Differences

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--- indigo2_impact_psu [2026/02/28 22:45] – Added some cap dimensions. iliketech
+++ indigo2_impact_psu [2026/02/28 23:09] (current) – Fixed wording. iliketech
@@ Line 122: / Line 122: @@
 As this unit was received, it would not attempt to power up the machine or start. Measuring the standby rail with the unit removed from the computer showed that it measured 5 V as expected. Attempting to jumper the power-on pin to the 5 V supply would cause the fan in the supply to twitch briefly, but the supply would not start, with or without load.
+[{{:power_supply_endview.jpg?400 |End view of supply, showing internal arrangement of PCBs.}}]
 To disassemble the supply, the external screws were removed. With the screws removed, the supply separated into two halves, one with the primary PCB attached and one with the secondary PCB attached. These boards can be removed from the halves of the chassis by removing the 5 screws that hold them in place. There are also two screws attaching the power connector and filter assembly to the back of the chassis. They must be removed to detach the primary PCB from its part of the chassis.
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 Replacing all of these capacitors did not restore operation to the supply. It was determined that one of the operational amplifiers for the mag-amp that regulated the 12 V rail was missing entirely. This chip was located directly underneath a large, leaky electrolytic capacitor, and likely fell off the board at some point during disassembly, though it was never located. Additionally, a resistor in the same area was also missing. The LM358 op-amp and the resistor were replaced using photos from [[https://forums.sgi.sh/index.php?threads/sgi-indigo-2-impact-psu-repair.148/|this]] thread, which shows corrosion in a similar location, though not the same components.
+[{{ :power_supply_partially_disassembled.jpg?400|Low-voltage board on partially disassembled supply.}}]
 After replacing these components (and clearing an accidental solder bridge) along with all electrolytic capacitors in the supply (the large FA series capacitors were the only ones obviously defective, but all should be replaced), the supply would power up and all rails were within appropriate specification when jumpered to power up and plugged in. However, the supply did not seem to start if shut down and reconnected. It was at this point much time was wasted determining that this was expected behavior, as the computer likely drives the power control input with a push-pull output.
-It was also determined that the supply can be forced to power up by removing the yellow wire closest to the red wire from the control cable. This disables the power-down circuitry, causing the primary side to start switching. This results in the supply running with no over-voltage protection. Before the mag-amp circuitry was rebuilt, this resulted in approximately 22 V being present on the 12 V rail. This resulted in the failure of a diode in another portion of the control circuitry. It is not recommended to operate the supply in this manner, especially not connected to anything other than dummy loads. This over-voltage protection is likely why the supply would attempt but fail to start before repair. It is likely that the rail voltages began to rise before the 12 V rail would trip the over-voltage protection of the UC2903, and the supply would shut down.
+It was also determined that the supply can be forced to power up by removing the yellow wire closest to the red wire from the control cable. This disables the power-down circuitry, causing the primary side to start switching. This results in the supply running with no protection of any kind. Before the mag-amp circuitry was rebuilt, this resulted in approximately 22 V being present on the 12 V rail. This resulted in the failure of a diode in another portion of the control circuitry. It is not recommended to operate the supply in this manner, especially not connected to anything other than dummy loads. This over-voltage protection is likely why the supply would attempt but fail to start before repair. It is likely that the rail voltages began to rise before the 12 V rail would trip the over-voltage protection of the UC2903, and the supply would shut down.
 It should also be noted that this power supply is difficult to rework, especially around the section with the primary low-voltage filters. This section of the board has several large power planes, and the four-layer board requires a lot of heat to clear the holes from the capacitors. This problem is exacerbated by corrosion of the solder joints. Multiple pads were damaged during the repair process. Avoid repeated rework if possible.
+[{{:power_supply_primary_corrosion.jpg?400 |Main low-voltage capacitors removed, revealing corrosion.}}]
 ==== Capacitor List ====