Centipede Repair Log

The game played fine but failed to save high scores. Centipede saves the top three high scores and stores them in an Electrically Alterable Read Only Memory chip known as an EAROM. I changed the EAROM with a known working one but this did not fix the problem. The EAROM is located in E5 on the PCB and can be a NI-7055 or an ER-2055.

I read the Centipede manual and found that the game could be put in test mode in order to see if the EAROM was working. This meant that I did not have to continually set a high score in order to test the EAROM. When the game is in test mode and the EAROM is not working you will see 4 FF in the upper left corner of the screen. Also the number of plays and the game time will be missing.

I then studied the Centipede schematic and found that the EAROM required approximately –30VDC to operate. My voltage at the –30VC test point was only –14VDC! I rebuilt the –30VDC line by replacing the 1uf caps at C84 and C86, the 10uf cap at C85, both 1N4001 diodes at CR4 and CR5 as well as the 555 timer at A11. I retested the –30vdc line at the test point and now had a solid –28VDC. Excellent! I put the game in test mode but again saw the 4 FF upper left corner of the screen. This meant the EAROM was still not working.

I could see that this was going to be a more difficult repair and it was time to do a more in depth study of the board. I again studied the schematic and saw that the WRITE2 signal went through a 74LS32 at A4. I connected a HP 10529A Comparator to the chip and found that there was a logic fault! I replaced the chip at A4 and was confident the problem was fixed. I again put the PCB in the game and put it in test mode and was again greeted with the 4 FF.

Back to studying the schematic! I could see that there was only one chip between the output of A4 and the EAROM and that was the buffer at H4. H4 is a 20 pin chip that unfortunately could not be checked with the comparator as it only checks chips with 16 pins or less. Checking H4, which is a 74LS374 chip, with a logic probe didn't reveal anything significant however since the chip took input from A4 and sent it to the EAROM I decided to replace it. 

I put the board back in the game and put it in test mode. Good sign! No longer was there a 4 FF in the upper left corner and the game now properly displayed the number of plays and the game time! However the real test came when I set a high score, turned the game off, turned it back on and the score would still be there. I commenced to setting a high score!  I did so and entered my initials. Then I turned the game off, turned it back on and let it go to attract mode without putting it in test mode.

Success!  My high score was there! I quickly set another high score and like the last one it was saved too! So what is normally a simple replacement of an EAROM became a very involved repair but gave me a good knowledge base should I ever encounter this problem again. Now it is time to obliterate some centipedes!

The track ball assembly had seen many years of wear and tear and needed to be completely rebuilt. The bearings were bad and the shafts were worn which led to poor control of the archer. Dust, debris and probably a few beverages were spilled through the housing at one time.

Everything was thoroughly cleaned and new shafts and bearings were installed. A few drops of 3-in-1 oil were added as well.

Once assembled the track ball rolled very smoothly and was ready for action!

The front of this game was a mystery to me.  At one point somebody thought the front of the game was beat up enough that it was necessary to wipe the entire front surface with a rag dipped in black paint to "improve" it atheistically.  After years of wear and tear the black paint was in horrible condition to the point I thought for sure the black laminate would have to be replaced.

I tried many stain removal products but the paint proved stubborn and did not come off easily, however, with some serious elbow grease Goof Off was starting to remove it.

I was surprised that after cleaning off the lower section it did not appear to be heavily damaged or even worn! The original black laminate surface appeared to be in excellent condition.  I decided to go further and rub down the entire front surface.

To my astonishment the front laminate was in pristine shape and after a good polishing it looked like new!  Why it was ever painted over in the first place remains a mystery!

The game now looks as good as it plays!