The 8 to 20v is for charging the 700/720 battery

Remove the Power Supply
   This threw me for a loop, but I eventually pegged onto it. Undo the three captive thumbscrews at the rear of the 3550, remove the bezel. Remove the top by pushing it forward about an inch, then lift up (like a 50/70/90). Unplug any device in the 5.25" bay.  Pull off the right side of the spring bezel by pulling straight out. Unplug P1 and P2 on the right side of the systemboard. Move all three cables to the front so you can pull them through the enclosure.

  IMPORTANT! Remove the two phillips screws along the lower outer edge of the PSU (on the die cut extensions) and remove the single phillips screw fron the upper right corner above them. This single screw will stop you!
 
 

   Look at the back of the case. Notice the two black angles sticking out, and immediately below them, a catch. Lift up on each catch and slide the PSU out of the case. It's loose enough so you can undo one side, push the PSU out a bit, then undo the other catch, then slide the PSU out.

Opening the PSU
   See above to remove the PSU from the case. Now sit and try to figure out howthehell is this going to work? A VERY important point. Look at the "step" right above the cable outlet. See that small phillips screw right next to the inner vertical edge? You must remove that screw, because it fastens a heatsink from the PCB to the case. Remove the two T-15 security TORX at the front lower edge of the PSU.
   Look at the upper rear edge of the PSU case. Note that the upper piece of the PSU case is hooked under the back edge. Pry the upper piece forward until it clears the retaining lip. Now rotate the upper piece forward on the wire bundle for the power conductors only about an inch. Now twist the cover towards the side of the wire bundle so it can uncover the PSU guts.

Power Systems Checkout
   If the “power good” light is not on and if the power-supply fan is not running, check the power cord for continuity and proper installation.
   If the power cord is not the problem, either the power supply is defective or another component is defective and is causing the power supply to shut off. To verify that the power supply is operating correctly, do the following:
1. Power-off the 3550 Expansion Unit and disconnect the 3550 Expansion Unit power cord.
2. Remove all power supply connectors (power supply connector P1, P2 and drive connector 1) from the system board of the 3550 Expansion Unit and SCSI device.
3. On power supply connector P2, short-circuit pin 4 to pin 6 and short-circuit pin 7 to 8.
4. Connect the 3550 Expansion Unit power cord.
5. Check the power supply voltages using the figures on the next page.

Drive Connector 1
 Standard 4 pin molex

Pin	Signal	V dc Min.	V dc Max.
1	+12 volts	+11.52	+12.6
2, 3	Ground
4	+5 volts	+4.8	+5.25

If any of the voltages are not correct, replace the power supply.
   If all voltages are correct, the power supply is working properly. Another 3550 Expansion Unit component might be causing the power supply to shut off. 

Power Supply Connector P1 

Pin	Signal	V Min	V Max.
1 2 3 4	+5 volts	+4.8	+5.25
5 6 7 8 10	Ground
9	+20 volts	+19.0	+21.0

Power Supply Connector P2 (by HMM, see Possible Error In HMM)

Pin	Signal	V Min.	V Max
1,2	+12 volts	+11.52	+12.6
3	Power Good	+4.0	+5.25
4	On/Off Signal
5	-12 volts	-10.92	-13.2
6,7	Ground
8	Global Signal

Possible Error In HMM
 John Seniura wrote:
Just a correction for an error in the IBM HMM for the 3550 expansion unit.

   To check out the power for the expansion unit, you have to short some pins on the P2 connector from the power supply. The diagram in the HMM has the pins numbered backwards. I discovered this just before I was about to throw out my expansion unit because the power supply appeared to be totally dead. On a hunch, I tried shorting the pins in the
opposite direction and I got good readings for all voltages.