10.6 Selftest Failed Selftest failed Distortion poor Noise poor
Because of the low residual levels and the very stringent tolerance specification it is
important that good quality screened leads are used (with the screen connected to the
LA102 XLR ground, pin 1) and that the 10k
input impedance is selected for this test.
The cause of the failure can be found by printing the results (section 3.4) or examining
them on the LCD (section 3.2) to find the failed measurement marked with an asterisk
(section 5.1).
Distortion and noise failure can be caused by a noisy DC-DC converter and it is often
worth swapping the converters between the two units to see if it affects the results. Level
errors should be check manually as explained in section 10.11. Poor noise and low
frequency (40Hz and 100Hz) crosstalk and distortion can result from mains interference
and it is often worth repeating the test with the mains, oscilloscope and RS232 leads
disconnected to ensure that there are no earth-loops picking up the mains power
frequency.
Selftest failed Failed selftest Mains hum
10.7 5V Power Supply Check Battery failure NiCd battery Removal of the battery
Power supply voltages may be checked using the negative side of C12 (2200µF) on the
top board as a 0V ground reference (see circuit diagram, appendix D.2). The main +5V
supply can be picked up on pin 20 of any 74HC373 chip, and should be within ±5%. If
within tolerance, measure the voltage on pin 28 of IC1 (using the same ground reference).
This is the non-volatile RAM +5V supply which should also be 5V ±5% If it isn’t then
the processor will not operate and the LM2931 5V regulator fitted at IC20 has almost
certainly failed.
IC20 5V regulator LM2931 regulator
If the main +5V supply is not working disconnect each of the following in turn and
measure it again: battery connector (PL4), bottom board connector (PL2), DC-DC
converter (TR2). It is possible that one of these systems is loading the 5V supply to such
an extent that it is preventing the unit from operating. If the the 5V supply works without
the battery proceed to section 10.9. If it works without the bottom board or DC-DC
converter proceed to section 10.8.
If there is still no +5V supply check the voltage across the C12 capacitor. It should be at
least 12V. If it is, suspect the 7805 IC18 regulator and the diode D3. If not remove the
fuse F1 and test it. With the fuse removed and with the mains disconnected measure the
resistance of the transformer primary and secondary by removing the bottom board and
removing the plastic plate covering the mains connection to the top board. The two
primaries in series should measure around 720
cold) - 840 hot)and the two
secondaries in series should be around 2.8±1
.
161
10. Service Information
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