Dismantling Instructions And Servicing Notes - TTI LD300 Service Manual

Dismantling Instructions and Servicing Notes

Service Handling Precautions
Service work or calibration should only be carried out by skilled engineers. Please note the
following points before commencing work.
Most of the integrated circuits are CMOS devices and care should be taken when handling to
avoid damage by static discharge. Most devices are also surface mounted miniature components
with very fine leads on small pitches. These components must be removed and replaced with
great care to avoid damage to the PCB. It is essential that only the proper tools and soldering
equipment as recommended for surface mount components are used.
Dismantling the Instrument
Disconnect the instrument from all voltage sources before it is opened for adjustment or repair.
If any adjustment or repair of the opened instrument is inevitable it shall be carried out only by a
skilled person who is aware of the hazards involved.
Remove the six screws retaining the top cover and lift off.
Control and Display PCBs
To give better access to the screws which retain the Control and Display PCBs the front panel
can be separated from the chassis by removing the four screws which secure it (two a the bottom
edge and one at each side). Pull off all the control knobs before attempting to remove the Control
PCB, noting the positions of the nylon spacers used on the shafts of the multi-turn potentiometers
(LEVEL A and LEVEL B).
Power Supply PCB
Note that the wires to the mains switch are clipped to the underside of the PCB; ensure that they
are not trapped by the pillars when replacing the board. If a regulator is replaced, ensure that
there is no gap between the clip-on heatsink and the tab of the device; this can happen if it is
pushed on too far. Check also that the heatsinks are not touching one another. Reconnect the
leads to both fans.
Main PCB
This can be removed from the chassis without removing the PSU PCB first. Take note of the
position of all the cable connections, especially the two connections to the front panel. The sense
connection from the load terminals is PJ13, the output to the Current Monitor sockets is PJ7 (on
the right). Ensure these wires are dressed away from the mains connections to the switch.
Power Wiring
Before dismantling the power stages, note the arrangement of the wiring. The cables between the
FETs and the rear panel terminals are 6mm
only rated at 30 Amps). All these cables should be tri-rated (UL, CSA and EU) types. The internal
voltage sensing circuit is connected to the front terminals (if the load is connected to the rear
terminals there is no voltage drop across the internal wiring to the front).
All the crimp terminals are high conductivity ("Solistrand") types – they should not be replaced by
the common plastic insulated types as their crimp force is insufficient and their resistance too
high.
10
WARNING!
2
, those to the front terminals are 4mm
2
(as these are