Psion Series 5mx Capacitor Replacement Guide

Short answer: the Psion Series 5mx is not a recap candidate, and you should not open one to change capacitors speculatively.

The 5mx is not a recap candidate. Its faults are mechanical and connector-related, and opening one to "recap it on principle" does more harm than good.

This page exists because the question comes up, and because knowing why a machine is not a recap candidate is as useful as knowing the values for one that is.

Why the 5mx is different

Most of the machines with capacitor guides on this wiki are from the late 1970s and 1980s, run from mains-derived rails, and use large leaded aluminium electrolytics for bulk smoothing and decoupling. Those parts dry out, leak and take the board with them.

The 5mx is a 1999 design running from two AA cells at low voltage. It is a dense, double-sided surface-mount board: the decoupling is ceramic and tantalum chip parts, not aluminium electrolytic cans. There is no bulk smoothing stage of the kind that ages badly, and — unlike the Psion Organiser II, where the LZ/LZ64 whistle is a well-documented capacitor fault — there is no community tradition of recapping Series 5 or 5mx machines, because it does not fix anything people actually report.

The faults these machines really suffer are:

• A cracked screen ribbon cable — mechanical fatigue
• Broken hinge-wire hooks — embrittled plastic
• Corroded boards and keyboard flexis — leaked AA cells
• Connector and switch trouble — the CompactFlash door switch, the case-open microswitch, the ZIF sockets

None of those is a capacitor.

Psion's own position reinforces it: they recommended that repairs be carried out at circuit board level rather than at component level, because of the high-density surface-mount construction.

It is also telling what Psion blamed for excessive standby current in their own factory test procedure. Fail Ticket code 07 lists the causes as the DRAM on the PCB, dry joints and floating inputs, and shorts caused by metallisation of the plastics onto the PCB. Capacitors are not on that list.

When capacitor work is justified

There are three situations, and all three start with evidence rather than with a recap kit.

1. Corrosion damage

If AA cells have leaked, the electrolyte tracks across the board and attacks whatever it reaches — including passives. Here you are not recapping, you are repairing corrosion damage: neutralise, clean, inspect every component and track in the affected area under magnification, and replace only what is actually damaged. Green vias are a bad sign and often mean the board is beyond saving.

See Battery Explosion, Capacitor or Corrosion Damage.

2. A tantalum that has failed short

Tantalum capacitors fail short rather than open, and when one does the machine is completely dead and draws heavy current immediately. This is a real failure mode on any SMD design of the period.

To find it:

1. Power the board from a bench supply at 3 V with the current limit wound right down — 200 mA is plenty.
2. If it goes straight into current limit, you have a short.
3. Find the hot part. At a low current limit the shorted component warms measurably before anything else does. A thermal camera makes this trivial; a fingertip or a drop of isopropyl alcohol watched for where it evaporates first also works.
4. Measure across the suspect part — a shorted tantalum reads a few ohms or less.
5. Remove it and re-measure to confirm the short has gone, then fit an equivalent part of at least the same voltage rating.

Do not simply remove a shorted decoupling capacitor and leave the position empty. It was there for a reason.

3. Excessive standby current with no other explanation

If the machine works but drains cells abnormally, and you have already ruled out the operating system (a soft reset), an unsuitable battery brand and a stuck case switch, then a leaky capacitor is one candidate — but so are the causes Psion listed, which are statistically far more likely. Check those first.

The in-machine check is Shift-Ctrl-P in the System screen: "Battery current now" should read below 50 mA with no external power and the backlight off.

No published values exist

Psion did not publish a component-level parts list for the Series 5 or 5mx mainboard. The service manual is written around spares kits and assemblies — LCD assembly, keyboard assembly, chassis assembly, main PCB, ROM PCB — and gives part numbers for those, not for individual passives.

So there is no capacitor table to give you, and this page will not invent one. If you need to identify a specific part, read it off the board and measure it out of circuit.

If you do open the board

• Back up first. Removing both cells without external power erases the internal memory.
• Anti-static precautions throughout.
• Do not bend the mainboard or the daughter PCB — Psion warn that both are prone to fracture.
• Do not bend the LCD module, and take care of the fragile drivers along its bottom edge.
• Use hot air or a fine-tipped iron with proper flux. This is not a board for a big chisel tip.
• Check the daughter PCB first for intermittent faults. Psion specifically note that dry or faulty joints on the daughter PCB cause certain intermittent faults, and that swapping it is a quick way to confirm — but a replacement must be the same OS build number and language, checkable with Ctrl+H in the System screen.

Related pages

• Psion Series 5mx
• Psion Series 5mx General Maintenance
• Psion Series 5mx Troubleshooting Guide
• Psion Series 5mx Hinge Repair
• Psion Organiser II Capacitor Replacement Guide — a Psion machine that is a recap candidate, with published values
• Capacitor Failure Symptoms · Battery Explosion, Capacitor or Corrosion Damage