(from William R. Walsh (original HERE)
The IBM PS/2 Model 25 personal computer is an all-in-one unit. The monitor
and computer are in one case. In order to get a feel for what your particular
system is capable of, you need to find out what is inside. In a perfect world,
all the model numbers would be unique and well documented. This being IBM,
however, that isn't the case. For the best possible assessment of what you have
(or don't have), the best thing you can do is open your system unit (see the
instructions here) and take a look at it. This will
answer your questions and builds your character (maybe).
Three specific systems and one upgrade model number exist as provided by
IBM. All four systems differ greatly in planar (mainboard) design, processing
speed, display, storage capability, and upgrade parts that can be used. I've
seen conflicting labeling on systems and sometimes people do strange things,
such as swapping monitor assemblies with a lower end system.
Should you desire to see your system's type-model and serial number, you
will find them on a label underneath the lower left hand corner of the monitor
bezel. The system should be facing you, as in the picture below.
An earphone connector is provided at the rear of the system unit to allow
the user to disable the internal beeper (speaker) and listen to the sound
output through a set of earphones. The internal beeper is disabled whenever a
plug is inserted into this connector.
The connector will accept any 1/4-inch diameter audio plug. A monophonic
earphone with an impedance level of 15 to 35 ohms is recommended. Some
earphones with an impedance level as high as 100 ohms may also be acceptable.
The drive level is fixed so that the sound level will be directly related to
the sensitivity of the earphones. Earphones with 1/8-inch plugs may be used
with adapter plugs.
Note: This is not Line-Out, but a 33 KΩ
series resistor can be used to drop the audio signal down to a line level. Watch
IBM PS/2 Model 25 Earphone to Line Level
(by Hakemon Mike) for more info.
The IBM 8525 security, auditability and control features include a
bolt-down feature that allows the user to physically secure the system
unit to a table or desk. Three holes are provided in the system unit.
Using the center hole only allows the user to rotate and tilt the system
unit. If the two outside holes are used, the system unit will tilt, but
will not rotate.