StrongARM RISC PC Maintenance Guide

StrongARM Risc PC SRP26 with the top cover removed: a PC card and the 233 MHz StrongARM card at top left, the two-slot backplane, and the 103 W supply on the right (photo: Chris Whytehead, CC BY-SA 3.0)

The StrongARM RISC PC was launched in September 1996. It is a Risc PC with Acorn's StrongARM SA-110 processor card and RISC OS 3.70 in ROM. The two machines Chris Whytehead documented both have the issue 3 main board, part 1208,000, and processor card 1111,150 issue 3. The case, supply, drives and main board are those of the Acorn Risc PC 700, so the general procedures on Acorn Risc PC 600 Maintenance Guide (opening the case, the supply connector, cleaning) apply.

Safety

The power supply is a mains switch-mode unit in the base of the case. Unplug the mains lead before taking the top off. Warning: the supply's primary reservoir capacitor can hold a dangerous charge after unplugging; discharge it and measure it before touching the primary side of an opened supply.

Acorn's StrongARM fitting instructions ask for an anti-static wrist strap clipped to a metal earth point such as the shell of the serial port, and warn against touching the pins of the processor.

Backup battery

The main board keeps the clock and CMOS RAM alive from a 1.2 V rechargeable cell, trickle charged from +5 V while the machine is on. On StrongARM machines this cell leaks as it does on other Risc PCs:

• One of Chris Whytehead's SRP26 machines had a corroded battery, and the other had had its cell replaced with a rechargeable AAA cell wired to the board.
• LinuxJedi's double-slice machine, sold with the issue 3 board and a 233 MHz StrongARM card, had a leaking battery. He cut it off, cleaned the area, and started the board without a cell, holding Delete at power-on to reset the CMOS settings.

Remove a leaking cell at once and clean the board before powering it. The circuit around the cell (D2, R212, C70 and the PCF8583 clock chip) and the cleaning method are on Acorn Risc PC 700 Maintenance Guide. See also Battery Refurbishment and Battery Explosion, Capacitor or Corrosion Damage.

The StrongARM processor card

The card sits in the front processor slot, marked PROC I/F slot 0; the rear slot takes an optional PC card. Take out any expansion cards before changing the processor card, as Acorn's instructions do, and avoid touching the pins of the chips and components on the card.

Card 1111,160 with an Intel SA-110 and no switches fitted (top), and card 1111,150 issue 3 with a Digital SA-110 and the four-way switch block SW1 at the top left (bottom) (photo: LinuxJedi)

Switch block

The card has a four-way switch block, SW1, at one end. Acorn's Application Note 297 says all four switches should be down, and that the middle two are also shorted by solder links on the reverse; a card whose links have been changed has been altered by a user. Not every card has the switches fitted: the Intel-built 1111,160 in LinuxJedi's photograph has an empty footprint. An owner of two 233 MHz 1111,160 issue 1 cards found switch 2 on one of them and the rest off; the reply on the RISC OS Open forum was that the switches set the clock speed but do nothing until tracks on the card are cut. Acorn's own stability test does use the leftmost switch to slow the card (see StrongARM RISC PC Troubleshooting Guide).

R25 and R35

Acorn's timing modification moves these two resistors on the processor side of the card from the perpendicular to the parallel position relative to the main connector. Note which way they sit before chasing a fault.

Cleaning a contaminated card

LinuxJedi bought a card whose metal parts were coated with deposits, which he thought came from a battery or capacitor leak in its previous machine, or from mould. An ultrasonic wash in Safewash Super, a rinse in deionised water and a spell in a drying oven cleaned it, and he replaced its electrolytic capacitors before testing it. It then ran. The card's capacitors are listed on StrongARM RISC PC Capacitor Guide.

ROMs and ROM patches

RISC OS 3.70 is in two ROMs: part 1203,191-xx in the socket marked ROM 1 and 1203,192-xx in ROM 2, fitted with the dimple towards the left of the case. Use a ROM extractor to remove them.

Acorn issued software patches for RISC OS 3.70 that load from the boot sequence. Application Note 297 tells engineers to confirm they are installed, by looking for a directory called ROMPatches in !Boot.Choices.Boot.PreDesk, before treating any crash as a hardware fault. Keep that directory when rebuilding a hard disc.

Memory

The VRAM SIMM sits upright at the back, in line with the PC card and just in front of VIDC20. The two sloping sockets in front of it take main memory; the rear one is the most significant and should hold the larger SIMM. Aleph One gives these points for buying main-memory SIMMs: 72-pin, 32-bit, 70 ns or faster, square array, no more than 16 chips, fast page mode and CAS-before-RAS refresh.

Aleph One also reports that the Samsung VRAM SIMMs fitted to early Risc PCs "do not work well" with the StrongARM. An upgraded machine that crashes with the StrongARM card but not with its old processor card should be tried with a different VRAM SIMM, or with none. Acorn's own procedure removes the VRAM and then tries a single SIMM known to work with the StrongARM.

Power supply

Acorn fitted a 70 W supply for machines with no backplane or the two-slot backplane, and a 103 W supply for the four-slot backplane. Chris Whytehead's single-slice SRP26 with a two-slot backplane has the 103 W unit, so read the rating on the supply's label.

Supply outputs (Technical Reference Manual p. E-1)

Output	Nominal	Tolerance	Minimum load	Ripple and noise (pk-pk)
+5 V	+5.1 V	+2 %, −4 %	0.50 A	50 mV
+12 V	+12.0 V	±7 %	0.10 A	100 mV
−12 V	−12.0 V	±7 %	0	50 mV

The supply will not regulate properly without its minimum load, so measure it in the machine or with a hard disc connected as a load. LinuxJedi tested a StrongARM machine's supply that way, with one meter on the 5 V line and one on the 12 V line and the boot drive as the load; both were in range. Inside the supply he found thick dust and broken pieces of the case, which he thought had gone in through the fan, and the fan bearing was noisy. Blow the dust out, check the fan turns freely, and replace a noisy fan. The connector pin-out is on Acorn Risc PC 600 Maintenance Guide.

Case and backplane

The spring that holds up the front flap has broken on many Risc PCs, including one of Chris Whytehead's StrongARM machines. LinuxJedi's machine arrived with both flap-spring mounts snapped and several of the plastic teeth that hold the case slices together broken off.

The backplane carries three 47 µF 16 V axial electrolytics (C5–C7 on the two-slot board, C1–C3 on the four-slot board) and a 7905 regulator for the −5 V supply to expansion cards. On LinuxJedi's machine a previous owner had screwed an earth wire from the supply and case top to the backplane; it had broken the leg off one of these capacitors. Look for loose leads and damaged capacitors on the backplane whenever the case is open.

Preventive maintenance checklist

1. Inspect the backup cell and remove it if it shows any leakage.
2. Check that the StrongARM card's switches are all down (1111,150 cards) and that R25 and R35 are present and soldered.
3. Reseat the processor card, the SIMMs and the VRAM.
4. Measure +5 V, +12 V and −12 V with the machine running, against the table above.
5. Clean the dust out of the supply and check the fan.
6. Inspect the backplane capacitors and any added wiring.
7. Confirm that the ROMPatches directory is present in !Boot.Choices.Boot.PreDesk.
8. Back up the hard disc.

Related pages

• StrongARM RISC PC
• StrongARM RISC PC Troubleshooting Guide
• StrongARM RISC PC Capacitor Guide
• StrongARM Risc PC Documentation
• Acorn Risc PC 700 Maintenance Guide
• Acorn Risc PC 600 Maintenance Guide