ARII oddity with -5v line...

Jeff F

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I rebuilt the ARII in an abused Centipede I picked up a few days ago and everything went well except for the 7905 which controls the -5v line.

After I first rebuilt it, everything worked fine and then I had some issues with the sound and come to find out the -5v line was measuring -10.8v which took out the LM324 on the main Centipede PCB.

I replaced both and had -5v and then after a few hours I measure again and it was back to -11v. The LM324 did not blow but I replaced the 7905 again and it was fine.

Today I measure the -5v line again... and yep... -10.8v.

I've gone over both the ARII and the main PCB and for the life of me I can't figure out what is going on. :dunno:

Any ideas would be appreciated.

Thanks in advance,
Jeff
 
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I read Bob's writeup about the 7905's under load and it makes sense.

The spec sheets only state the limits for the regulated output when the output current is between 5 and 1000 mA. Slapping a 1K resistor across the output would keep it from going far from -5VDC.

It wasn't a problem for Battlezone, because the -5V line isn't even used. It is routed to a few empty sockets and is available via a jumped/cut trace connection for alternate EPROM chips.

Your problem is real though. Chips are blowing. That one op-amp is the only thing I see on the schematic that uses the -5V. How does the +12V line look? It's hooked up to the same chip.

Can you see if replacing the regulator results in it blowing even without the board hooked up? What is the resistance to ground at pin 11 of chip location K10?

The other amps on the chip are in the same audio circuit. Could the speaker be drawing too much current? Didn't they make some single voltage LM324's? Maybe the chips you're using can't handle/don't expect a negative voltage on that supply pin?

It is an interesting problem at least.
 
I'll take a look this week. First thing I want to do though is test the voltage when the game is at normal volume to see if it makes a difference. I really don't think it should, but you never know. ;)
 
Only had a few minutes to look tonight but I get around -12v loaded and unloaded at the -5v TP on the ARII.

I'm going to order a 1.5A 7905 and I'm also going to try adding a resistor across the regulator just to see if that brings it back to around -5v... maybe there is not enough load from the LM324?

Can't do anything tonight so it'll have to wait. ;)
 
I'll ask the (potentially) insulting question...
Are you sure there's no short between the metal tab on the regulator and the heatsink?
cheers
/Tim
 
Nah, not insulting at all. I didn't check this time because I measured -5v once the new 7905 was installed, but as soon as I read your post I thought maybe the insulator broke down so I had to rush to check. No luck... it's still isolated.

Whatta stinker. ;)

And the thing that gets me is the LM324 isn't blowing this time. I had the game on for an hour or so before and when I checked, the LM324 didn't even get warm.
 
I have the same problem with mine, without a fix. I have not yet added a resistor mainly because it was not needed before, why bandage... but if thats what it takes as a last resort. Anyway looking forward to your findings.
Rick
 
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