Acorn System 2 Troubleshooting Guide - Revision history

Josh: Link the System 2 Technical Manual documentation page

2026-10-01T18:04:11Z

Link the System 2 Technical Manual documentation page

← Older revision		Revision as of 19:04, 1 October 2026
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	[[File:Acorn System 2.jpg\|thumb\|360x360px\|Acorn System 2 in its 19-inch card frame]]		[[File:Acorn System 2.jpg\|thumb\|360x360px\|Acorn System 2 in its 19-inch card frame]]

	A working '''[[Acorn System 2]]''' shows "Acorn Cos" and an asterisk prompt on the display when the reset key is pressed.<ref name="s2tm">Acorn Computers, ''System 2 Technical Manual'', issue 1, January 1980. Text transcription by Ben Harris (1998), hosted on this wiki as [[:File:Acorn System 2 Technical Manual (text transcription).pdf]]. Parts lists pp. 2-1 to 2-4; CPU card set-up and COS ROM link pp. 3-6 to 3-8; memory card p. 3-10; backplane links p. 3-12; power supply and keyboard pp. 5-1 and 5-2; cassette and display pp. 5-2 to 5-4; Cassette Operating System commands and messages, chapter 6.</ref> Fault-finding starts from that sign-on. The CPU and cassette cards are [[Acorn System 1]] boards, so the [[Acorn System 1 Troubleshooting Guide]] covers their circuits. The System 2 sets them up differently, as below.		A working '''[[Acorn System 2]]''' shows "Acorn Cos" and an asterisk prompt on the display when the reset key is pressed.<ref name="s2tm">Acorn Computers, ''System 2 Technical Manual'', issue 1, January 1980. Text transcription by Ben Harris (1998), hosted on this wiki as [[Acorn System 2 Technical Manual]] ([[:File:Acorn System 2 Technical Manual (text transcription).pdf\|PDF]]). Parts lists pp. 2-1 to 2-4; CPU card set-up and COS ROM link pp. 3-6 to 3-8; memory card p. 3-10; backplane links p. 3-12; power supply and keyboard pp. 5-1 and 5-2; cassette and display pp. 5-2 to 5-4; Cassette Operating System commands and messages, chapter 6.</ref> Fault-finding starts from that sign-on. The CPU and cassette cards are [[Acorn System 1]] boards, so the [[Acorn System 1 Troubleshooting Guide]] covers their circuits. The System 2 sets them up differently, as below.

	== No sign-on ==		== No sign-on ==

Josh: Rewrite from Acorn's System 2 Technical Manual: COS sign-on and messages, COS ROM link, keyboard and backplane links, VDU checks; remove the +/-12 V VDU rail (System 2 uses 5 V only) and uncited claims

2026-10-01T08:12:27Z

Rewrite from Acorn's System 2 Technical Manual: COS sign-on and messages, COS ROM link, keyboard and backplane links, VDU checks; remove the +/-12 V VDU rail (System 2 uses 5 V only) and uncited claims

← Older revision		Revision as of 09:12, 1 October 2026
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	[[File:Acorn System 2.jpg\|thumb\|360x360px\|Acorn System 2 in a 19-inch ~~Eurocard~~ card frame.]]		[[File:Acorn System 2.jpg\|thumb\|360x360px\|Acorn System 2 in its 19-inch card frame]]

	~~This guide gives component-level diagnosis for the~~ '''[[Acorn System 2]]''' (~~1980~~). ~~Because the System~~ 2 ~~is a~~ set ~~of Eurocards on an~~ 8-~~slot~~ backplane, ~~diagnosis is by card: isolate the faulty card, then probe it against its circuit diagram~~. The CPU and cassette cards are ~~the~~ [[Acorn System 1]] ~~cards~~, so the [[Acorn System 1 Troubleshooting Guide]] ~~applies to them directly~~.~~<ref name="cat">[[Acorn~~ System ~~Catalogue]] and [[Acorn System 1 Technical Manual]]~~, ~~Acorn Computers — hosted on this wiki~~. ~~Source for the Eurocard set and the shared CPU/cassette/VDU/memory card detail.</ref>~~		A working '''[[Acorn System 2]]''' shows "Acorn Cos" and an asterisk prompt on the display when the reset key is pressed.<ref name="s2tm">Acorn Computers, ''System 2 Technical Manual'', issue 1, January 1980. Text transcription by Ben Harris (1998), hosted on this wiki as [[:File:Acorn System 2 Technical Manual (text transcription).pdf]]. Parts lists pp. 2-1 to 2-4; CPU card set-up and COS ROM link pp. 3-6 to 3-8; memory card p. 3-10; backplane links p. 3-12; power supply and keyboard pp. 5-1 and 5-2; cassette and display pp. 5-2 to 5-4; Cassette Operating System commands and messages, chapter 6.</ref> Fault-finding starts from that sign-on. The CPU and cassette cards are [[Acorn System 1]] boards, so the [[Acorn System 1 Troubleshooting Guide]] covers their circuits. The System 2 sets them up differently, as below.

