Power Supply
   Remove PSU (P70 and P75 similar)
      Open PSU
      Remove PSU Switch Guard
      Remove PSU Switch
      Fuse
   Metal Screws to Reassemble PSU
   Power Supply Checkout (Square plug pinout)
   Plasma Screen Power Plug Pinout (3x4 connector plug)
   Power Supply Fan (Remove/Install)
ECA 068 (Modified P70 Bus Riser)
   Modified Bus Riser Outline
ECA104 (Copper Tape on P70 PSU)
   Copper Tape and RFI Trivia)

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. 

  General impression of P75 PSU- pretty well built for a PSU with a plastic case. The thin metal shield is only for EMI and it is quite thin. The layout of the internal components is fairly open. Which certainly helps when you are trying to blow out the dust bunnies.
   The heat sinks are 3/32" COPPER. The only discoloring on the main PCB was below the resistors R030 and R033 (56ohm, 5%, 2W wirewound). I looked at the traces on the bottom of the PSU, and they weren't burnt.

Remove Power Supply

   Remove the green/red wire and the ground strap from the left of the fan. The forward mounting screw is in the black foot underneath the rocker switch. 
#2 Use the latch on the side of the square, white power plug to unfasten it. Disconnect the small, black plug for the fan. Use diagonal cutters to remove both the nylon wire ties holding the black tubing on the plasma screen power plug. Work out the kinks and pull the tubing towards the PSU. Squeeze the tabs on the ends of the black power plug to unfasten it.
NOTE: On the P75, the floppy cable is attached to the PSU. Easier to just cut the barbed retainer, then use a nylon cable tie to fasten it.

#3 Unscrew the retaining screw from both sides of the fan (white plastic webs that come up from the PSU, mounts directly to the P75 case). 

#4  NOW you can pull the power supply straight out. Trust me, it's so much easier when the front screw is not threaded into the PSU... I found that opening the floppy and pushing down on the power switch allowed me to easily pull the PSU out.

Opening Power Supply
   First, drill out the rivets. Next, take out all the screws, including the black security torx for the fan shroud (P75 only) and the ones used to attach the ground(s) on the left of the PSU. You can't remove the top metal shield unless those screws are out!

Remove the power switch guard .Pop the top of the metal shield off. Use a knife or a small standard screwdriver to gently pry the flange over the catches. You will have to pull up on the screw retainers that are on the top of the mounting flanges
. 

Remove the top plastic half. This gives you access to the entire inside of the PSU. You can stop disassembling the PSU and clean it.

IMPORTANT! Notice that the part I marked as "AC Plug Frame" is now only held on by the wires out of the back of the AC plug. On the P75 / late PSU, you can detatch the AC Plug Frame by looking under the black tubing covered ferrite and squeezing the top half of the white plastic plug. The entire AC Plug Frame will now be free.

With the AC Plug Frame either removed or just swung away from the PSU, you may now pull the bottom half of the metal shield off.

NOTE! To reassemble the PSU, the bottom half of the exterior metal shield goes between the AC Plug Frame and the plastic frame! It sort of fits the other way, but them the rivet holes WILL NOT LINE UP.

Remove Power Switch Guard

  Far simpler than it looks. Get a small jeweler's screwdriver. On the underside of the switch guard, there is a small cutout on either side. Stick the screwdriver inside the cutout and press the latch inward. Pull up on that side of the guard so the latch is loose from the PSU frame. Repeat on other side. Now with both latches loose, pull up on guard.

Remove Power Switch
   To pull the main PCB out, you must remove the power switch (ALPS SDT-7, 5A/80A250V~). Carefully remove the two mounting screws. Be aware that the mounting posts are darned near ten years old, they're fairly thin, and they twist when you turn the screw too fast. Don't complain to me if you snapped one off, I haven't any spares.

Fuse
5A 250V~ Marked SOC SD4 D250V5A. Please remember if the fuse blew, it had to be due to SOMETHING. 

Metal Screws Used to Reassemble PSU
  After you drilled and pried everything open, you have to put it back together. Get six #8 3/8ths" metal screws (called Pan Heads). Not perfect, but they hold good enough for what we're doing. 
NOTE! For the two screws on the Power switch end (under the floppy port) you MUST grind them down by 1/8th" or they WILL bottom out against the plastic PSU frame.

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. 

P75 Rectangular Plug

12 White 11 20GA Black 10, 9, 8 18GA Black 7, 6 N/C

5, 4, 3 Red 2 Orange 1 Blue 0

 

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. 

Plasma Screen Power Plug Pinout

Remove P70 PSU Fan
   For the P70 / early PSU, there is no metal bracket over the fan. Instead, there are two black Torx in the bottom two holes.

Remove/Install System Fan, P75 / late PSU
   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. 

 NOTE: On a 8573-121, SN 1065064, the bus interface assembly lacked the big electrolytic can.
 P/N 56F9047, FRU 56F9101, but had the copper tape on the PSU. I noticed slight wavering in the characters on screen in text only. 
    On an 8573-121, SN1031526, the bus interface assembly has the big electrolytic can. FRU
 65X1567. The image is rock steady in text mode. Both riser cards are silkscreened with
 iGH0256DA, 65X1308. 

Modified Bus Interface Card 

 C1 1,000uF 16v, 85c Electrolytic capacitor 
 CN2 16-bit MCA Socket (faces down) 
 CN1 32-bit MCA Socket (faces top) 
    The mod riser has the capacitor glued directly to the riser. The negative lead is soldered to the top left pin from CN1, the positive lead is soldered to the eleventh pin from the top left. 
   Both of the P70 risers that I have (one mod, one stock) have the teeny SMD capacitor and
 resistor to the end of CN2. 

ECA104 
    -diskette or disk errors/hangs, loading/reading. Indicates an RFI problem with the P70's that
 caused intermittent drive failures during reading and writing. Basically, the fix was to insulate part of
 the power supply with 1/2" wide copper tape to prevent RFI leakage. There was two seperate
 pieces of copper tape- one the full length of the front horizontal seam, the second about 1" long on
 the seam from the corner of the ventilated end over to the long piece. 
    There is a quick way to check to see if you need this ECA, if you are familiar with opening up
 your machine. Open up the back of the machine and check the bus interface card, P/N 65x1567,
 for a 1000Mf capacitor installed on the bottom of the card. If you have this capacitor then you don't need ECA068. The capacitor apparently helps eliminate data transfer noise that can prevent the loading of multitasking operating systems. 

Technical Trivia on Copper Tape 
Don Hills wrote: 
    The copper tape is relatively expensive and has to be manually (expensively) applied. To justify
 using it, they would have discovered significant RFI leakage from the joint in some of the supplies. A long, thin slot makes an effective antenna for wavelengths similar to the slot length. You need effective bonding of the two metal surfaces all along the joint length. They may have tested OK when new but failed testing after the surfaces corroded slightly in service. They probably discovered it while chasing the RFI problem with the diskette drives. 

    To understand how a hair-thin slot can radiate more RF than a "swiss cheese" perforated panel, consider a half-wave dipole antenna (VHF TV antenna is a good example). It's a rod of metal surrounded by air, with the feed line connected in the middle. Now reverse the components and you have a "rod" of air surrounded by metal. Connect the feed wires to each side of the slot at the
 midpoint and there you are again - a half wave dipole. (The electromagnetic theory is actually a bit
 more involved than that, but it illustrates the concept).