Space Invaders Board Problems

[ForKChop]

Ordered a logic probe today, we'll see if that helps find the cause of the missing credits.

Unfortunately, the 1N4004 below the 1000uF cap did not fix the sound. I have ~16V at the power supply and off the pins on the sound board; a good connection to the speaker as well. There is a hum when the speaker is connected, but no sounds.

Before, when it was garbage on the screen, there was an occasional sound - note that it was sporadic. It sounded a lot like the explosion sound, but now I'm not getting anything. I suppose it's possible, but how likely is it that all the LM3900s are bad?

 

[EclipseEye]

Hats off to you dude for getting this far, for wanting to learn and more importantly taking the time to learn. No pain no gain!

 

[ForKChop]

Here's where a logic probe comes in handy. You put it on the board where the wiring harness connects - in particular on the input you wish to test, in this case one of the coin up lines. Trip the switch and see if the line momentarily changes logic states.

I'd have to look at the schematics for an SI but for standard JAMMA the pins are held high and when you push the switch they are grounded. If the pins aren't held high then there's a board issue. If they aren't grounding then it's a switch or wiring harness issue. If they are stuck grounded then it could be wiring, switch, or a short in the inputs on the logic board.

They aren't hard to troubleshoot if you have the wondrous little logic probe.

RJ

PS: Get one with audio capabilities so you don't have to keep staring at the LEDs.

Logic probe came in today, pin 3 on the 11 pin connector, which I believe is the coin pin, goes from high to low when the switch is closed and back to high when it's open. That seems like it should be normal behavior. Suggestions?

Hats off to you dude for getting this far, for wanting to learn and more importantly taking the time to learn. No pain no gain!

Thanks for the encouragement, I never realized that Space Invaders was before a lot of the troubleshooting features I took for granted.

 

[channelmanic]

Follow the circuitry!

See where that input goes... find the chip it leads to... and see if the input signal is making to the chip. If it isn't, find out why. If it is, check the chip's outputs. Follow that signal!

 

[ForKChop]

Okay, I have the signal solid through the pin 8 of the 1K SIP resistor just above the 7931 at C2. Past that it's gone, I believe it should be following out of the left side pin 1, marked with the white dot, but no. None of the other pins change state. Pin 2,3,4 are ~0.5K, while 5-9 are 1K. It seems odd that the other pins wouldn't change state, I would think it should fail at the next IC?

Also really glad that I got a probe with audio, as it's bad enough that I have to crane around inside the cab trying to follow the circuit, but at least I can clip a lead on and hear the difference when I walk around to trigger the coin switch.

 

[channelmanic]

I'll have to look at the schematics and see where it goes. That'll be fore another day. It's late here in Texas.

 

[kb0jjn]

Okay, I have the signal solid through the pin 8 of the 1K SIP resistor just above the 7931 at C2. Past that it's gone, I believe it should be following out of the left side pin 1, marked with the white dot, but no. None of the other pins change state. Pin 2,3,4 are ~0.5K, while 5-9 are 1K. It seems odd that the other pins wouldn't change state, I would think it should fail at the next IC?

Also really glad that I got a probe with audio, as it's bad enough that I have to crane around inside the cab trying to follow the circuit, but at least I can clip a lead on and hear the difference when I walk around to trigger the coin switch.

Any progress on this issue?

 

[ForKChop]

None yet, I was busy yesterday and didn't even look at it. I did get some parts from digikey so I'll try to fix the sound today.

 

[ForKChop]

Okay, looked at the coin loop in the schematics. It should enter at pin 9 on the 741 (I called it 7931) at C2 and exit at pin 8. Pin 8 does not change state when pin 9 does. It's low the whole time, and sounds like garbage. By that I mean that it doesn't create a clean sound like a constant high or low state but it also doesn't seem to be creating a pattern either.

Am I correct in guessing the next step is to replace that IC?

