Ardent Tool of Capitalism

PC720 Power Supply

Specifications
Power Supply Connectors Pinout
   Planar Power Connectors P1/P2
   Planar Power Connector P9
   Planar Power Connector P11 - 3.3 V
   Riser Power Connector P10 - 3.3 V
Remove Power Supply
Open Power Supply
Power Supply Inside
Power Supply Fan

Content created by Tomáš Slavotínek and Tatsuo Sunagawa.

Specifications

The PC720 power supply is a compact unit with some relatively modern features. It has a low-voltage power switch (known from later PS/2 and PS/55 machines), a stand-by 5 V supply and a 3.3 V rail. The integrated fan has an automatic speed regulation.

Power Consumption

Maximum: 70 W
Typical: 40 W

PSU Model

Model NJD-4125
IBM P/N 84G6245
FRU P/N 84G6246
EC NO. D60734
Nagano Japan Radio Co. Ltd.
Made in China

Input

100 - 125 V ~ 1.8 A
200 - 240 V ~ 0.9 A
(manual voltage switch - 115/220 V)
50/60 Hz

Output

+3.3 V @ 4.5 A
+5 V @ 11.5 A
+5 V @ 0.02 A (stand-by?)
-5 V @ 0.2 A
+12 V @ 1.2 A
-12 V @ 0.25 A

Total 57.5 W Max. for +3.3 V and +5 V combined.

Power Supply Connectors

PSU cabling & connectors (non-modular; permanently attached):

• 4 planar connectors (2x AT style, 1x 3.3 V, 1x SB/PS-ON)
• 1 riser connector (3.3 V)
• 4 drive connectors on 2 separate leads (2x Molex and 1x Molex + 1x Berg)

P1, P2, P10 & P11 are Molex 90331 plug w/ Molex 08-50-0276 contacts.

Power Supply Pinout

Planar Power Connectors P1/P2

The P1/P2 connectors use the "standard" PC AT PSU pinout.

Planar Power Connector P9

Planar Power Connector P11 - 3.3 V

Supplies power to the CPU socket (when set to 3.3 V) and possibly some other minor components (U18 is a 5 V part but one? pin is connected to 3.3 V too).

Riser Power Connector P10 - 3.3 V

Supplies power to the PCI slot and the PCI-MCA bridge (U8 - 10G7808).

Remove Power Supply

1. Undo the two large flat-head screws holding the top cover of the machine. Remove the cover by sliding it back and then pulling up.
2. Undo the phillips screw (black pan) holding the ISA-like PCMCIA adapter. Slide the PCMICA adapter out of its slot and remove it.
3. Unplug all power connectors from the planar (four), riser (one), and drives (up to four).
4. Unplug the IDE data cable from the hard drive.
5. Undo the phillips screw (black pan) holding the hard drive mount and swing it up and towards the back of the machine to remove it.
6. Remove the optional CD-ROM drive and anything else installed in the drive bays that might prevent the PSU from being slid forward (see the next step).
7. Undo the two phillips screws (black pan) holding the PSU to the back of the chassis. Slide the PSU towards the front of the machine and then pull up to remove it.

Open Power Supply

1. Remove two phillips screws (flat) on the side with no vents.
   
2. Remove two phillips screws (pan) on the other side of the PSU.
   
3. Remove the two bottom phillips screws (top on the picture below) holding the fan. The screws are quite long and go through both parts of the chassis. Leave the other two fan screws in place.
   
4. Crack open the bottom cover on the side where the vent is (use a flat-head screwdriver or some other tool if you have trouble separating the two parts).
   
5. Hinge open the bottom cover and remove it (the "hinges" are on the side with no vents).
   

Power Supply Inside

The PSU consists of one main board with two smaller PCBs plug into each other.

The fuse is 250 V, T3, 15 A with leads and is soldered in.

Power Supply Fan

The rear portion of the PSU where the fan is enclosed is taller than the rest of the unit. The fan pulls air not only through the PSU but also around the hard drive that is normally installed right above. The fan will slow down if the total system power consumption is lower than 50 W, and will stop to spinning completely if below 30 W. Thus the cooling fan usually stays idle.

Fan Model

Specifications (datasheet)

Model: FBA06A12L1A
Nominal speed: 3200 RPM
Max.air flow: 0.40 m3/min, 14.1 CFM
Max. air pressure: 3.10 mmH20, 30.4 Pa
Noise: 24.0 dB-A
Dimensions: 60.0 x 60.0 x 25.5 mm

Fan Speed Control

Sandy's experience and fan modification (edited):
   When I got the 6860 I've immediately noticed that the fan wasn't running at all. As I didn't know about this unique feature I thought there must be some fault on the circuit board of the PSU. I disassembled PSU and soldered 12V line directly to the fan assembly. Only latter on have I learned about this unique function. The jumper wire is still there... Some machines have input AC voltage selector. Mine has a "fan mode selector", summer mode with direct connection and winter mode with original connection.

The machine works fine under DOS. But under Win95B it usually freeze at start up after the Win95 logo appears on the screen. I solved this symptom by modifying MSDOS.SYS to have a start up menu with default boot for Win95 after 60 seconds of delay time to boot. Maybe mine needs some delay for stable DC output for planar, a hard drive and a CD-ROM. I'm sure that this symptom is not related to the above mentioned fan cable modification, 'cause I have the same problem even when I switch back to the original cable connection.