Access To Pcbs In Other Outercases; Replacement Of Cpu Pcb; Replacement Of Power/Phase Assembly - Siemens ST750 General Handbook

To open the hinged plate, the 64-way Phase Control Ribbon cables at
the front of the ST750 Rack Assembly must be disconnected.
Disconnection of the cable disconnects all logic power from the CPU
Warning
and therefore prevents the ST750 operating the lights, but it does not
switch the incoming mains off. The mains must be isolated at the
master switch before any disconnection of cables or fuse replacement
takes place.

7.4.4 Access to PCBs in other Outercases

All ST750 Rack Assemblies installed in cabinets with 19" mounting kits provide full
access to the front face of the ST750 and have sufficient room to allow the hinged
front plate to swing open, giving access to the internal PCBs.
Access to the PCBs is the same as described in section 7.4.3.

7.4.5 Replacement of CPU PCB

Ensure that the replacement PCB has the correct firmwares fitted and they are of
the correct variant and issue. Refer to the Works Order Specification for details.
See notes on CPU compatibility between ST750 and ST700
Important
On NO ACCOUNT is it permitted to downgrade an ST750 site to
an ST700 site by changing the CPU board and associated
Warning!
firmware as an ST750 site will conform to TR2500A which the
ST700 DOES NOT and CAN NOT
The mounting pillars on the CPU are a nylon clip type and only require the clipping
mechanism to be released to allow the board to be removed. The pillars are retained
on the hinge plate by means of an integral clipping mechanism on the other end.
When the CPU is replaced, make sure that the clipping mechanism of the mounting
pillar is fully engaged.
Warning Care must be exercised when removing or inserting the green connectors
and their cables from the PCB, as strong extraction and insertion forces are
required. Be careful not to over flex the board, which could cause tracks to fracture
and the board to fail.

7.4.6 Replacement of Power/Phase Assembly

The Power/Phase assembly may be removed once the following have been carried
out:
• Isolate the Mains to the ST750 at the master switch
667/HB/33750/000
contained in section 2.1.7
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