Acorn Atom Troubleshooting Guide

The Acorn Atom (1980–1983, main PCB 202,000) has no self-test or error codes, so faults are found by symptom, by measuring the supply, by probing against Acorn's circuit diagram and by swapping socketed ICs. Designators are Acorn's, from the Acorn Atom Technical Manual parts list and the circuit diagram.

Power-up sequence

A working Atom with 8 V DC on SK3 does the following:

1. The two LM340T-5 regulators produce +5 V: IC53 feeds the "+5 V B" half of the board and IC54 the "+5 V A" half.
2. X2 (4.00 MHz) runs an oscillator on IC45 (74LS04). The 74LS393 counter IC44 divides it to the 1 MHz clock on the 6502 (IC22) pin 37. X1 (3.58 MHz) runs a second oscillator on IC9 (74LS04) that clocks the MC6847 video generator (IC31).
3. The reset from power-up or the BREAK key starts the 6502 in the 8 KB operating system and BASIC ROM (IC20).
4. The 8255 (IC25) scans the keyboard through the 7445 (IC26), and the MC6847 reads the video RAM (IC42, IC43) to build the picture.
5. The screen shows the ATOM sign-on and the > prompt.

Acorn's Circuit Diagram for Atom Microcomputer, used for the test points in this guide

No picture, no signs of life

Dead machine

Symptom	Probable cause	Action
Nothing on screen	No supply, or wrong supply	Measure 8 V at SK3, then +5 V at the outputs of IC53 and IC54. SK3 has two soldered lugs and a wire link for the third connection; check all three joints.
+5 V on one half only	One regulator failed, or a short on that half	IC53 feeds +5 V B and IC54 feeds +5 V A. With the power off, measure the resistance of the dead half to 0 V and look for a shorted decoupler (C14–C22 on B, C23–C28 on A) or IC.
+5 V good, still dead	No processor clock	Check X2 and the IC45 oscillator, then the 1 MHz output of IC44 at IC22 pin 37.
+5 V and clock good, still dead	Processor held in reset, or not running	Check that IC22 pin 40 (reset) goes high after power-up. The reset comes from the BREAK key network (R38, C11 10 µF) through two IC9 inverters. Then check that the address lines toggle; reseat IC22 and IC20.
Regulator very hot, supply drops out	Missing heatsink, or overload	Check that the heatsink is fitted between the regulators and the PCB and that the fixing screw does not short to a track. The LM340 has internal thermal overload protection and shuts down when it overheats.
Dead or unstable only when fully expanded	Adaptor overloaded	Acorn's adaptor delivers up to 1600 mA, which is what a fully expanded Atom with low-power 2114-L RAM draws. Atoms supplied before October 1980 used normal-power RAM and draw more when fully expanded. Use a larger 8 V supply, or fit links LK6 and LK7 and run the board from a regulated 5 V supply (see Acorn Atom General Maintenance).

Each regulator needs at least 7.5 V at its input to hold regulation, so measure SK3 with the machine running, not on an open-circuit adaptor. If 8 V is present at SK3 and +5 V is missing at a regulator output, lift that regulator's output and measure again. If the output recovers, something on that half of the board is pulling the rail down.

Picture faults

Picture present but wrong

Symptom	Probable cause	Action
Characters corrupt in a repeating pattern	Video RAM	IC42 holds data bits 0–3 and IC43 bits 4–7 of the first 1 KB of video RAM (8000–83FF). Swap each with a known-good 2114 in turn.
Text screen fine, graphics modes corrupt	Expansion video RAM	The extra graphics RAM sits in pairs IC40/IC41 down to IC32/IC33 (8400–97FF). In each pair the even-numbered IC holds bits 0–3.
Sign-on appears, then the machine hangs or crashes	Block zero RAM, ROM or lower text RAM	IC51 and IC52 hold block zero (0000–03FF), which the processor uses for its stack. Reseat IC20 and the RAM. If lower text space RAM (IC10–IC19) is fitted, the Atom uses it automatically at reset; remove the pairs to see whether the fault goes.
No picture on the TV, machine otherwise working	Modulator or video buffer	Try a monitor on the composite output, PL4 pin 9 (0 V on pin 10), 1 V into 75 Ω. If composite is good, the fault is the UM1233 modulator (SK1). If both are dead, check the transistor buffer Q1 and Q2 between the MC6847 Y output (pin 28) and the outputs, then X1 and the MC6847.
Picture rolls or will not lock on a UK set	60 Hz output	The MC6847 produces only a 60 Hz picture, which some European sets cannot display. Try another set or a monitor before suspecting the board.
Monochrome picture on a colour set	Normal	The standard Atom gives a monochrome picture even on a colour set. Colour needs Acorn's PAL encoder board fitted inside the case.