Thanks
Vanessa

 

[kb0jjn]

Okay, looked at the coin loop in the schematics. It should enter at pin 9 on the 741 (I called it 7931) at C2 and exit at pin 8. Pin 8 does not change state when pin 9 does. It's low the whole time, and sounds like garbage. By that I mean that it doesn't create a clean sound like a constant high or low state but it also doesn't seem to be creating a pattern either.

Am I correct in guessing the next step is to replace that IC?

Thanks
Vanessa

I don't usually advise this but sometimes it is worth it.

You may want to temporary cut the connection to/from pin 8 and test that pin (for output). Sometimes the next stop on the road will be bringinig the output pin(8) down. If you do cut a trace or connection it is usually best to do it near the feed through and small enough that a small amount of solder will replace the connection. I have read this and cannot remember where but I think it was the Atari Book.

This will tell you if that chip is bad or if something else in the connection is the bad guy.

 

[ForKChop]

I cut a trace right past the solder point on pin 8 and made sure there was no connectivity. Pin 8 now has no response to anything, it's neither high nor low, just dead. That implicates that chip, right?

On that note, these don't seem to be as common to find. Anyone have a good source? Any known good substitutes?

 

[kb0jjn]

I cut a trace right past the solder point on pin 8 and made sure there was no connectivity. Pin 8 now has no response to anything, it's neither high nor low, just dead. That implicates that chip, right?

On that note, these don't seem to be as common to find. Anyone have a good source? Any known good substitutes?

Bad chip. When you replace the chip you should see that go low when you add a credit.

I think channelmaniac has them but not shure.

http://www.arcadecomponents.com/

and MikeArcade

http://www.mikesarcade.com/

 

[G-P-E]

Okay, looked at the coin loop in the schematics. It should enter at pin 9 on the 741 (I called it 7931) at C2 and exit at pin 8. Pin 8 does not change state when pin 9 does....

Thanks
Vanessa

Alright. I'm kinda confused on this one. You said this IC is a 741? A 741 is typically an 8-pin op-amp with nothing on pin 8. Or is there just a digit missing from the posting?

-Ed

 

[ForKChop]

Alright. I'm kinda confused on this one. You said this IC is a 741? A 741 is typically an 8-pin op-amp with nothing on pin 8. Or is there just a digit missing from the posting?

-Ed

It's a 7414, typo. Sorry

 

[kb0jjn]

Bad chip. When you replace the chip you should see that go low when you add a credit.

I think channelmaniac has them but not shure.

http://www.arcadecomponents.com/

and MikeArcade

http://www.mikesarcade.com/

The input should go low and the output shoud go high.

Pin 9 low = pin 8 high

 

[MonsterBash]

FYI Bob Roberts has the 7414 (or a equivalent replacement)...I didn't see them on is site but asked for them.

I cut a trace right past the solder point on pin 8 and made sure there was no connectivity. Pin 8 now has no response to anything, it's neither high nor low, just dead. That implicates that chip, right?

On that note, these don't seem to be as common to find. Anyone have a good source? Any known good substitutes?

 

[EclipseEye]

FYI Bob Roberts has the 7414 (or a equivalent replacement)...I didn't see them on is site but asked for them.

I take it a 74LS14 is fine to use as a replacement?

 

[kb0jjn]

I take it a 74LS14 is fine to use as a replacement?

7414, 74ls14, and 74f14.


http://www.zimmers.net/anonftp/pub/cbm/schematics/drives/new/1571/manual/14.html

Don't forget to repair the trace when you are done.

 

[ForKChop]

I got the 74LS74 yesterday and popped it in today. It works!

Coins up and most sounds working. As they should be since I replaced a lot of the audio stuff while waiting to figure out the coin up problem. The shooting sound is just a high pitched squeal and when you die there's an audible dip in the sound but no new sound. From what I'm reading it looks like both of those could be the 4006, or possibly the 4030 for the death sound.

Next step cap kit, this monitor is dim.

Side note: it's missing the L-shaped board supports, is there an existing source for those or should I rig something up?

Thanks to all for your help, it feels great to bring this one back from the other side!

 

[channelmanic]

Happy to help where I can!