System 1 (MarbleMadness) audio from alternative universe

Altan

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System 1 (MarbleMadness) audio from alternative universe

Over a year ago, I purchased an "untested" Atari System 1 (TTL) main board from eBay. It was before I even had a working Marble Madness.

Tonight I decided to try it out... and... drum roll... it works!

Well, mostly.

The audio is funky. You can hear all the normal audio. But it sounds like it's from an alternative funky universe. It has some static, it is has more bass, the music and effects don't seem mixed right, and it just sounds weird.

I'll pull out the System 1 schematic later, but I was wondering if anyone has run into this?

... Altan
 
Well, generally speaking it's probably the LM324's or the caps in that area, or even a resistor.

Jeesh, that's just about everything.

I forgot to write that the audio was really loud... I needed to turn down the pot on the AR-III board.

Nothing wrong with the AR-III board... I put my original (known good) Sys 1 PCB back and the audio was fine (after turning it back up).

I'll see if I can tell whether it's the left channel, the right channel, or both. That should provide some direction (I hope).

... Altan
 
Took the brute force approach and replaced the 3 LM324's. Good news? I didn't break anything. Bad news? It has the same behavior.

At least one potential cause has been eliminated!

... Altan
 
Just verified the voltage going into all 3 LM324s... Each is about +/- 15v. Reading between pins 4 and 11 I get ~30 volts.

So the voltage is right. This doesn't surprise me since the 15v seems to come from the AR-III board. But there are a bunch of resistors/caps in the path on the main board...

Wondering if it isn't a problem in the amp section but rather a problem in the "sound effect" section...

... Altan
 
As you can tell, I'm reporting as I'm doing... I'd welcome any comments.

I thought that, perhaps, the Pokey was bad. So I removed it and booted the game without it. Oddly, everything worked EXACTLY the same. It would appear Marble Madness doesn't even use the Pokey. Possible? It seems that way. Self test was quite happy without it... it did the music test and the sound effect test without the Pokey (although still distorted and too loud).

Anyway, it's not the Pokey. Or perhaps it is the pokey... and it's totally dead and somehow messing with the audio. Let me see if I can swap it.

... Altan
 
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Ok, so MM does use the Pokey. I swapped the Pokey chip, putting a known working on in the questionable board. I was able to hear some parts of the audio test that I couldn't hear previously... but the sound was still distorted.

Interestingly, I don't think the pokey sounds were distorted. But it's hard to tell without a sample from a working board near by.

So it appears I have a bad Pokey, but that's not the entire problem. Still hunting down the cause of the distortion.

BTW, not having a Pokey installed WILL make your audio louder. The output of the pokey (pin 37, "pks") goes to the op amp where it is compared to +5v. If it is anything other than +5, this will cause the amp circuit to increase the volume on all the audio as this gets mixed in with the other music/sounds.

... Altan
 
There hasn't been much interest in this thread, but I'm thinking there might be some lurkers out there :) I hope this is helpful to someone. I'll write this whole investigation up more clearly with more details on my web site in a bit.

Good news! It's fixed! Or at least I know what the problems are. I've got bad YM3012. This is the D/A converter that connects to the YM2151 audio generator. While it clearly didn't just up and die, it's certainly doing it's job quite poorly.

After replacing the 324s (op amps) and digging around into the Pokey (which, per earlier, i found was also bad but not the cause of the distortion), I decided to see if both channels were messed up or only 1 channel. I was pretty sure both were messed up but wasn't 100% sure.

I cut a lead from cap C111. This stopped all left channel audio (from the YM2151). The right channel still sounded bad. I then cut a lead from cap C110, stopping the right audio. I then connected C111 back (temporarily) and verified that the right channel was bad also. It was --- so both channels are problematic.

With this additional info, I started looking upstream. The first common (to left and right audio) part upstream is the YM3012. Happily, it is socketed. I swapped it with the 3012 from my working board and the audio was perfect!

So I need to do the following to finish up

1) replace C110 and C110 since I cut them
2) get a new YM3012, install it
3) get a new Pokey, install it

I'll start looking, but anyone know sources for the YM3012 and the Pokey?

... Altan
 
I placed an order from arcadechips.com today.

I've written up this investigation with some more details on my web site. If anyone is interested, check out

http://www.aaarpinball.com/MM/MM.htm#System1Audio

I even drew a picture by hand! Makes it more 80's-like :)

img023.jpg


... Altan
 
Final closure...

Got the YM3012 and Pokey form Arcadechips... Installed them... both Atari System 1 boards are working great.

... Altan
 
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