Keyboard, sound and tape

Keyboard

The 8255 (IC25) drives the 7445 (IC26) from port A bits 0–3. The 7445's ten outputs drive the rows of the key matrix in turn, and the 8255 reads the columns on port B. The REPT key is read on port C bit 6.

1. Whole keyboard dead, picture fine: reseat IC25 and IC26.
2. One row of keys dead: follow that row from its IC26 output. One column dead: follow it to its IC25 port B input and its 4K7 pull-up (R17–R25).
3. One key dead: check its two solder joints, then the switch. The key leads are soldered into large plated-through holes; do not fill the holes with solder when reflowing them, because Acorn warns that contracting solder can break the plating.

No sound

Port C bit 2 of the 8255 drives the loudspeaker, through an IC45 inverter and the 22 µF coupling capacitor C5. The speaker is stuck to side 1 of the board to the right of the keyboard and wired to two holes beside C5. Check the speaker wires, then C5, then the inverter.

Cassette will not load or save

The tape interface records CUTS tones of 2.4 kHz and 1.2 kHz at 300 bits per second. The tones come from the 4 MHz crystal through IC44 and the 74LS161/163 counter IC47. On replay the signal passes through the two halves of the LM358 (IC46), used as an amplifier and a Schmitt trigger, to the 8255.

1. Use a 3-pin DIN lead. Pins 4–7 of SK2 carry 8255 port C outputs, and Acorn warns that they may damage a recorder with a 5- or 7-pin connector.
2. Check the recorder, tape and lead with a known-good tape first. The Atom loads only 300 bit/s Atom tapes; BBC Micro cassettes run at 1200 baud and will not load.
3. Both levels are set to 300 mV RMS. Change R29 to alter the recording level (the recorder's meter should read 70 %, −3 dB), and change R34 from 47 kΩ to 220 kΩ for a hi-fi deck that needs more input gain.
4. If saving fails with good equipment, look for the tones on the cassette output, SK2 pin 1, while saving. If loading fails, trace the replay signal from SK2 pin 3 through IC46 to the 8255.

Expansion faults

• If the machine fails only with a card or cable fitted, remove it and inspect the connector: PL6 is the 64-way Acorn bus connector (with PL7 in parallel for one internal Eurocard), PL5 is the 26-way printer header and PL8 is the 20-way extension plug on side 1. A card that draws current can also overload the adaptor (see above).
• The 6522 VIA (IC1), the printer driver (74LS244, IC50) and the bus buffers (IC2–IC5) are optional fits. On a basic Atom the sockets are empty and the ports do nothing.

Probing a dead machine

With the adaptor connected and nothing on the screen:

1. Measure 8 V at SK3, then +5 V at the outputs of IC53 and IC54. A missing or low rail goes back to the dead-machine table above.
2. Scope X2 (4.00 MHz) at the IC45 oscillator.
3. Scope the 1 MHz clock at IC22 pin 37. If X2 runs and there is no 1 MHz, suspect IC44.
4. Check that IC22 pin 40 (reset) is high.
5. Scope X1 (3.58 MHz) at the IC9 oscillator and the MC6847 clock input (pin 33).
6. With clocks and reset good, check that the 6502 address lines toggle. If they are static, reseat or swap IC22, then IC20.
7. With the processor running but no picture or a corrupt one, swap IC42 and IC43, then the MC6847 (IC31).

Pin numbers are from Acorn's circuit diagram.

Related pages

• Acorn Atom
• Acorn Atom General Maintenance
• Acorn Atom Capacitor Replacement Guide
• Acorn Atom Technical Manual
• Capacitor Failure Symptoms