1. Strongly push below latch to release. 
2. Work right to left. Lift edge of rear cover outward to clear catch.
3. After releasing all the five latches, lift and remove the rear cover.
 
 

NOTE: There are two additional catches, one to each side of the carrying handle.When you lift up the rear cover, it pivots on them.

 
 

Detach power cable from system board.
Detach SCSI cable from system board.
Remove both screws from the HD Bracket.
Remove black screw from foot.
Pull drive cage up.
 
 
 
 
 
 

External Storage Device Cable
           A new cable (#2716) (23F2716) is available for attaching appropriate external backup devices and diskette drives.  The cable features Hoshiden connector to attach to the P70 with an industry standard 37 pin D Shell connector that connects to the external powered devices.  The length of the cable is 35.5cm (14 inches). 
  Mondo 6 pin wide by 5 pin high rectangular connector. Still slogging through IBM Technoslavian, it's referred to as a "B drive" like on the Model 70 that does not take up the B letter as one would expect, but appears as "D".

External Storage Device Cable Source
  Thanks to the power of Capitalism, the good folks at  Prime Electronic Components, Inc. have them, it's 23F2716 for $9.95. They look like THIS

From Peter
   Looks like a slightly buggy power supply -or- systemboard. The PSU is a little sensible against aging and overheating after many years, because it is a known "dust collector". The PSU fan sits awkwarly mounted over the PSU and only blows half (or less) through the PSU, which causes dust particles to stay in the PSU. I would recommend that you remove it from the machine and give it a good blow with a compressor and air-nozzle to get out the dust bunnies from it. Make sure the fan is in working order. 

On various occasions I saw failing mainboards. They refused to come on - and I don't know why. They simply died from one day to the next and could only be replaced. Back in the "good old days" with full IBM support and machines under warranty that was no big problem. Today things look different. If cleaning the power supply doesn't help - and if removing all adapters and testing with the bare sysboard doesn't change the misbehaviour: suspect the mainboard to be damaged and look for a replacement. 

Under rare circumstances a poorly seated card-riserboard caused trouble. Have the small two-slot riserboard removed, clean the contacts and spray a good amount of TV-tuner cleaner in the board connector. Let it dry for 10 minutes and use a piece of stiff card pushed into the connector with one edge and move it up / down the connector. Then spray the connector once again and let it dry for an hour. You might also repeat these steps on the two MCA slots on the
riserboard. Especially on the slot pointing towards the top of the unit, which collects dust when left with no card installed for a longer time.

Some older units also developed a sort of "creeping short" along the FDD cable where the FDD cable enters the "flip out" FDD bay. There was an ECA for the FDD cable from IBM for the earlier models, where FDD cables broke, shortened DC-voltages to GND or FDD signals and caused a dead systemboard. (Few only - IBM fixed it in an early stage).

Another potential problem has been reported with the memory modules. The system takes only 1 and 2MB modules anyway, but certain 2MB modules installed on the sysboard connectors with double sided 10 chips layout (20 chips total - 2 x 8 data + 2 x 2 parity - 16 bigger and 4 smaller chips) caused overload of the memory driver lines. This usually resulted in either a 200-something memory error -or- a machine failing to come up at all. 

I take from the 16MB you'd mentioned that the upper slot contains a memory card (the lower is too short anyway). So you should probably check the card connector as described further above and check if you cannot use memory modules with lower amount of chips. Usually the memory installed on cards causes a 200-series memory error in first place before dumping the machine to total darkness - but you never know. 

I have several P70 in my collection and all of these run with double-sided 2MB chips ... but I had a customer machine in '93 or so, which didn't. We used later 2MB/70ns 9-chip modules "hardware-recoded" (with soldering) to 2MB / 85ns and the system worked like a charm.
 

Power Systems Checkout
To check the power supply voltages, do the following:
Note:  If you can't power-on the computer, the hard disk drive motor-start jumper might be on .
Remove the motor-start jumper.
1. Power-off the computer.
2. Unplug the two power-supply connectors.
3. Power-on the computer and check for the voltages listed below.
If the voltages are not correct, check the power cord for continuity.
If the power cord is good, replace the power supply.

Vdc Min   Vdc Max   Ground (–) Pin   Positive (+) Pin
+4.8       +5.2      6, 7, or 8       3 ,4, or 5
+11.5     +12.6      6, 7, or 8            2
-11.0     -12.9      6, 7, or 8            1

Pin positions 6 and 7 are unpopulated.

Remove/Install System Fan
   You will need a T-10 security Torx. Unfortunately, you need to get at the rear of the fan cage to get at the screws. If you don't want to disassemble the sytem, use a 1/4" drive ratchet and a hex bit adapter (assuming your security torx bit is a hex insert) to remove the two screws in the top two holes.
   Now pull the top outer edge out. Notice that the fan will pivot on the two white pivots at the base of the power supply. Carefully pull the power cable throgh the fan cage (you will have to turn the power connector to fit thru the opening).
   Personal opinion- I noticed that the factory routing of the fan's power wires routed them through the cutouts in the thermoplastic impeller housing. But notice that the right fan cage mount for the right screw extends past that cutout, thereby allowing you to tighten the fan housing against the extension with the wires between them. Not a good design. Fix- I pulled the wires out of the cutout and ran it down the side of the housing. There is enough room between the side of the fan cage and the fan housing for the wires to fit without being crushed.

System Unit Fan: 
  A non-functioning fan can cause heat to build up resulting in intermittent problems. If the fan is not running, replace the power supply.

With the pins shorted, power-on the computer. This erases the power-on password. Remove the short after POST is finished.