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Service Manual
5A-18
Table 5A-6. Ohms Open-Circuit Voltage (2635A)
Range Voltage
300Ω
3 kΩ
30 kΩ
300 kΩ
3 MΩ
10 MΩ
3V
1.3V
1.3V
3V
3V
3V
Table 5A-7. Ohms HI Troubleshooting (2635A)
Checkpoint Signal Description Possible Fault
A3U8 pin 14 Ohms Source A3U8
A3R10 HI SRC Ohms Source A3R10, A3K16, A3RT1, A3Z4, A3Q10
Channel HI Ohms Source A3K1 through A3K14, A3U4, A3U5, A3U11, A3U12, A3L1,
A3L2, A3L3
A3U8 pin 23 Ohms Source A3R11, A3K17, A3R42, A3C32
A3U8 pin 58 Ohms Source filter output A3U8, A3Q2
5A-14. Digital Kernel Troubleshooting
At power-up, if the display does not light or lights up and fails to report errors or begin
operation, use the following troubleshooting procedures.
To determine the relative health of the MC68302 Microprocessor (A1U1), first check for
a valid system clock (SCLK) at TP11. Use an oscilloscope to check for the SCLK clock
at A1TP11. Look for a 12.288-MHz square wave that transitions from 0 to 5V dc (VCC).
• If this signal is present, check for a similar waveform at pinA1U25-30 of the FPGA.
If a 12.288-MHz square wave is not presentthere, resistor A1R107 is probably bad.
• If the SCLK signal (A1TP11) is something other than a 12.288-MHzsquare wave, it
is most likely that the problem is related to thecrystal oscillator circuit (A1U1,
A1Y1, A1C3, A1C8, or A1R2).
If the SCLK signal is good, check the Display Clock (DCLK) output from A1U25-19.
DCLK should have a frequency of 1.024-MHz (period of 976 nanoseconds). The DCLK
signal is not symmetrical; use an oscilloscope to verify that it is high for about 325
nanoseconds and then low for about 651 nanoseconds. The operation of the Display
assembly depends on the DCLK signal. Missing segments, intensified digits, a strobing
display, or a blank display can be caused by a faulty DCLK clock.
• If the DCLK signal (A1U25-19) is not present but the SCLK signal iscorrect at
A1U25-30, the problem may be that A1U25 was notconfigured correctly at power-
up or A1U25 is defective.
• If the OCLK signal (A1U25-22) is a 3.072-MHz square wave but the DCLKsignal is
wrong, A1U25 must be defective.