Acorn Electron Troubleshooting Guide

The Acorn Electron (main PCB 205,000) has no self-test display. Acorn's Electron Service Manual gives a fault-finding order for a dead machine: power supply, oscillator, ULA, processor, ROM, DRAMs, then video, cassette, keyboard and sound, with test figures and pin numbers for each step. Designators are Acorn's.

Acorn's dealers had two test aids: the "PIT test" (production inspection test), which can point to the area of a fault and tests the keyboard, and the "Electron Watchdog" soak tester for intermittent faults. Acorn also advises keeping a known-good ULA for substitution, because the ULA is the quickest part to check by swapping.

Acorn's Circuit Diagram for Electron Microcomputer Main PCB, drawing 105,000/C

Power-up sequence

On a working machine the power supply board produces +5 V and −5 V, the 16 MHz oscillator on IC8 runs, and the ULA (IC1) divides it to clock the 6502A (IC3). The ULA holds the processor in reset at power-up through its power-on-reset input (pin 54, timed by C9) and then releases it, and the processor starts the MOS from the ROM (IC2). The machine beeps and shows the BASIC prompt.

Power supply

The tests use an AC voltmeter, an oscilloscope, a 10 Ω 2.5 W resistor and a 47 Ω resistor.

1. With the adaptor plugged into a working Electron, measure the AC across the two grey leads where they join the power supply board. It should be at least 17 V RMS. If not, replace the adaptor.
2. Unplug the supply connector from the right side of the main PCB. Put the 10 Ω resistor across the red and black leads (about 500 mA load). On the oscilloscope, +5 V must be between 4.75 and 5.25 V, with no more than 50 mV of noise.
3. Keeping that load, put the 47 Ω resistor across the blue and black leads and check −5 V. A −5 V fault shows up as a cassette fault.
4. If the adaptor is good and +5 V is dead or out of limits, the fault is on the power supply board. Acorn notes that the board is easy to work on and can be repaired from its circuit diagram.
5. Remove both resistors, reconnect the main PCB and measure the current from +5 V. It should be 700–900 mA.
6. Zero current with 5 V present means an open circuit on the main board: check the connectors and tracks. Zero current with 0 V means the supply has cut out because of a short on the main board. The supply is short-circuit protected; unplug the adaptor for at least 15 seconds to reset it.
7. Current too high with the voltage above 5.25 V: replace or repair the supply board. Current too high with the voltage in limits: a component on the main board is shorting. Switch off and feel for the one that is hot.
8. The −5 V current should be about 25 mA. Zero means a short, or a broken track to the LM324 (IC13).

All of the above is Acorn's procedure. The +5 V track runs in a ring around the board, so a track would have to break in two places to cut off any section.

Before deeper work, check the joints of the power input socket on the supply board. Adam Wilson found all three cracked on one Electron, and the Classic Acorn repair notes give a worn joint where the power lead enters the supply board as a cause of crashes and failure to start.

Oscillator

Check that a 16 MHz clock of 4 V peak to peak reaches ULA pin 49. If it does not, check IC8 pin 8 and the track between the two, then 5 V across IC8 pins 14 (+) and 7 (0 V). If the oscillator is dead, suspect IC8 (74S04), the 16 MHz crystal X1 or the passive parts around them.

ULA

On issue 2 and 4 boards the ULA (IC1) is in a 68-way socket under a square metal clip; on issue 6 boards it is soldered in. On socketed boards, Classic Acorn reports that pressing on the ULA with the machine running often makes a dead machine start, which confirms a socket contact fault. The cure is to clean the socket fingers and ULA contacts, and to check each socket contact for a broken pin. Wikipedia also reports a continuous tone at switch-on from a lifted ULA. The removal method is on Acorn Electron Maintenance Guide.

With a ULA fitted, Acorn's checks are:

• Supply: +5 V on pins 9, 43 and 48; 0 V on pins 51 and 68.
• Clock out on pin 60: varying between 1 and 2 MHz in modes 4, 5 and 6, and absent for part of each line in modes 0–3.
• The ROM select on pin 61, and RAS and CAS on pins 52 and 53, must be changing, not stuck at one level.

If RAS is stuck at logic 0, the DRAMs are very quickly destroyed. Switch off at once.

Processor, ROM and RAM

Acorn's device checks (Service Manual pp. 10–11)

Device	Supply pins	Signals to check
IC3, 6502A	+5 V pin 8; 0 V pins 1 and 21	Clock in on pin 37 (as ULA pin 60); IRQ pin 4, NMI pin 6 and R/W pin 34 changing; reset pin 40 at +5 V, falling to 0 V when BREAK is pressed
IC2, ROM	+5 V pin 28; 0 V pin 14	Output enable pin 22 and the address lines changing
IC4–IC7, 4164 DRAM	+5 V pin 8; 0 V pin 16	RAS pin 4 and CAS pin 15 changing
IC8, 74S04	+5 V pin 14; 0 V pin 7	16 MHz out on pin 8; 17.7345 MHz on pin 6

If IRQ, NMI and R/W on the processor are static, check the reset line. If it is held low, there may be a short on the board or the ULA may be faulty. If RAS and CAS reach the ULA side but not the DRAMs, a track is broken.

