modessitt
Volunteer: Encyclopedia Submission Moderator
So today, a fellow member brought over a few things to test. After verifying he had some bad chips on his Asteroids PCB and a bad ARII in his Missile Command, we moved on to monitors.
He brought two monitors:
1) A K7000 that only displayed red.
2) A K6100 that was blowing fuses.
Let's take a look at what was wrong:
K7000
He had already rebuilt the monitor with new caps, HOT, VR, flyback, etc. He brought the entire tube and frame, so the first thing I did was hook it up to my test bench to see for myself what it was doing. Yep, only red. Checked the voltages at the input and outputs of IC1 and they were okay. Checked the input to the neckboard and they were okay. Went to check the voltages at the drive transistors and - WHAT WAS THIS?
NOOB ALERT!
Apparently he never noticed that there were no drive transistors installed for the green or blue circuits.
He had forgotten that he had robbed them to fix another monitor and had never replaced them. I put new ones in and lo and behold he had all his colors!
K6100
Well, he picked up a Star Wars from a guy who tried fixing it and could never get it to work. It had the caps all done, HV section rebuilt, and low-voltage upgrade board installed. My buddy swapped his Tempest deflection board into the Star Wars so he could play it, but needed this fixed for his Tempest. Found one of the rectifier diodes shorted (which was blowing the fuses), so I replaced it and it came up - but without the -28. Only one LED was lit on the low voltage board. A quick check found that the black wire from the LV board to the chassis had come unsoldered. Apparently this will allow the +28 to work, but not the -28. I reconnected it and found the -28 was now working, B+ was perfect, but the spot killer stayed on. However, it came up with a vertical line on the screen. Swapped it into my tempest and got the same result, so I knew it was on the deflection board.
Well, I spent some time on the X-Amplifier circuit. Swapped out the Q703 and the Q704, but no change. Hmm. Swapped out Q700, Q701, and Q702. no change. All the diodes were good. What was causing it? Time to start tracing the signal through the entire circuit.
NOOB ALERT!
R700 was okay. R701 - WHAT WAS THIS? Apparently at some point R701 had been replaced with a 5W resistor for some reason, but instead of replacing it with a 1.3K-ohm resistor as was called for, they put in a 1.3-ohm resistor instead.
As the other side of this resistor was tied to ground, the X-Amplifier signal was just shunting straight to ground and never making it to the yoke. Replaced the resisitor with the proper 1.3K-ohm resistor and we have a perfect picture!
So - I guess the lesson here is:
Whenever working on a monitor that someone else tried unsuccessfully to fix, it's best to go over EVERY SINGLE PART that might have been replaced (look for newer solder) and verify that not only is it the right part, but that it is in properly.
My buddy is very happy that he can play his games now...
He brought two monitors:
1) A K7000 that only displayed red.
2) A K6100 that was blowing fuses.
Let's take a look at what was wrong:
K7000
He had already rebuilt the monitor with new caps, HOT, VR, flyback, etc. He brought the entire tube and frame, so the first thing I did was hook it up to my test bench to see for myself what it was doing. Yep, only red. Checked the voltages at the input and outputs of IC1 and they were okay. Checked the input to the neckboard and they were okay. Went to check the voltages at the drive transistors and - WHAT WAS THIS?
NOOB ALERT!
Apparently he never noticed that there were no drive transistors installed for the green or blue circuits.
K6100
Well, he picked up a Star Wars from a guy who tried fixing it and could never get it to work. It had the caps all done, HV section rebuilt, and low-voltage upgrade board installed. My buddy swapped his Tempest deflection board into the Star Wars so he could play it, but needed this fixed for his Tempest. Found one of the rectifier diodes shorted (which was blowing the fuses), so I replaced it and it came up - but without the -28. Only one LED was lit on the low voltage board. A quick check found that the black wire from the LV board to the chassis had come unsoldered. Apparently this will allow the +28 to work, but not the -28. I reconnected it and found the -28 was now working, B+ was perfect, but the spot killer stayed on. However, it came up with a vertical line on the screen. Swapped it into my tempest and got the same result, so I knew it was on the deflection board.
Well, I spent some time on the X-Amplifier circuit. Swapped out the Q703 and the Q704, but no change. Hmm. Swapped out Q700, Q701, and Q702. no change. All the diodes were good. What was causing it? Time to start tracing the signal through the entire circuit.
NOOB ALERT!
R700 was okay. R701 - WHAT WAS THIS? Apparently at some point R701 had been replaced with a 5W resistor for some reason, but instead of replacing it with a 1.3K-ohm resistor as was called for, they put in a 1.3-ohm resistor instead.
So - I guess the lesson here is:
Whenever working on a monitor that someone else tried unsuccessfully to fix, it's best to go over EVERY SINGLE PART that might have been replaced (look for newer solder) and verify that not only is it the right part, but that it is in properly.
My buddy is very happy that he can play his games now...