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Psion Series 3mx Troubleshooting Guide

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Fault-finding for the Psion Series 3mx (1998).

A Series 3mx. If it will not read an SSD in either slot, suspect the door microswitches before the motherboard — that mistake has cost people a lot of wasted board-swapping.

The full symptom table, the SIBO power states and the supply rail chart are in the Psion Series 3a Troubleshooting Guide, which applies here too. This page covers the 3mx-specific faults — and one of them is important enough to lead with.

The SSD door microswitch fault

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Symptom: the machine reports "not ready" and does not recognise an SSD in either the A or the B slot. Everything else works normally.[1]

It looks exactly like a motherboard fault, and it is not. One documented case involved buying a second 3mx to test with, swapping motherboards, and finding the fault followed the case rather than the board — and that the previously-good board then failed in the same way once installed in the damaged machine.[1]

Cause

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The 3mx senses the state of its two SSD slot doors with a pair of microswitches set at an angle — a different arrangement from the 3c, which uses PCB tracks and a small metal connector on the door hinge.[1]

The plastic tips of the microswitch buttons snap off, almost certainly from prising up the motherboard with the SSD doors closed.[1]

Since the SIBO architecture generates an NMI from the SSD door microswitches and uses them to decide whether a pack is present, a broken switch means the machine never believes a pack is there.[2]

Repair

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  1. Recover the broken plastic tips — they are usually still inside the case.
  2. Re-glue each tip onto its switch button with superglue. This is straightforward but very fiddly: any glue on your fingers or tools makes it impossible to hold the pieces together while it sets. Once set, the joint is reported as strong.[1]
  3. For a tip that is missing entirely, make a new one from a scrap of ABS. This is documented as repairing as well as the re-glued ones.[1]
  4. Watch the tiny spring that each switch presses against — it pops out easily and is very easy to lose.[1]
  5. Repair both switches. This matters: in the documented case, fixing one was not enough — SSDs were only recognised once both switches were repaired and refitted.[1]

Prevention

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Open both SSD doors before lifting the mainboard. Every time.

Other 3mx-specific symptoms

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Symptom Cause Action
Will not hold data through a battery change Wrong backup cell. The 3mx takes a CR2025; the Series 3, 3a and 3c take a CR1620 Fit a CR2025. A CR1620 is both narrower and thinner and will not make reliable contact.[3]
Eats batteries The backlight, or the battery lead Every 3mx is backlit and it is the biggest discretionary load. Check that first, then go to the battery lead.[3][4]
Serial link will not connect at high speed Mismatched expectations The 3mx serial port runs up to 115 kbit/s, double the 3c. Confirm both ends agree on the rate.[3]
3a serial cable will not fit Expected. The 3mx uses the 3c/Siena connector Use the right cable. Travel Modem connections also differ between 3/3a and 3c/3mx.[3]
No sticky case Normal. The 3mx is matte silver painted, not rubberised The sticky-coating problem belongs to the 3c.[3]
Vertical lines on the display Screen ribbon deterioration Replace the cable or fit a donor lid. See Psion Series 3mx Hinge Repair.[3]
Button bar dead Ribbon adrift from the lower PCB Push it home with a paperclip, lid half closed.[4]
Back edge crooked when opening Broken hidden hinge See Psion Series 3mx Hinge Repair.

Everything else

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For the four SIBO power states, the supply rail chart including the +16 V Vh rail that Flash SSD writing depends on, the battery lead fault, the keyboard matrix, the full symptom table and the assessment of what is realistically repairable, see Psion Series 3a Troubleshooting Guide.

Note that the 3mx runs the V30MX at 27.684 MHz rather than the V30H at 7.68 MHz, but the SIBO architecture, rails and power states are the same.[3]

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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Richard Zealley, "Psion Series 3mx SSD door microswitch repair", Retro Coding, 25 May 2020. Source for the symptom, the diagnosis, the microswitch arrangement, the damage mechanism, the requirement to repair both switches and the repair method described here.
  2. ↑ Psion PLC, SIBO Hardware Reference, version 2.10, 3 February 1995, in the Psion Documentation Project. Source for the NMI generation from SSD and expansion port door microswitches.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 "Psion Series 3", Wikipedia. Source for the CR2025 backup cell on the 3mx, the universal backlight, the fast serial port and the documented clamshell problems.
  4. ↑ 4.0 4.1 Psion Series 3/3a community FAQ material and the comp.sys.psion newsgroup archives. Source for the battery lead problem and its fix, and the button-bar ribbon reseating trick.


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