Facebook
Google
GitHub
Linkedin
Hi there!
This board landed in my shop for repair. The processor works fine except display does not work. Upon closer inspection, I checked signals going to the display and there were none!
The circuit feeding signals to the display is a 74AC14 hex Schmitt Trigger inverters with Vcc=5V designated as U9 on the board. Signals going into each inverter are coming from a 74LS14 hex Schmitt Trigger inverters with Vcc=3.3V designated as U2. These signals have about 2.6V to 2.7V for high level. I replaced this circuit and still no signals coming out of three inverters, signal is coming out of one inverter (but not faithfully inverted, as if having bandwidth problems at this low frequency), and the remaining two inverters are used to feed signals from the display to U9 then back to U2. Thinking that maybe this AC14 was probably bad, I replaced it again with another new AC14 and still same behavior, except that another inverter also showed signal. See attached photos.
This board seems that have been in another shop before based on the solder joints, however, I am in doubt whether this 74AC14 (U9) is the correct circuit as probably somebody replaced it instead of what I am starting to believe was another IC. Probably another 74LS14? AC14 doesn't seem to be compatible with input voltage at high levels coming from 74LS14 at 2.6 to 2.7V. That may explain why the outputs of AC14 don't follow the inputs.
Checking datasheets of both shows that AC14 may need a minimum of 3.15V as a high input voltage when operating at 5V. 74LS14 at 5V Vcc accepts 2.0V as minimum high voltage which means it could handle the incoming signals at 2.6V.
I am unsure of what IC may go as U9 in the original board, so if anybody knows I will appreciate it very much if I am let know.
I will order LS14 and try. I'll keep you posted.
Have a great day!
rlarios
This board landed in my shop for repair. The processor works fine except display does not work. Upon closer inspection, I checked signals going to the display and there were none!
The circuit feeding signals to the display is a 74AC14 hex Schmitt Trigger inverters with Vcc=5V designated as U9 on the board. Signals going into each inverter are coming from a 74LS14 hex Schmitt Trigger inverters with Vcc=3.3V designated as U2. These signals have about 2.6V to 2.7V for high level. I replaced this circuit and still no signals coming out of three inverters, signal is coming out of one inverter (but not faithfully inverted, as if having bandwidth problems at this low frequency), and the remaining two inverters are used to feed signals from the display to U9 then back to U2. Thinking that maybe this AC14 was probably bad, I replaced it again with another new AC14 and still same behavior, except that another inverter also showed signal. See attached photos.
This board seems that have been in another shop before based on the solder joints, however, I am in doubt whether this 74AC14 (U9) is the correct circuit as probably somebody replaced it instead of what I am starting to believe was another IC. Probably another 74LS14? AC14 doesn't seem to be compatible with input voltage at high levels coming from 74LS14 at 2.6 to 2.7V. That may explain why the outputs of AC14 don't follow the inputs.
Checking datasheets of both shows that AC14 may need a minimum of 3.15V as a high input voltage when operating at 5V. 74LS14 at 5V Vcc accepts 2.0V as minimum high voltage which means it could handle the incoming signals at 2.6V.
I am unsure of what IC may go as U9 in the original board, so if anybody knows I will appreciate it very much if I am let know.
I will order LS14 and try. I'll keep you posted.
Have a great day!
rlarios