Each DRAM holds two bits of every byte: IC4 bits 0 and 1, IC5 bits 2 and 3, IC6 bits 4 and 5, IC7 bits 6 and 7. One failed DRAM therefore corrupts the same two bits throughout memory, including the screen.

Video

Acorn's video procedure starts by trying all three outputs (RGB, UHF and composite):

Video faults (Service Manual pp. 11–13)

Symptom	Probable cause	Action
No picture on any output	No video from the ULA	Replace the ULA and test again.
RGB works, UHF does not	Modulator, or the luminance or colour circuits	Scope the modulator input (50 mV/div, 10 µs/div). If the waveform is right, replace the UM1233 modulator. If only the colour burst is missing, see the colour row below. Otherwise make LK4 temporarily and check the composite output (about 1 V peak to peak). If composite is good, the fault is in the UHF luminance circuit (Q5, D1–D3, C13); check that D1–D3 are the right way round. If composite is bad, check the video luminance circuit (Q8).
RGB works, UHF or colour composite in black and white only	PAL colour circuit	Measure IC8 pin 6 with a frequency counter: 17.7345 MHz ±200 Hz, adjustable with VC1. Check it reaches IC11 pins 3 and 11; IC11 must be a 74S74, and a 74LS74 there can stop the circuit working. Check 4.4336 MHz at IC11 pins 9 and 6, and signals at IC12 pin 5 (about 7.7 kHz) and pin 9. Check L1 for open circuit, C21 for a short, and Q7. Then check IC14, IC15, IC16, IC10 and R50–R57.
RGB picture distorted and out of sync	Sync polarity	Change LK3 to suit the monitor.
RGB blank, UHF and composite good	RGB sync buffer	The ULA's composite sync (pin 65) reaches the RGB socket through a 74LS86 gate in IC14 (output pin 11), with LK3 on the other input. Adam Wilson found this output stuck low and cured it by replacing IC14.

Cassette

Acorn's checks, in order:

1. 16 MHz at IC9 pin 2, and the divide-by-13 clock (812 ns period) leaving IC9 pin 11 and arriving at ULA pin 55. If it is missing, check IC9's supply (pin 16 +5 V, pin 8 0 V).
2. The LM324 (IC13) supplies: pin 4 +5 V, pin 11 −5 V.
3. Output: SAVE a section of memory (the ROM is best) and check for a synthesised sine wave of about 1.8 V peak to peak on ULA pin 50.
4. Input: press BREAK and measure the quiescent voltage on IC13 pins 7, 8 and 14. None should be above 100 mV; if one is, replace the LM324. Then LOAD a known-good tape and check for a signal of 0.5 to 2 V peak to peak at ULA pin 59.

The recorder motor relay RL1 is driven from ULA pin 64 through Q6.

Keyboard

Acorn's PIT test checks the keyboard. Without it, use an ohmmeter and the keyboard circuit diagram: look for shorts, dry joints and lifted tracks, and replace faulty keys. Check that the ribbon cable is undamaged and makes good contact at both ends, that its connector on the main board covers all the pins and is not offset, and that the keyboard diodes and resistors are fitted the right way and are the right values. The Electron will not run with the keyboard disconnected. Unreliable keys can often be cleared with contact cleaner sprayed into the switch and worked.

Sound

Give a SOUND command and check for a signal on ULA pin 62. If there is none, replace the ULA with a known-good one. If there is, check the amplifier to the loudspeaker, especially Q3 and Q4. The speaker is coupled through C11 (10 µF 35 V).

Quick reference

Common symptoms

Symptom	First checks
Dead, no beep	Adaptor AC at the grey leads; power socket joints; +5 V and current draw
Crashes when warm, fails to start, garbled display (socketed ULA)	ULA socket contact; power socket joints
Supply cuts out	Short on the main board; unplug for 15 seconds to reset
No cassette load or save	−5 V rail, IC9, LM324 quiescent voltages
No colour on UHF	17.7345 MHz at IC8 pin 6, IC11 type and outputs
No RGB picture, composite good	IC14 pin 11 and LK3
Machine will not run at all with the case apart	Keyboard ribbon connected and not offset

Related pages

• Acorn Electron
• Acorn Electron Maintenance Guide
• Acorn Electron Capacitor Replacement Guide
• Acorn Electron Service Manual
• Capacitor Failure Symptoms