Acorn System 1 Troubleshooting Guide
The Acorn System 1 has no self-test. The monitor's first sign of life is eight dots on the display after RST is pressed, so a machine that does not show them is checked from the supply inwards: power, clock, reset, then the processor, monitor PROMs and RAM.[1] Designators are from Acorn's parts lists and circuit diagrams.[2]
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Acorn's first check
[edit | edit source]If a newly built or repaired System 1 does not work, Acorn's manual says to switch off at once and feel each IC for one that is exceptionally hot, which usually means it is in the wrong way round. Then check the power connections and the assembly.[1] IC1, the 6502, faces the opposite way to its neighbours on the CPU card.[1]
Acorn also warns against desoldering parts with four or more leads, because the board can be damaged; cut the faulty part out and remove its leads one at a time.[1]
No eight dots
[edit | edit source]- Supply: 7 to 35 V DC on edge connector pin 31 (positive) and pin 32 (0 V).[1] Then +5 V at the output of IC13 (LM340-T5) and on pin 20 of the strip to the keyboard card.[2]
- Clock: the 1 MHz crystal runs in an oscillator built from IC11 (74LS04) and feeds the 6502. Check for the clock at the 6502.[2]
- Reset: the RST key and the optional reset switch pull the reset line low through an RC network. Check that the 6502's reset pin goes high when RST is released.[2]
- Address lines: with clock and reset good, the 6502 address lines should be changing. If they are static, reseat or substitute IC1.
- Monitor PROMs: the monitor is in IC5 ("blue") and IC6 ("yellow"), two 74S571 PROMs of 512 × 4 bits each.[1] IC10 (74LS139) and IC9 (74LS20) decode block F for them.[2] Reseat both PROMs and check that they are in the right sockets.
- Address selection: issue 2 and later CPU cards have the standard address map in tracks on the ADR SEL position. Issue 1 cards need wire links fitted there before they will work.[1]
- RAM: IC3 and IC4 (2114) hold page 00, which the monitor uses, and page 01, the 6502 stack. Substitute them if the machine resets but the display is corrupt or the monitor crashes.[1]
Display and keys
[edit | edit source]| Symptom | Probable cause | Action |
|---|---|---|
| Display dark, +5 V present on the keyboard card | 8154 (IC2 on the CPU card), the 20-way strip, or the 7445 (IC8 on the keyboard card) | Check the strip joints; reseat IC2 and the 7445[2] |
| One segment dead on every digit | Segment drive line from the 8154 | Follow the segment line from IC2 through the strip to the display |
| One digit dead | Digit drive from the 7445, or the display | Check that 7445 output; check the 16-way display strip joints[1] |
| Keys not recognised | Key domes, key sense lines, or IC2 | Check the domes click and sit in the cage; check the sense lines back to IC2[1] |
The 8154 on the CPU card (IC2) both drives the display segments and reads the key sense lines, and the 7445 on the keyboard card selects the digit and keyboard column, so a fault in either chip affects both the display and the keys.[2]
Cassette
[edit | edit source]The cassette circuit is on the keyboard card: CMOS counters and gates (IC1 to IC6) make and decode the 2.4 kHz and 1.2 kHz tones, and the LM358 (IC7) amplifies the playback signal.[1][2]
- Try the recorder and a known-good tape first. Acorn recommends a recorder with line-level input and output.[1]
- On playback, IC7 pin 7 should show more than 2 V peak to peak without clipping. Adjust with R12 if the level is wrong; R3 sets the recording level.[1]
- C5 (15 µF), the only electrolytic on the keyboard card, decouples the LM358's bias divider (R6, R7); check it if the amplifier output sits away from mid-rail.[2]
Faults when used in a rack
[edit | edit source]A CPU card converted for a rack has IC13 and C10 removed and a link fitted, and takes 5 V from the backplane. If the card is dead in a rack, check the link and the rack's 5 V supply. The keyboard card can stay connected by its strip, or be wired to the backplane as described in the Technical Manual.[1]
Related pages
[edit | edit source]- Acorn System 1
- Acorn System 1 Maintenance Guide
- Acorn System 1 Capacitor Replacement Guide
- Acorn System 1 Circuit Diagrams
- Acorn System 1 Technical Manual
References
[edit | edit source]- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 Acorn Computers, Technical Manual, System 1 Micro-Computer, 1979 (parts lists issue 2, July 1979), hosted on this wiki. Switching on and first fault checks (construction procedure); power supply p. 13; cassette interface p. 14; address selection p. 15; parts lists pp. 25–30.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 Acorn Computers, circuit diagrams 200,000/C (6502 microcomputer PCB, two halves) and 200,001/C (keyboard, display and cassette interface PCB), 1979–1980. Hosted on this wiki as Acorn System 1 Circuit Diagrams.