	== ~~Method: isolate the card~~ ==		== No sign-on ==

	A ~~racked system~~ is ~~best diagnosed by reducing it~~ to a ~~minimum~~ and ~~adding cards back~~:		# Supply: 5 V at the backplane, at least 1.5 A (2 A with 8 KB of RAM). No card has its own regulator.<ref name="s2tm" />
			# Reset: the reset button on the keyboard connects to 0 V on pin 19 of the CPU card's keyboard connector.<ref name="s2tm" />
			# COS ROM: the Cassette Operating System is in IC7 on the CPU card. It needs a track broken on side 2 and a wire link from pin 5 to pin 9 of the ADR SEL position; check both on a card that has been changed or swapped from a System 1.<ref name="s2tm" />
			# Clock and processor: check the 1 MHz clock and the 6502 as on the System 1 card.
			# Display: if the processor is running but nothing appears, check the VDU card (below).

	~~# Confirm the backplane rails (+5 V~~, ~~and any ±12 V used by~~ the ~~VDU) at an empty slot.~~		To narrow a fault down, run the system with the CPU, VDU and memory cards only, then add the cassette card.
	~~# Run~~ with ~~only~~ the CPU ~~card~~ and ~~the~~ memory~~/BASIC card; confirm the machine starts.~~
	~~# Add the VDU card~~, then the cassette card~~, retesting after each.~~
	~~# A fault that appears only when one card is fitted isolates to that card (or a dirty slot)~~.

	== ~~No display~~ ==		== Display faults ==

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	{\| class="wikitable styled-table" style="width:100%; text-align:left;"		{\| class="wikitable styled-table" style="width:100%; text-align:left;"
			\|+'''VDU card faults'''
	! Symptom !! Probable cause !! Action		! Symptom !! Probable cause !! Action
	\|-		\|-
	\| No ~~video at all~~ \|\| VDU card~~, its rails, or the monitor~~ \|\| ~~Confirm +5~~ V/&~~plusmn~~;12 ~~V at the VDU slot; reseat the MC6845 and SAA5050; try another monitor~~		\| No picture \|\| Video lead, monitor or VDU card \|\| Check the composite output on pin 15 of the VDU card's front connector (0 V on pin 14), 1 V into 75 Ω<ref name="s2tm" />
	\|-		\|-
	\| ~~Garbage /~~ wrong ~~characters~~ \|\| ~~SAA5050 or video RAM~~ on the VDU card \|\| ~~Reseat~~ the ~~SAA5050; check the VDU card's RAM~~		\| Unstable characters, or wrong width \|\| Preset R3 on the VDU card \|\| Adjust R3 with the COS running<ref name="s2tm" />
	\|-		\|-
	\| ~~Dead machine, no activity~~ \|\| ~~CPU~~ card ~~(clock~~/~~6502)~~ or ~~no rails~~ \|\| ~~Apply~~ the ~~System 1 CPU~~-~~card checks; verify~~ the ~~backplane supply~~		\| Colours shown as greys \|\| Normal on composite \|\| Colour needs the card's RGB and sync outputs and a TTL RGB monitor or colour encoder<ref name="s2tm" />
			\|-
			\| Wrong or corrupt characters \|\| SAA5050, the two 2114-3 RAMs or the MC6845 \|\| Reseat and substitute<ref name="s2tm" />
	\|}		\|}

	== ~~Memory~~ and ~~BASIC~~ ==		== Keyboard ==

			The ASCII keyboard connects to port B of the 8154 (IC2) on the CPU card. It needs 7-bit data, +5 V on pin 20, 0 V on pin 1 and a strobe that goes low while a key is pressed. On the optional 20-way header the pins are different; match the pin 1 arrows on header and socket.<ref name="s2tm" /> COS commands must be typed in capitals.<ref name="s2tm" />

