Centipede

[DanP]

Symptom: Totally Dead

Action: Hook up to Fluke 9010 and run ram tests for regions 0000-03FF (work ram) and 0400-07FF (video ram). 0000-03FF fails with BTS FF error. Try piggybacking a 74LS244 onto B1 and C1, now passes some of the ram test.

Fix: Replace 74LS244 buffers at B1/C1

 

[DanP]

Symptom: Dead

Action: Run Fluke RAM tests on 0000-03FF, fails at some addresses in the range

Fix: Replace 2114 ram at F2/H2

 

[DanP]

Symptom: Still dead

Action: Run Fluke ROM checksums and find one ROM fails

Fix: Replace socket at bad ROM location E1

 

[DanP]

Symptom: Game plays blind but screen is black with some distortion

Action: Determine we have no HSYNC. Trace signal back to suspec 74LS163 at M2. Piggyback another IC and we have sync

Fix: Replace 74LS163 @ M2

 

[DanP]

Symptom: Game plays but sprites are mostly missing

Action: Check IC's in motion object circuitry

Fix: Found bad 74LS83 @ H6, replaced and sprites are back

 

[guddler]

Symptom: Game plays but no sound.

Action: Bruce points his fingers at some chip and says "I think it's that one". I stick my scope on the output of said chip, then the input of said chip and concur.

Fix: I cut out and replaced that chip.

Obviously I can't remember what "that chip" was, but it was the final op-amp in the sound circuit so I think it was the LM-324

 

[Equites]

Symptoms:
The GAME ran but there was lots of alphanumeric garbage around the middle of the screen. All sprites including the Archer had erratic vertical movement.

Fixes:

The garbage was resolved by replacing a bad video RAM. The erratic movement of sprites turned out to be a bad POKEY chip, that will teach me to check socketed stuff first - sheesh!

 

[DanP]

Symptom: Game plays but some mushrooms are letters "H".

Actions: Self test reports 5 beeps (video RAM L7 bad). Fluke 9010 on an L7 address (x0410) reports ERR BTS 08). Try replacing socketing and replacing L7, no change. Replace J2 (74LS139) as it seems flaky, no change. Try removing Graphics ROMS at F7 and H7 and the Pokey and clean their legs. After putting them back the RAM issue is now gone and the screen looks fine!?!?

Fix: Clean and reseat Graphics ROMs and pokey. Will replace sockets in due course.

Bizarre one this one, but it caught me out for ages.

 

[DanP]

Symptom: The game appears to run OK, the colours are ok also but all the text is mangled and the mushrooms all appear to be displaced by 4 pixels. The gun and spider are also odd. When they are in the middle of the screen they look ok but when they are at the left hand side they appear upside down and when the're at the right side thay are mangled or displaced like the mushrooms. The centipedes head is also not behaving correctly. It always looks to the right even if it's moving to the left.

Fix: On self test beep code suggests Ram @ K5 is at fault. Checking with a scope showed that an output on the 74LS153 data multiplexor @ K6 was stuck low, changing this cured the ram problem but the graphics where still scrambled. Checked the character rom addressing logic and found that address line ref: MGA3 from 74LS86 @ E7 was giving the inverse of what it should have been, changed the chip and all is well with the world.