Arcade Collecting > Restorations & repair
I don't believe it, another classic added => Centipede ! FINISHED !!!!
Level42:
I replaced lots of chips today, regretfully without any result.
The only one's I didn't do yet are C8 and the main RAMs, but there are 8 and I only have 2, and they all measure the same values....piggy backing these RAMs didnt do anything at all.
I tried piggy backing C8, and there were already more details on the screen then without it, but the PCB is still not "running". Just a steady garbage screen with game characters....
I also replaced one of the startbuttons, just to take a look. Of course, the holes in the metal have been made larger to fit those ugly fake fruitmachine buttons, but the cones do cover the holes. However, they did a "proper" job and there's a small "key" on the side of the hole that shows when using the cones.
Should be OK when the CPO is replaced though this will cover it and it's so tiny, it won't be harmed.
Level42:
Wow , I really had to dig up this thread :)
Worked on the Centipede PCB today and I think I fixed it :) :laugh: :laugh:
It was no RAM error.
Last week I checked the EPROMs at work, we have an EPROM programmer there. They all checked out just fine when compared to the "files I have floating around"....
I also checked out all the pins of all the EPROMs to the solder side, each and everyone beeped out just fine.
So today I decided to go to work like a pro should:
I got out my Oszifox (http://picotechitaly.8k.com/oszifox1/oszifox.html) and connected it to my laptop for easier reading.
I already had ordered a Centipede to Jamma adapter from Y-plus technologies http://www.arcademvs.com (who are GREAT, they deliver lightening fast and are CHEAP) just for the test work. Hooked it up to my Jamma cabinet and, there was the same static garbage screen like I had before.
I thought I smelled something and when checking all the parts for heat (fingers are a great instrument :) ) I discovered that Q2 and Q3 were really running hot. I then discovered through some more measuring that pin 5 and 6 were shorting on L2 (for those who want to know full details, let me know). I cut both pins, fired up the board, still garbage, but now it was actually responding (a bit) to the reset button. When pressed some garbage changed to other garbage, and back to the original garbage when released :D Since L2 is controlling the reset line of the CPU (6502) I took a gamble and shorted the +5V to pin 40 (the reset pin) of the CPU and YESSSSSS I had a play field. Even better, as long as I short the pin to +5V the game runs fine in attract mode !!! :cheers: :cheers:
Set the dipswitches to Free Play, and yes I can start a game and fire....moving is a bit hard without trackball though :D
Sooooo, I went to my local electronics shop to get a new 74LS90 (=L2) but of course they didn't have it in stock :banghead:
Ordered 2, pretty insanely expensive, over €2,- each for a simple 74LS.... but what the heck, I need it and I want it fixed.....
I guess I could simply hard-wire pin 40 to +5v but then there's no way to reset it anymore....
Anyway, I was very excited to see the game running in attract, really gives you a boost to know you found the problem and (most likely) fixed it.
I guess all the RAMs and CPU etc. have been replaced for nothing, but o well, another good lesson learned ! First measure ! :)
SirPeale:
--- Quote from: Level42 on November 03, 2007, 11:08:11 am ---
I thought I smelled something and when checking all the parts for heat (fingers are a great instrument :) ) I discovered that Q2 and Q3 were really running hot. I then discovered through some more measuring that pin 5 and 6 were shorting on L2 (for those who want to know full details, let me know).
--- End quote ---
Of course you should post those details.
Level42:
OK, sometimes I have the feeling my posts are too detailed and I'm boring people....
Here goes:
What you need to have for this are the schematics and a little understanding how these things work.
The schematics you can find at http://arcarc.xmission.com/
This one was of use the most for this situation:
http://arcarc.xmission.com/PDF_Arcade_Atari_Kee/Centipede/Centipede_DP-182-5th-01B.pdf
Since my PCB was completely frozen I had replaced a lot RAMs before and replaced the CPU. Also some more 74LS chips near the CPU.
On the KLOV forum, Mark Speath pointed me into the direction that this was probably NOT a RAM problem, since the game doesn't actually need any RAM to start-up. It makes sense, first thing a computer (which an arcade PCB is of course) when it starts up is read the very first adress for it's first CPU instruction. My PCB didn't seem to get that far...
Here's a pic how it looked when started up:
I could push the reset button, but it remained completely static, like it was frozen.
When you check the schematics, you can see that everything "starts" with the clock generator, using a 12 Mhz crystal and a transistor Q1. I checked those with the Oszifox and it was oscillating happily. The clock signal is routed to some counters/dividers. One branch of these make the H-sync and V-sync signals. Since I had a stable picture, I figured these were OK.
The other branch of course goes towards the CPU. I checked the clock signals with the Oszifox on the test-points and on the CPU pins and they seemed OK. I had also checked the ROM select signals (there are test-points for each one) and these were constantly high. This meant the CPU was not addressing the ROM.
So, I figured something was holding the CPU. I also felt the overheating Q2 and Q3 so I measured the transistors and they seemed good. Then I measured (diode-check setting of DVM) across R7 and got a beep (=short). I re-measured it in Ohms setting: 0 Ohms. It should be 10k. I desoldered one leg of the resistor and it measured out fine, the short was somewhere else (usualy, the passive parts (like resistors, condensators etc. don't go bad, the active parts like diodes, transistors, IC's are most likely to go).
Checking the schematics I figured it could only be L2 since pins 6 and 7 are parallel to R7 and +5V. I cut off pin 6 and 7. The short across R7 was gone. Checked between pin 5 (+5v input of L2) and pin 6 and there was the short !
I fired up the PCB again with the two pins cut off. Now I could see something happening when pressing the reset button. (The garbage changed when pressed, and changed back when released....)
L2 controls the reset signal going to the CPU.So I checked the reset pin on the CPU (pin 40). When this is low, the CPU is in reset, so it should measure +5V when running normally. I checked it and it was low. So the CPU was constantly in Reset.
Then I figured "what the heck" and shorted a +5v testpoint to pin 40. First I did it very briefly. But it was enough to get the complete playfield :) Then I shorted it longer and the attract ran....
Hello, everyone still awake ? :P
P.S. Anyone needs a almost brand-new (two times used !!!!) Centipede to Jamma adapter ?
ChadTower:
--- Quote from: Level42 on November 03, 2007, 01:59:11 pm ---Hello, everyone still awake ? :P
--- End quote ---
Hell yeah, great post. :cheers:
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version