9
Diagnostic Notes
This instrument uses many small surface mount components in critical high frequency circuits.
Component level fault finding and repair is not possible and service is normally performed by
identifying the faulty PCB assembly and returning it to the manufacturer or their field service
agent for repair or replacement. The only field repair possible is the replacement of the
rechargeable battery cells. The following notes are intended to help in identifying which of the
PCBs is faulty.
AC Adaptor
The output voltage of the adaptor can be checked with a DVM: the outer sleeve is the negative
terminal and the voltage should be around 5.2 to 5.3V.
Initial Checks
First check that all keys are free to operate; if any one is stuck down, then the unit cannot
correctly identify keystrokes.
Now remove the top cover of the instrument as described up to step 4 of the disassembly
procedure above.
First try disconnecting and then reconnecting the ribbon cable from the back of the main PCB.
Inspect the insulation displacement connectors and check they are clamped together tightly. If
intermittency is suspected, remove the Charger PCB and check the solder joints on the transition
header.
Otherwise, identify the faulty subsystem by checking the following items in order.
Batteries
Measure the voltage of each cell; they should be similar and between 1.0V (discharged) and
1.45V (fully charged). If any cell delivers below 0.5V it is probably damaged and all three cells
must be replaced, either by field repair (see below) or service exchange.
There is a self resetting fuse in series with the battery. This component is a last resort protection
against fire and any failure indicates that another major fault has occurred.
Note in particular that this component MUST NOT be hand soldered as that will
almost certainly cause an internal short circuit which would negate the protection.
If the cells are charged, but the unit will not function then the fault is almost certainly on the main
PCB. To check, use a DVM to measure the battery voltage on the charger PCB between the test
points T0V (to the right of the USB socket) and TV (in front of the large gold area on the PCB).
Then measure the voltage on the main PCB between T0V (in the bottom left hand corner) and
the hole for pin 1 of the unfitted power socket on the right hand edge of the PCB. If the same
voltage is present here, then the fault lies on the Main PCB.
Battery Charger
Connect the AC adaptor to the unit and switch it on. Check that the input voltage appears
between the test point T0V (to the right of the USB socket) and the input to D2 on the rear-most
of the three gold heatsink areas.
There is a 2A surface mount fuse in the input circuit to this point. The AC adaptor supplied with
the instrument does not have sufficient current capacity to blow this fuse, so if it is open circuit
this indicates that an incorrect AC adaptor has been attached to the unit. Major consequential
damage should be assumed, so replace the whole PCB; do not just replace the fuse.
If the battery voltage is below about 3.3V the charger should start automatically, and the yellow
lamp should show on the panel. The charger may also be started by pressing SW1 on the PCB.
If the charger is active, it will be possible to feel that D2 and Q1 are warm within a few minutes. If
the charger is running but the lamp is not showing then the fault might be in the drive circuit, the
ribbon cable connections or the LED itself.