A "You're not as smart as you thought you were" moment (Discs of TRON audio hum)

i86time

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A "You're not as smart as you thought you were" moment (Discs of TRON audio hum)

I purcahsed 30FathomDave's (and a few other's here) UDOT with the speech mod and I've thoroughly been enjoying it. The only problem was the audio. There's a sweet spot, about 30% volume, where there is no hum, but the audio is too quiet for me to enjoy. If I turn it up, I get annoying audio hum. But if I also turn it down to play quietly the hum also increases in volume and is louder than the audio. There was a switching PS in it to drive the -5V needed for the Squawk n Talk. After reading all the posts here about how switcher + MCR boards= hum, I figured I'd be a genius and pull the switcher and use an original -5V auxillary board. So I wire it all up and now there's a constant hum at all levels. I pulled the -5V board out of circuit to check and when I fired it up, guess what I found. The loud hum was gone but the sweet spot in the audio was still there. It wasn't caused by the switcher at all but by something else in the system. If I had just pulled the connectors from the S&T earlier, I'd have probably realized that.

So, how do I get rid of this hum? I did notice that if I only pull the +5 unreg from the power chassis that goes into the -5V board, the hum does subsude a bit. Could that be anything? I did have to use piggyback connectors on the fuse clips, but everything spec'd out.
 
You could be picking up an AC harmonic from a bad ballast in a florescent tube. I'd try disconnecting (don't just pull the lamp - the ballast still comes online) each ballast to see if you can find the source (if it is a ballast, and there is more than one).
 
I'll give that a shot tomorrow. I'm still perplexed as to why the hum got worse with the switcher taken out. I read in another thread that the +12V being too high causes hum, but mine was actually a bit low, 11.6 @ the power board. I upped to a flat 12V and no change in the hum.
 
No go on the light fixtures. I unplugged them all and the hum was still there.

I'm pretty sure I've determined the hum is coming from the Squawk & Talk circuit. With all the pulling and re-seating of connectors, I broke one of the leads for the audio out of the S&T to the jump on point on the SSIO. When fixing it I noticed that there were two caps, 1uF & 2uF, connected to two resistors, both 33ohm. Looking the S&T threads and the schematics, there's only supposed to be a single 1uF cap and one 13K ohm reistor. The 33ohm reisistors are the ones already on the SSIO that the audio is supposed to jump on to, not the ones in the S&T speech hack cable. I'm hoping that since 33<<13K that the increased resistance will further supress the noise and hopefully eliminate the hum. I just need to order the parts.

But even if that works, I still need to find the source of the low volume hum....
 
Well, I got rid of the hum, to a point. In the schematics, it looks like the S&T J2-7/9 is hooked up to the cap which is hooked up to the resistor then to the jump on points. The original hook up was reversed with the resistor first, but in these threads:

http://forums.arcade-museum.com/showthread.php?t=167126

they've been hooked up either way. So I went by the schematics and instead of having each jump on line with a cap and resistor, I combined them together, also like the threads mentioned above. Well, the DOT didn't like that. With both jump on ponts connected, I sometimes got speech and sometimes didn't, and when I did it was fuzzy. All other sounds worked great. I found that if I removed the conector from R220 (didn't matter which of the jump on connectors I used), the speech was good, but only came out of 1 speaker. The excessive hum was gone- down to the typical MCR level, so that was a plus. The other sounds were also good out of 1 speaker but were fuzzy from the other. When both connectors were removed, sound was fine out of both channels, but of course no speech. I went back and forth testing and finally decided to change the circuit back to a resistor/cap for each separate jump on line. This cleared up the fuzzy sound problems and I have speech in both speakers, but the original problem with the hum is still there. It's not as bad as it was with the 33ohm resistors, but it's still there. I'm filing this one under 'Done for now.'
 
old thread but..

Go back to the tutorial link in the last post. I updated it with a ton of info and also how to use a pinball Squawk & Talk in EDOT/DOT.
 
yeah I was a little bent that I was 2nd in line for 30fathomdaves udot, lol. Glad a Cali guy got it though. I would like to be on your short list for trade/sale consideration if it ever occurs. sweet game!
 
I'm certain that whatever hum this has now is just typical MCR hum, it's the same with my EDOT. Still odd about the crazy sound coming out of the one speaker when the cap/resistor order was switched.
 
hum

If your squawk & talk is using the pigtail to the 2 resistors instead of 2 separate lines to the SSIO that's causing noise and also keeping your sound from being in stereo.

Put the game into test mode and do a channel test - 1,2,3 are left speaker and 4,5,6 are right. If you get 1-6 in only one or both speakers that's a give away you have it hooked up wrong.

It should be like this from J2 of the squawk & talk to the SSIO:

Pin 7 - Wire to a 13k resistor (12k followed by 1k is fine) then to Neg of a 1uf 50v cap.
Then attach + of the cap goes to SSIO R220 (separate lines for stereo sound)

Pin 9 - Wire to a 13k resistor (12k followed by 1k is fine) then to Neg of a 1uf 50v cap.
Then attach + of the cap goes to SSIO R209 (separate lines for stereo sound)
 
Yeah, that's basically how I have it. The only difference is I have a single lead from J2-9 and from there it's separated into 2x the resistors-cap-SSIO leads. It still gives me stereo separation (rear sounds from right speaker, front from left, speech in both). It was only when I hooked it up wrong (mentioned in post #5), that's when everything was funky.
 
Squawk

Is your Squawk & Talk the model for an EDOT or a pinball? Mine is a pinball one and It only works properly with separate lines. If I pigtail from pin 9 then channels 1-6 are in both speakers and there's noise on the line. Running separate lines with pin 7 & 9 fixed it.

Interesting that you get 1-3 on left and 4-6 on right with just the one line. I wonder what you would get if you ran the second line?


There are several Squawk & Talk revisions...
 
Yeah, it's a MCR S&T. I'd have to check the wiring to confirm, but in the EDOT, the lines from the S&T output the same signal to each of the amps. But the non-speech sounds are fed to the amps separately from the SSIO to produce the front and rear separation. Since the UDOT has only 1 amp and the jumper/ROM mod tells it that it's an EDOT, it causes the separation, I guess. My UDOT came with the S&T installed, so I don't know if a standard UDOT separates the channels like that.
 
Interesting

Will have to investigate more to find out of the difference lies in the various Squawk & Talk revisions vs the SSIO with or without panning...

I have a DOT & EDOT here so I will look into this but won't be for a few weeks until I have room to pull the EDOT from the wall...
 
Yes, if you can check your UDOT w/o the S&T installed, that would help. My UDOT has the freeplay/environmental ROMs installed, so just removing the wires from SSIO connector won't tell anything definitive since it *thinks* it's an EDOT.
 
I was working on a spare UDOT boardset I got (who'd have thunk the pull and reseat chip method would actually work?!) and I checked the sound. It is the same as my mod'd UDOT w/ speech board, channels 1,2,3 on left, 4,5,6 on right? I did notice 2 cool things:
1) With that UDOT board and no SnT, the MCR hum is nearly non-existant. During the game, I could not even hear it.
2) When Sark throws the energy pellets/chasers, the lame 2 note music in the background completely disappears and the gameplay is super quite. It's quite eerie, and makes me wish they did away with the music for the whole game.
 
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