrad monitor with no picture on a television therefore points at the encoder, the modulator or their supplies; no picture on either points at the cartridge, the ASIC, the processor or the RAM.

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Picture faults'''
! Symptom !! Probable cause !! Action
|-
| Blank screen on every output, power LED on || Cartridge not seated, dirty cartridge or CE101 contacts || Switch off, reseat the cartridge, clean its contacts and CE101
|-
| Blank on every output with a known-good cartridge || ASIC (IC1), CPU (IC2), RAM (IC10, IC11), crystal X1 || Check +5&nbsp;V at the chips and the 39.9025&nbsp;MHz crystal; see the chips section below
|-
| Monitor good, no RF picture || Tuning, modulator coil, IC17, IC102 || Tune to about channel 36, then adjust the modulator coil (p. 23)
|-
| Monitor good, no Peritel picture || Peritel lead, TV input selection, +10&nbsp;V line, IC101 || Check that the TV switches to its Peritel input. The +10&nbsp;V line feeds the Peritel socket through R153 and R139 (p. 30).
|-
| One colour missing on Peritel || That channel's output path || The RGB outputs pass through 470&nbsp;µF capacitors C102, C103, C104 and 75&nbsp;Ω resistors R106, R129, R130 (p. 30); check that channel's parts
|-
| Picture rolls or has bars || Hold controls, tuning || Adjust the TV or monitor hold controls (user instructions p. 6)
|}

== Sound ==

The rear sound switch SW101 selects a 6.0&nbsp;MHz (X101) or 5.5&nbsp;MHz (X102) sound carrier for the RF output (p. 30).<ref name="smgx" /> If a television gives a picture but no sound through the RF lead, change the switch position first, as the user instructions say.<ref name="uigx" /> The monitor socket carries left and right sound on pins 6 and 7, and the 3.5&nbsp;mm socket feeds an external stereo amplifier.<ref name="uigx" /> Sound present on the monitor or amplifier but missing through RF points at the RF sound circuit (Q107 to Q110 and the two ceramic oscillators); silence on every output points at the sound circuit around the AY-3-8912 (IC14).<ref name="smgx" />

== Cartridges and software ==

The parts list has no ROM on the main board; the program comes from the cartridge. Amstrad's instructions are to switch off with the power/interlock switch before inserting or removing a cartridge and never to use force.<ref name="uigx" /> Clean the cartridge contacts and inspect CE101 before suspecting the board. The supplied cartridge assembly is part 41088.<ref name="smgx" />

== Paddles and other controllers ==

Each paddle socket has up, down, left, right, two fire buttons and a common line; on the second paddle socket pin 9 is not COM2.<ref name="uigx" /> A direction that never works on one paddle but works on the other is the paddle or its lead; one that fails on both paddles points at the console. The paddle part number is 270792.<ref name="smgx" /> Some games use the 15-pin analogue joystick socket or the RJ11 light-gun socket instead.<ref name="smgx" />

== Chips ==

The main logic is the AMS40489 ASIC (IC1), the Z8400AB1N processor (IC2), two KM41464AP-12 RAMs (IC10, IC11), the AY-3-8912 (IC14) and the AMS40464 video DAC (IC15), the same parts as in the Plus computers.<ref name="smgx" /> Amstrad expected these to be diagnosed with the RP11 cartridge.<ref name="smgx" /> The [[Amstrad 464 Plus Troubleshooting Guide]] covers the common chip set.

== Related pages ==

* [[Amstrad GX4000]] · [[Amstrad GX4000 Maintenance Guide]] · [[Amstrad GX4000 Capacitor Replacement Guide]]
* [[Amstrad 464 Plus Troubleshooting Guide]] · [[Amstrad 6128 Plus Troubleshooting Guide]]
* [[Capacitor Failure Symptoms]] · [[CRT Discharge Procedure]]

== References ==
<references />

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