pcjason
New member
Hello,
Just thought I would post details on a few of the repairs that I just performed on a Centipede cocktail.
Background: This game was sitting in a garage without climate control in Lake Tahoe for over two years. It was placed there after it stopped powering on.
Issue #1: No power to the game.
Diagnosis: After ensuring that the power switch and interlock switch were functional with a meter, I checked the power cord for continuity from the plug prongs to the power brick. It was found that one wire had no continuity.
Repair: Replaced power cord and we now have power.
Issue #2: R30 was smoking on the AR II board.
Diagnosis: Should not be a surprise, but the edge connectors were not making very good contact, causing one of the sense lines used for feedback to the AR II to be floating.
Repair: For now, I have shorted the sense lines right at the AR II. New edge connectors are on order.
Issue #3: The monitor showed garbage and was waving all over the place. Almost looked like a loss of the sync signal.
Diagnosis: As with any first cut debugging, I started with checking the supply rails. The +5V rail measured 4.4VDC at the board. Before adjusting the AR II to raise this, I measured the AC ripple, as I did not want to fry anything if there was excessive ripple. The AC ripple measurement showed 800mVAC on my Fluke meter, which is *way* beyond a reasonable ripple value.
The power supply design is very simple, consisting of a step down transformer, a bridge rectifier, and a large filter cap to produce an unregulated 10.6VDC that is later regulated. Testing the unfiltered supply showed the excessive ripple there as well. There are only two things that could cause this. Either the bridge rectifier has an open diode, meaning a part of the AC cycle would be missing and the filter cap would have to do more work than it is capable of, or the filter cap is bad. Testing the bridge rectifier showed no issues, so the issue had to be the filter cap.
Repair: Replaced the large filter cap ("Big blue"). After replacing this, the +5V rail measured 5.15VDC at the board and the showed only 15mVAC ripple. This also caused the monitor waving to stop. At this time, I adjusted the supply down to 5.05VDC at the board.
Issue #4: The game shows non-static garbage on every power up. The garbage consists of random letters, numbers, and parts of sprites all of the screen. The background color changes randomly, garbage tones are heard, and the coin counter randomly increments.
Diagnosis: Firstly, I confirmed that all supplies were at the board and within spec. All supplies were fine. I had a working Centipede board on hand, so I threw that in just to verify that power was okay. The working board also worked in this machine, so power issues were ruled out.
Next, I swapped all socketed components between the two boards. After this exercise, the working board still worked, and the broken board was still broken.
Next, I did a thorough visual inspection of the board for bent leads, broken traces, or broken components. The visual inspection showed nothing abnormal.
Finally, I started probing the board. I have misplaced my logic probe, so I had to use my scope instead (poor me
). Probing the reset line on the CPU showed that the watchdog was firing continually, so the program was not running correctly. A quick probe on the clocks showed the correct frequency. Also, the address and data bus showed activity, so there was nothing grossly wrong there. I started to probe the program ROMs, and found that the /CS for the ROM at E1 was only toggling between 1V and 0V, so this ROM was always active! This is driven by the 74LS139 at J2. I removed the ROM in order to rule it out as the cause of the signal being pulled down. Even with the ROM removed, the signal still showed a 1V peak-to-peak swing, so the culprit had to be the 74LS139 at J2.
Repair: Replace J2. The board now boots and runs correctly!
Hope that helps somebody!
-Jason
Just thought I would post details on a few of the repairs that I just performed on a Centipede cocktail.
Background: This game was sitting in a garage without climate control in Lake Tahoe for over two years. It was placed there after it stopped powering on.
Issue #1: No power to the game.
Diagnosis: After ensuring that the power switch and interlock switch were functional with a meter, I checked the power cord for continuity from the plug prongs to the power brick. It was found that one wire had no continuity.
Repair: Replaced power cord and we now have power.
Issue #2: R30 was smoking on the AR II board.
Diagnosis: Should not be a surprise, but the edge connectors were not making very good contact, causing one of the sense lines used for feedback to the AR II to be floating.
Repair: For now, I have shorted the sense lines right at the AR II. New edge connectors are on order.
Issue #3: The monitor showed garbage and was waving all over the place. Almost looked like a loss of the sync signal.
Diagnosis: As with any first cut debugging, I started with checking the supply rails. The +5V rail measured 4.4VDC at the board. Before adjusting the AR II to raise this, I measured the AC ripple, as I did not want to fry anything if there was excessive ripple. The AC ripple measurement showed 800mVAC on my Fluke meter, which is *way* beyond a reasonable ripple value.
The power supply design is very simple, consisting of a step down transformer, a bridge rectifier, and a large filter cap to produce an unregulated 10.6VDC that is later regulated. Testing the unfiltered supply showed the excessive ripple there as well. There are only two things that could cause this. Either the bridge rectifier has an open diode, meaning a part of the AC cycle would be missing and the filter cap would have to do more work than it is capable of, or the filter cap is bad. Testing the bridge rectifier showed no issues, so the issue had to be the filter cap.
Repair: Replaced the large filter cap ("Big blue"). After replacing this, the +5V rail measured 5.15VDC at the board and the showed only 15mVAC ripple. This also caused the monitor waving to stop. At this time, I adjusted the supply down to 5.05VDC at the board.
Issue #4: The game shows non-static garbage on every power up. The garbage consists of random letters, numbers, and parts of sprites all of the screen. The background color changes randomly, garbage tones are heard, and the coin counter randomly increments.
Diagnosis: Firstly, I confirmed that all supplies were at the board and within spec. All supplies were fine. I had a working Centipede board on hand, so I threw that in just to verify that power was okay. The working board also worked in this machine, so power issues were ruled out.
Next, I swapped all socketed components between the two boards. After this exercise, the working board still worked, and the broken board was still broken.
Next, I did a thorough visual inspection of the board for bent leads, broken traces, or broken components. The visual inspection showed nothing abnormal.
Finally, I started probing the board. I have misplaced my logic probe, so I had to use my scope instead (poor me
Repair: Replace J2. The board now boots and runs correctly!
Hope that helps somebody!
-Jason