			== Cassette ==

			COS messages during a tape load are "Play drive 0", "Rewind drive 0" if the tape was started part way through the file, and "sum" if a block fails its checksum. After a checksum error, rewind to before the failed block and use FLOAD to continue loading.<ref name="s2tm" />

	The memory card carries the RAM and the Acorn System BASIC ROM. Corrupt operation or crashes point to the RAM; failure to enter BASIC points to the BASIC ROM or its select. Reseat the card, then substitute RAM. Adding the optional second 4 KB RAM or the floating-point ROM changes the memory map, so verify links are set per the card documentation.<ref name="cat" />		If tapes will not load at all:

	== CPU and cassette cards ==		# Check the four backplane links between the CPU and cassette cards: A29 to B11 (φ2), B19 to B12 (CASIN), B20 to B13 (CASOUT), B21 to B14 (NHITON).<ref name="s2tm" />
			# Check the cassette card's link from IC6 pin 3 to B14.<ref name="s2tm" />
			# Set the playback level so that IC7 pin 7 on the cassette card shows more than 2 V peak to peak without clipping (R12 sets the input gain, R3 the output).<ref name="s2tm" />

	These are the System 1 cards. Use the [[Acorn System 1 Troubleshooting Guide]]: verify the 1 MHz clock and 6502, the monitor ROMs, the 2114 RAM, and the LM358 cassette read level. In the System 2 the keypad and LED are unused; the CPU card's keyboard interface drives the external keyboard instead.		== BASIC ==

	=~~= Backplane~~ and ~~connectors =~~=		The BASIC command starts the interpreter; if BASIC is missing, the system crashes and must be reset.<ref name="s2tm" /> On the memory card, the BASIC ROM is in IC17 with a link at ROM 6; issue 1 cards hold BASIC in two EPROMs, IC17 (low) and IC18 (high). The 4 KB of user RAM in IC3–IC10 needs the link at RAM 1.<ref name="s2tm" />

	* A machine that works only when a card is pressed or wiggled has a dirty/loose edge connector — clean the fingers and the backplane socket.		== COS error messages ==
	* Check the backplane for cracked solder at the connectors, especially around the power-input pins.

	== ~~Common fault catalogue ==~~		{\| class="wikitable styled-table" style="width:80%; text-align:left;"
			\|+'''Cassette Operating System messages (chapter 6)'''
			! Message !! Meaning
			\|-
			\| Com? \|\| Command not recognised
			\|-
			\| Syn? \|\| Command recognised but syntax wrong
			\|-
			\| sum \|\| Checksum failed at the end of a tape block
			\|}

	* '~~''Dead, no rails''' — power supply or backplane power wiring.~~		Source: Acorn's System 2 Technical Manual, chapter 6.<ref name="s2tm" />
	* '''Dead, rails present''' — CPU card (clock/6502/ROM); apply the System ~~1 checks.~~
	* '''No display''' — VDU card, ~~its rails, or the monitor; reseat MC6845/SAA5050~~.
	* '''Crashes / corrupt''' — memory card RAM; reseat/~~substitute.~~
	* '''Intermittent when jostled''' — dirty edge connector or backplane joint.

	== Related pages ==		== Related pages ==
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	* [[Acorn System 2 Maintenance Guide]]		* [[Acorn System 2 Maintenance Guide]]
	* [[Acorn System 2 Capacitor Replacement Guide]]		* [[Acorn System 2 Capacitor Replacement Guide]]
	* [[Acorn System 1 Troubleshooting Guide~~]] · [[Acorn System Catalogue~~]]		* [[Acorn System 1 Troubleshooting Guide]]

	== References ==		== References ==

Josh: Add machine photo

2026-06-17T20:54:20Z

Add machine photo

← Older revision		Revision as of 21:54, 17 June 2026
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			[[File:Acorn System 2.jpg\|thumb\|360x360px\|Acorn System 2 in a 19-inch Eurocard card frame.]]

