SGI Tezro Capacitor Replacement Guide

Tezro tower from the right with the side panel off (SGI Hardware User's Guide, Figure 2-1). The power supply is in the top rear quadrant; the main board regulators discussed below sit under the fan wall on the other side.

SGI published no component list for the SGI Tezro. The tower Hardware User's Guide treats the power supply, the interface (main) board and the node board as parts for SGI service, and gives no values. What is known about the Tezro's electrolytic capacitors comes from owners who have opened the power supply and probed the boards. Elf's survey of the power rails on the Silicon Graphics User Group forum is the main source for the board regulators, and weblacky's rebuilds on the IRIXNet forum for the supply. Elf asks that anyone reusing his findings link his original post, "Tezro voltage regulators and rails", and credit him by name. Take every value from the part on your own board.

Power supply

The tower supply provides 3.3 V and 5 V at 24 A each, three +12 V outputs (8, 12 and 18 A), −12 V and 5 V standby, with the 3.3, 5 and 12 V outputs together limited to 500 W. Owners' findings:

• The supply is made by Delta Electronics; Geoman found a near-identical Delta unit in an HP xw8400 workstation. Elf recalled the Tezro unit using Nippon Chemi-Con capacitors.
• It has a high-voltage and a low-voltage board with daughterboards. The controller board carries a PIC16 microcontroller and a 24C02 EEPROM. A five-pin control cable from the system, custom to SGI, can raise the 3.3 V rail and changes the fan speed.
• weblacky replaced every electrolytic in his supplies, including those on the daughterboards, with mostly Nichicon parts and one Rubycon of the original series. Three of the capacitors on the DC output, connected to the 12 V output, are 3300 µF 10 V; he used Nichicon UPS series parts there after trying 3300 µF 16 V substitutes. One low-voltage daughterboard carries an SMD electrolytic. The primary filter capacitor is glued on its side.
• weblacky found the low-voltage daughterboard has four trimmers: the upper three marked 105 (1 MΩ), of which two adjust 12 V and 5 V and the top one probably 3.3 V, and a heavily glued one marked 101 (100 Ω) whose purpose he could not establish.

Tezro power supply capacitors reported by owners (incomplete; IRIXNet and SGUG forums)

Position	Value	Notes
DC output filter, three parts on the 12 V side	3300 µF 10 V	Replaced by weblacky with Nichicon UPS 3300 µF 10 V
Low-voltage daughterboard	SMD electrolytic, value not given	Reachable without removing the daughterboard
Primary filter	Value not given	Glued on its side; weblacky melted the adhesive and lifted the small board first

Working on the supply

weblacky's notes on the rebuild:

1. Expect it to take hours. The board has very small vias in wide power tracks, and wick will not clear them. He used a desoldering gun at about 700 °F with added 63/37 solder and a 3-4 second dwell, then a 1 mm drill in a pin vice for vias that stayed blocked.
2. Cut the white silicone stabiliser with a knife tip at about 450 °F, taking care around the DC output wires; several capacitors between them are glued together.
3. One capacitor lead sits under a metal screw mount on the underside, which has to be desoldered and refitted.
4. weblacky replaces 1-2 W carbon resistors in the power path with higher-wattage metal oxide parts, having found that such resistors change value after years of heat in older SGI supplies.
5. Refit with less stabiliser than the factory used, but enough to survive shipping.
6. Bench test on an ATX tester and note the readings before adjusting anything. Original and rebuilt Tezro supplies alike read 11.1-11.4 V on +12 V1 unloaded; weblacky treats this as normal for the design.

Warning: the supply rectifies mains onto its primary filter capacitor. Unplug it and let the capacitors discharge, and check with a meter, before working on the primary side.

Main board regulators

Elf traced two switching regulators on the interface board, hidden under the fan wall, both driven by one LTC3728EG controller: one gives the 1.8 V rail and the other the 2.5 V (or XIO 2.5 V) rail. A 29302BU linear regulator fed from 5 V standby supplies the 3.3 V AUX rails. The 1.8 V regulator uses a pair of MOSFETs (FDD6670A and FDD6612A), an inductor and an output capacitor, laid out like the typical application in the LTC3728 datasheet.

Both Elf and weblacky have Tezros that shut down with "1.8V low fault limit reached" followed by "brick auto power down". In weblacky's log the rail had fallen to 1.199 V. Elf found the 1.8 V output capacitor running warm (91.9 °F) and one of the MOSFETs overheating, and bought replacements for the controller, MOSFETs and capacitors of the whole block. weblacky suspected the output capacitor, since his rail fell over 15 to 20 seconds, or a failed gate diode. weblacky also says the regulator circuits on the main board use polymer capacitors, which he gives a life of about 15 years.

Node board regulators

The IP53 node board carries four regulator modules. Only the processor module (VRM 1, about 1.25 V) is removable; the others, for the 1.5 V I/O, 2.5 V SRAM and Bedrock rails, are soldered to the board on 0.1-inch headers. The VPro graphics board has its own 2.5 V regulator.

Recording your boards

1. Read the L1 env report first; it lists every monitored rail with its warning and fault limits, which tells you which regulator to examine.
2. Remove the supply or board and photograph it square-on in sections.
3. For each electrolytic and polymer capacitor, record the designator, capacitance, voltage, temperature rating, series and size.
4. Note the rail by tracing it to the output connector or to the regulator it serves.

Related pages

• SGI Tezro
• SGI Tezro General Maintenance
• SGI Tezro Troubleshooting Guide
• Capacitor Failure Symptoms
• Recommended Tools