Centipede question

wickedbass

Active member
Joined
Nov 15, 2008
Messages
2,586
Reaction score
8
Location
Massachusetts
Hey guy centipede has been acting funny so i ordered an A/R II rebuild kit and a Big Blue.
rebuilt the board and replaced the big blue today.. game was working fine for about 10 min..then "froze"

so i checked the V and got this
+5.2 B 2 +5.2
+11.7 C 3 +24.1 (should be +22)
-14.6(should be -5) D 4 -24.1 (should be -22)

any ideas on how to fix this?
-Thanks
 
whats the deal with these bad 7905s? http://www.therealbobroberts.net/7905.html
i have read Bob Roberts chicken story, but still confused..so could i have a bad 7905 or did i install it wrong..i dont think i did, its isolated legs look good..i dont know..confused on where to do next

That's interesting. Decent voltage regulators (like the 7905) are never off voltage by more than a couple hundred millivolts so long as the load is within spec. (i.e. not too high) That is in contrast to switch mode power supplies which will exhibit a good deal of voltage stray under very light current loads. (Usually there's a dummy load circuit on the output to prevent this!) Also, to say that you've gone decades without having a batch of bad parts from a supplier is amazing to me. I must have some bad luck then - in the few years I've been repairing I've replaced about 3-4 bad IC's that I just installed. It happens much more often than it should.

Now I'm not calling Bob Roberts a liar, or am saying that he's deceiving us or anything of the sort. I'm just contrasting my experience with his.

Although it is interesting that the majority of 7905 complaints Bob has gotten has been about the ARII. Maybe there's somethiing specific about that design that doesn't get along with the 7905? Although from looking at the schematic and the board itself, there's nothing that stands out.
 
Hey guy centipede has been acting funny so i ordered an A/R II rebuild kit and a Big Blue.
rebuilt the board and replaced the big blue today.. game was working fine for about 10 min..then "froze"

so i checked the V and got this
+5.2 B 2 +5.2
+11.7 C 3 +24.1 (should be +22)
-14.6(should be -5) D 4 -24.1 (should be -22)

any ideas on how to fix this?
-Thanks

-24.1 is fine for the -22VDC rail....this is an unregulated line.
24.1 is fine for the 22VDC rail.....this is also unregulated.

the -5VDC rail is defintely bad....though, this voltage is only used for audio. So, you've got other issues too.

Edward
 
-24.1 is fine for the -22VDC rail....this is an unregulated line.
24.1 is fine for the 22VDC rail.....this is also unregulated.

the -5VDC rail is defintely bad....though, this voltage is only used for audio. So, you've got other issues too.

Edward

Great thats not good news:(. im always having problems with this game dam it..
Thanks for the reply
 
Have you checked the ROMs and processor IC sockets. It was common practice for Atari to use crappy IC sockets for their socketed chips.

Edward
 
Have you checked the ROMs and processor IC sockets. It was common practice for Atari to use crappy IC sockets for their socketed chips.

Edward

No but the board was tested in my friends machine and working perfectly..so i figured it was my a/r board ..so i did a rebuild now im having this problem?
 
Can anyone help me figure this out... ive replaced the 7905 twice now and im getting -13 where i should be getting -5 at D
 
Looks like the 7905 problems are fairly widespread. I had distorted audio on a Missile Command - pretty sure it was a flaky TDA2002 amp. As I'm bone idle I ordered a nice fully refurbed AR2 instead of sourcing and fitting a new TDA200s, but rather than fix the distorted sound I ended up with no sound at all.

Sure enough -5V on the AR2 reads as -11V. The problem can only really be the 7905. So .... the AR2 has gone back for a repair - its likely I've also taken out the LM324 on the Missile Command board too. Grrr !!
 
The wierd thing is, there's nothing special about that -5V circuit, at least from a schematic point of view. There must be something inherent in either the physical layout or perhaps the loading of that circuit. Tonight I'll hook up an MC and measure the current draw from that voltage. There's gotta be a reason why we're seeing so many failures.
 