	This guide gives component-level diagnosis for the '''[[Acorn System 2]]''' (1980). Because the System 2 is a set of Eurocards on an 8-slot backplane, diagnosis is by card: isolate the faulty card, then probe it against its circuit diagram. The CPU and cassette cards are the [[Acorn System 1]] cards, so the [[Acorn System 1 Troubleshooting Guide]] applies to them directly.<ref name="cat">[[Acorn System Catalogue]] and [[Acorn System 1 Technical Manual]], Acorn Computers — hosted on this wiki. Source for the Eurocard set and the shared CPU/cassette/VDU/memory card detail.</ref>		This guide gives component-level diagnosis for the '''[[Acorn System 2]]''' (1980). Because the System 2 is a set of Eurocards on an 8-slot backplane, diagnosis is by card: isolate the faulty card, then probe it against its circuit diagram. The CPU and cassette cards are the [[Acorn System 1]] cards, so the [[Acorn System 1 Troubleshooting Guide]] applies to them directly.<ref name="cat">[[Acorn System Catalogue]] and [[Acorn System 1 Technical Manual]], Acorn Computers — hosted on this wiki. Source for the Eurocard set and the shared CPU/cassette/VDU/memory card detail.</ref>

Josh: Create Acorn System series page (cited; sourced from System manuals/catalogue)

2026-06-17T20:53:08Z

Create Acorn System series page (cited; sourced from System manuals/catalogue)

New page

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This guide gives component-level diagnosis for the '''[[Acorn System 2]]''' (1980). Because the System 2 is a set of Eurocards on an 8-slot backplane, diagnosis is by card: isolate the faulty card, then probe it against its circuit diagram. The CPU and cassette cards are the [[Acorn System 1]] cards, so the [[Acorn System 1 Troubleshooting Guide]] applies to them directly.<ref name="cat">[[Acorn System Catalogue]] and [[Acorn System 1 Technical Manual]], Acorn Computers — hosted on this wiki. Source for the Eurocard set and the shared CPU/cassette/VDU/memory card detail.</ref>

== Method: isolate the card ==

A racked system is best diagnosed by reducing it to a minimum and adding cards back:

# Confirm the backplane rails (+5 V, and any ±12 V used by the VDU) at an empty slot.
# Run with only the CPU card and the memory/BASIC card; confirm the machine starts.
# Add the VDU card, then the cassette card, retesting after each.
# A fault that appears only when one card is fitted isolates to that card (or a dirty slot).

== No display ==

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:100%; text-align:left;"
! Symptom !! Probable cause !! Action
|-
| No video at all || VDU card, its rails, or the monitor || Confirm +5 V/±12 V at the VDU slot; reseat the MC6845 and SAA5050; try another monitor
|-
| Garbage / wrong characters || SAA5050 or video RAM on the VDU card || Reseat the SAA5050; check the VDU card's RAM
|-
| Dead machine, no activity || CPU card (clock/6502) or no rails || Apply the System 1 CPU-card checks; verify the backplane supply
|}

== Memory and BASIC ==

The memory card carries the RAM and the Acorn System BASIC ROM. Corrupt operation or crashes point to the RAM; failure to enter BASIC points to the BASIC ROM or its select. Reseat the card, then substitute RAM. Adding the optional second 4 KB RAM or the floating-point ROM changes the memory map, so verify links are set per the card documentation.<ref name="cat" />

== CPU and cassette cards ==

These are the System 1 cards. Use the [[Acorn System 1 Troubleshooting Guide]]: verify the 1 MHz clock and 6502, the monitor ROMs, the 2114 RAM, and the LM358 cassette read level. In the System 2 the keypad and LED are unused; the CPU card's keyboard interface drives the external keyboard instead.

== Backplane and connectors ==

* A machine that works only when a card is pressed or wiggled has a dirty/loose edge connector — clean the fingers and the backplane socket.
* Check the backplane for cracked solder at the connectors, especially around the power-input pins.

== Common fault catalogue ==

* '''Dead, no rails''' — power supply or backplane power wiring.
* '''Dead, rails present''' — CPU card (clock/6502/ROM); apply the System 1 checks.
* '''No display''' — VDU card, its rails, or the monitor; reseat MC6845/SAA5050.
* '''Crashes / corrupt''' — memory card RAM; reseat/substitute.
* '''Intermittent when jostled''' — dirty edge connector or backplane joint.

== Related pages ==

* [[Acorn System 2]]
* [[Acorn System 2 Maintenance Guide]]
* [[Acorn System 2 Capacitor Replacement Guide]]
* [[Acorn System 1 Troubleshooting Guide]] · [[Acorn System Catalogue]]

== References ==

<references />

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