modessitt
Volunteer: Encyclopedia Submission Moderator
Symptom: Asteroids Deluxe came up, but with vector lines connecting everything like the brightness was up too high. Turning down the brightness or contrast would make everything go away.
Diagnosis: At first I thought it was the monitor (GO5-802) as I had recently replaced the HV diode and done some other repairs - and spent some time chasing non-existent monitor errors - but eventually found another working monitor (V2000) that I connected and got the same issue. Research suggested a problem in the Z-OUT circuit. I had another working AD board, so I put it in and everything was working normally. Considering I had a working PCB, I used it to check voltages all the way through the Z circuit (K9, H/J10, etc), then compared those readings to the bad PCB. I found that the trace that connects Q1, Q2, and Q3 was reading about 1vdc too high (1.9-2.4vdc instead of 0.9-1.4vdc). The other legs of those transistors had the proper voltages, so one of these was rectifying improperly. Checking the transistors with my meter didn't show any problems. Since they're cheap, I started swapping them out. I did Q3 (2N3904) first since it was the first in line for the VBLANK signal, but that didn't help. Then I did Q2 (2N3904) as it was next in line and the one right before the Z-OUT test point, but that didn't help either. So then I did Q1 (2N3906) which was the one on the SCALE inputs. YEP! That did it. Perfect picture now.
Solution: Replace Q1.
Figure this might help someone, as I wasn't finding a lot of "solution posts" in the few threads that deal with similar issues...
Diagnosis: At first I thought it was the monitor (GO5-802) as I had recently replaced the HV diode and done some other repairs - and spent some time chasing non-existent monitor errors - but eventually found another working monitor (V2000) that I connected and got the same issue. Research suggested a problem in the Z-OUT circuit. I had another working AD board, so I put it in and everything was working normally. Considering I had a working PCB, I used it to check voltages all the way through the Z circuit (K9, H/J10, etc), then compared those readings to the bad PCB. I found that the trace that connects Q1, Q2, and Q3 was reading about 1vdc too high (1.9-2.4vdc instead of 0.9-1.4vdc). The other legs of those transistors had the proper voltages, so one of these was rectifying improperly. Checking the transistors with my meter didn't show any problems. Since they're cheap, I started swapping them out. I did Q3 (2N3904) first since it was the first in line for the VBLANK signal, but that didn't help. Then I did Q2 (2N3904) as it was next in line and the one right before the Z-OUT test point, but that didn't help either. So then I did Q1 (2N3906) which was the one on the SCALE inputs. YEP! That did it. Perfect picture now.
Solution: Replace Q1.
Figure this might help someone, as I wasn't finding a lot of "solution posts" in the few threads that deal with similar issues...