Cheers Bit_Slicer - I'd be interested in your results.

The guy I bought the refurbed AR2 has a great reputation in the UK - he's not making any real money out of it, knows what he's doing, and does it for the love of the games. In short, he's a stand up guy. He reckons that he soak tested that AR2 in a Crystal Castles for a good few hours before he sent it out to me, and I believe him - so seems really odd that both Centipede and MC are having problems ....
 
centipede 7905 issue

guys,

I am pretty sure I know why there have been so many 7905 issues wuth the ARII...
That being said, I have a centipede and an ARII but no power brick to check my ideas.

Bit Slicer, could you tell me what the current reading was on the -5 Volt line? I will use this in my solution.
I have used thousands of 78XX and 79XX over the 30 years as a tech and never had a brand new part bad out of the box. I might be lucky but my history says otherwise! Also, companies no longer do testing on their mass-produced parts unless they have reason to believe there is a problem, so new stuff might not be as reliable.

Basically it looks like ATARI designed the -5V circuitry right at the maximum limit of the device. Get me some current readings and I will be better armed to say what I think is wrong.
 
Thurwor I think you and I are thinking along the same lines. I took a cursory reading of the -5V line on Centipede and it rang in steady @ 1.67A, which is just over the maximum recommended output current for modern parts with that base part number. I did manage to dig up an old spec sheet from Semiconductors Ltd. (presumed to have been bought out long ago!) and it put the max output current at 2.0A. So that gives creedence to your thinking and mine.

Unfortunately the LM7905CV part Bob sells falls into this category - it's spec'ed at 1.5A max output. With that, I must recant what I said earlier about him probably having a bad run of parts. :eek: We're just running them beyond their spec!

I'll hook up MC next and take another reading. I suspect it will be worse. :(

i guess the next step then would be to find a suitable replacement that can handle the current draw....
 
Whoa ..... so what's the theory here?

If I've got it right we're saying that this latest batch of Bob Roberts 7905s can only handle a current draw of up to 1.5A, where previously the limit was 2, and the Centipede draw (and probably MC) is at least 1.67A?

That's an interesting and potentialy very important discovery ....
 
Whoa ..... so what's the theory here?

If I've got it right we're saying that this latest batch of Bob Roberts 7905s can only handle a current draw of up to 1.5A, where previously the limit was 2, and the Centipede draw (and probably MC) is at least 1.67A?

That's an interesting and potentialy very important discovery ....

Not just Bob's parts, but all of the contemporary parts I've looked at so far - TI, National, SG, etc. It could at least partly explain why we're seeing so many failures.

BTW I want to strongly suggest that Bob is probably unaware of the issue and not just screwing with people.
 
Blimey ..... good job I've got several `old' 7905s from retired AR2s ..... I'll be keeping those safe until a suitable replacement is identified.

I've passed the info on to the chap that refurbs my AR2s - be interesting to see what he makes of it - I'll report back.
 
BTW, Crystal Castles uses an AR-II rev 1, and doesn't require a -5V. Sounds like the -5V wasn't tested.

Cheers Bit_Slicer - I'd be interested in your results.

The guy I bought the refurbed AR2 has a great reputation in the UK - he's not making any real money out of it, knows what he's doing, and does it for the love of the games. In short, he's a stand up guy. He reckons that he soak tested that AR2 in a Crystal Castles for a good few hours before he sent it out to me, and I believe him - so seems really odd that both Centipede and MC are having problems ....
 
Ah. Thanks. I hadn't realised that. Still, the -5VDC would have been present on the AR2 (and I'm sure would have been tested) but the -5 wouldn't have been subject to a load at that time.
 
Scratch that. I just checked the Crystal Castles schem. It only takes the 10.3VDC and regulates it down to +5. That's it as far as voltages go..... oops.
 
Back
Top Bottom