- Ea input is High/Low-pulsing...A0 & A1 are High/Low pulsing as well. However.....outputs 0o, 01 & 03 are sitting high...rest are High/Low pulsing...
I am assuming the following:
Ea=Enable pin=pin 15
A0=Select A1=pin 14
A1=Select A2=pin 13
O0=1Y0=pin 12
O1=1Y1=pin 11
O2=1Y2=pin 10
O3=1Y3=pin 9
if you look at the truth table for the 74ls139 your symptoms as you describe suggest that :
if pin 15 is pulsing, pin 14 is stuck low, and pin 13 is stuck high then outputs are p12=1,p11=1,p10=pulsing,p9=1 and match your symptoms.
Possible scenarios:
If as you say A0/A1 (pins 14/13) are pulsing it could mean that both the xor gate feeding pin 13 AND address bus line AB10 buffered by chip 2H signals are below TTL spec voltage levels for high level input - a less likely scenario than a faulty 74ls139. so possibly a dud chip. check for shorted tracks and replace the 74ls139 to start with.
Assuming that the outputs of the XOR ic and the AB10 line are good, If Ea (enable, pin 15) is pulsing, and output 2 is pulsing , then 1H driving pin 15 is prob ok. once again points to likely stuck inputs for A0/A1 on pins 14,13 of the 74ls139. check for shorted tracks and replace the 74ls139 to start with.
this shouldnt exlude:
- faulty 2H
- 5A pin 14 shorted to ground, possible faulty ic 74xx157 buffer providing buffered ab10 address line to pin 14
- 5A pin 13 shorted to 5v, possible faulty XOR ic feeding pin 13 of 5A
just some thoughts!