Wyse WY-60 Maintenance Manual page 39

Figure 3-1 Troubleshooting Flowchart, page 3 of 7
No Video/No Beep, Continued
7 . Chec k U20
(continued)
If the waveforem is correct, check
the character clock output at U20-8.
Look for 2.658 or 4.4122 MHz. If
the output is missing, replace U20.
Check for shorts on the video
output lines: U20-14, U20-15,
Q403, Q404, R25, and R26. Are
these lines open?
Check the video blank line, U20-9.
Check HSYNC: Is U20-7 toggling?
Look for 26.58 KHz. Is the line
open or short? Check R201.
Is HSYNC present from the video
controller at U20-10? If the signal
is present, replace U20. If the
signal isn't present, check U19-9.
Check MBC2 at U20-11, U23-11, and
U20-4. Is the signal toggling? Is
the MBC signal present at U20-4.
If the signal is present, replace
U20. If the signal isn't present,
continue to step 8.
8. Check U9 Is U9 receiving power? Look for
+5V at U9-40. U9-20 is ground.
Check character clock input. Use
the schematics in Chapter 7 to
check the circuitry between u20-8
and U9-16. Are there shorts? Is
any portion of the circuitry open?
If the signal is missing and you
find no open circuit, replace U20.
If the signal is missing and you
find no shorts, replace U9.
Check controller select activity at
U9-2. If no shorts and no open
lines, go to step 9.
Check interrupt lines:
U9-5 (toggles at 1.5 KHz)
U9-35 (toggles every 16.6ms)
Are they short or open?
3-8