Service manual

Service Manual
Repairs following functional
testing
The following notes refer to the functional test
procedures described in the previous chapter, and give
component level diagnosis and repair information
following a test failure.
Unless otherwise stated, always perform the simple
checks given in Part 4 - Fault diagnosis first, then refer
to the relevant component level information below.
Type/Model
Memory area fault— run the Memory Test (see the
section entitled Test ROMs on page 5-23) and repair as
necessary.
Memory
Repair as above.
Battery-backed RAM
If the NVM suffers data retention problems and the RTC
fails, then, with the computer power off, check for about
2.8 V on IC22 pin 8. If this voltage is not present, check
the charge state of the battery BT1 (1.2V).
If the NVM IC22 consistently fails on the same data bits,
change the device.
If the clock fails to run or runs inaccurately, check and if
necessary replace X3. LK8 allows access to the clock
signal.
Audio (Loudspeaker test and
Headphones test)
Test the audio with both headphones and internal
speaker. Do not forget to issue *SPEAKER ON and
*VOLUME 127 commands.
If only the speaker fails, check connections to the main
PCB via LK13 and check IC80 pin 5 for a signal of 3 V
amplitude. If no signal is present on pin 5 but can be
found on pin 3, change IC80.
If there is no audio at all, first check for +5 Von both
ends of L18. If this is open circuit, check the condition of
C2 before replacement. Check for -5 V on IC78 pin 11
and R49 and R43. Check for about 3 V on VIDC IC 54
pin 12.
A low-amplitude signal should be found on VIDC IC 54
pins 13, 14, 15 and 16. If not, change VIDC. These
signals can be traced through the peripheral circuitry and
out to 012 and 013. The signal amplitude at these points
should be about 1.5 V peak to peak.
Check for short or open circuit on signals SNDAK and
SNDRQ on VIDC IC 54 pins 9 and 24.
Monitor Screen
If the display breaks up around its edges and spurious
characters appear then investigate the system oscillator.
Check IC 51 and X1.
Check DRAM using the test ROMs (see the section
entitled Test ROMs on page 5-23).
With a full white screen, VIDC IC 54 pins 39, 40 and 41
should all have the same signal on them. If not, change
the VIDC IC 54.
Trace each signal through the periphery circuitry and out
to SK2 until the fault is found.
Unstable or scrolling display
The computer may have lost its configuration value for
SYNC. Type at the keyboard:
*CON. SYNC 1
press RESET and see if any change occurs. Check for
CSYNC signal on SK2 pin 4. If not present, trace back
through LK6, R96 and IC 63, finally changing VIDC IC
54.
Floppy disc drive
Make sure that the configuration items STEP and
FLOPPIES are correctly set. Check that the disc drive ID
selection switch is in the required position (usually 0).
Swap the disc drive for a known good drive and cable. If
this also fails, check the power supply connection for
+12V, +5V and 0V.
Serial port
If a fault is reported but the test is passed, see the Serial
Port Application Note in the Archimedes 440 Service
Manual for possible explanations, noting that the A500
and R200 series serial port is now based on the RS232
standard, and the patch (RS423 Drive version 1.24) is
no longer required.
Check for -5 V on IC 6 pin. Check for the clock on IC 2
pins 6 and 7; change X2 if faulty. If OK, change ICs 5
and 6.
Printer
If the printer fails completely, check for a STROBE signal
on SK 3 pin 1, trace back through R 122, 0 4 and R 29 to
IC 77. Also check for shorts or open circuits on PACK
and PBSY.
If the data printed is incorrect, check the continuity of the
data lines into and out of IC 79, though R 104, R83,
R203, R196, R186, R172, R157, R140 and onto SK 3.
If both the printer and the floppy disc drive fail, change
IC 43.
Part 5 - Main PCB fault diagnosis Issue 2, June 1991 5-27