The audio issue of AES revision 3-4

[MKL]

A few years ago, after buying a system from me, the customer (who considered himself an audiophile) complained that he could hear a buzzing noise coming from his TV speakers as well as from the headphone jack. He also mentioned that he had a low serial system that didn't have that issue at all.

Since I had another (unmodded) system with the same PCB revision (3-4), I tested it immediately and much to my surprise it was having the same issues described by the guy. I couldn't believe I had never noticed it because once it was pointed out to me it was impossible not to hear it.

To get a better grasp of the problem I ran tests with as basic a setup as possible: I ran the system without hooking up video, only with headphones plugged in and another system (revision 3-6) running next to it, also with only headphones in and only the 3-4 was having the buzzing issue. Both system were powered by the same power supply (without step-down transformer) so the PSU was ruled out as a possible cause.

When the guy sent the system back to me along with his other "silent" system (that turned out to be the very first PCB type NEO-AES) the tests confirmed that 3-4 had the issue and that NEO-AES was buzz-free just like 3-6. The thing was all the more puzzling as NEO-AES and 3-4, despite being different revisions, have the same audio circuit (while on revisions 3-5 and 3-6 it's partly different).

I was starting to put the blame on the different PCB layout of the 3-4 causing interference when I noticed the difference between the NEO-AES and 3-4 circuits: each audio IC (DAC, op-amps, headphone amplifier) on the NEO-AES has a decoupling (bypass) cap nearby (close to the VCC and GND leads) just like all the other ICs on the board which is what you would normally expect to see:

http://img404.imageshack.us/img404/9035/27727951.jpg

The 3-4 system doesn't have them though. Not a single bypass cap in sight:

http://img404.imageshack.us/img404/116/30702199.jpg

After adding 0.1uF (104 marking) caps across the 5V and ground leads of each IC (on the bottom of the board) the issue was gone:

The cap for the DAC (YM3016) seems to make the most difference.

According to the datasheet, the bypass cap recommended for the headphone amplifier IC is a 100uF electrolytic cap.
The value for this cap on MVS boards is 470uF/16V. I used 100uF/35V:

Of course you will have noticed the 1000pF (marking 102K) caps soldered to pin 1 and 16 of the IC. Those are other components that SNK left out of the headphone circuit on AES boards but they're supposed to be there according to the datasheet (they are part of a low-pass filter on the inputs) and are not missing from MVS boards:

http://img189.imageshack.us/img189/4275/46690809.jpg

A further example of the lack of attention in designing the audio circuit on this revision is the incorrect placement of two polyester caps in the L channel path:

http://img339.imageshack.us/img339/9559/84792566.jpg

To rectify things desolder and swap them.

How about the other revisions? Will 3-3 have the same issue as 3-4? I didn't have the chance to test one but judging by the lack of bypass caps for the audio ICs it's safe to assume it has the issue too:

I mentioned above that revision 3-6 is buzz-free. Let's take a look at the audio circuit of this board:

http://img683.imageshack.us/img683/8319/48387903.jpg

In this case the caps used for decoupling purposes are electrolytic instead of the usual multilayer ceramic ones. They're the 10uF caps numbered 1, 3 and 9 in the previous pics.

Revision 3-5 is almost identical to 3-6 but guess what, the bypass cap for the DAC (number 1 on 3-6) is missing:

I haven't tested a 3-5 system so I can't tell if the missing cap makes any difference.

It may be worth mentioning here that the headphone out on revision 3-6 has the R and L channels reversed. I had noticed this years ago and wasn't the only one:

http://www.neo-geo.com/forums/showthread.php?t=94456

Since then I have found why. It's another mistake of good ol' SNK designers. Apparently they realized it and made a correction on later boards by moving a couple of resistors:

 

[Xian Xi]

Wow, awesome info dude. Thanks a bunch for taking the time to do this.

 

[Anselm]

Very cool. This deserves a sticky or wiki entry. I'm sure this is going to help a lot of Neo Geo owners who didn't have a fix for this. Awesome work.

~Anselm~

 

[monouchi]

Yeah, thanks for that awesome post.
Great info.

 

[monouchi]

After adding 0.1uF (104 marking) caps across the 5V and ground leads of each IC (on the bottom of the board) the issue was gone:

So to fix it, this is what needs to be done.
Or should I add the cap for the DAC (YM3016) too?

Also, where can one find these caps and what voltage on them should it be?

Thanks.

 

[MKL]

So to fix it, this is what needs to be done.
Or should I add the cap for the DAC (YM3016) too?

Yes, that's what needs to be done and it's enough to get rid of the issue:

http://img404.imageshack.us/img404/149/31157064.jpg

The DAC is in the upper right part of the pic and it has a 0.1uF cap soldered across pin 1 and 2.

In the blank space there is the 4558 op-amp for the mono output on the A/V port. You can put a cap on that too while you're at it (although I seem to understand you're not using the mono out).

Also, where can one find these caps and what voltage on them should it be?

If you're referring to the electrolytic cap in this pic, that's on the headphone amp 2066D and it's 100uF/35V. The voltage rating isn't important here, 16-35V is OK and the size won't be excessive (on MVS boards it's 470uF/16V). But if you don't have one handy, a 0.1uF caramic cap (as shown in the first pic) will be enough: it's what you find on the buzz-free NEO-AES revision:

http://img571.imageshack.us/img571/7678/78002479.jpg

Any electronics store will have these capacitors. If you can't find them locally you have plenty of online options, for instance:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=490-5401-ND
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=P5551-ND

Note that electrolytic caps are polarized so you need to solder the positive lead to the 5V pin of the IC and the negative lead to the ground pin. For ceramic caps it doesn't matter.

 

[JMKurtz]

Great info MKL!

 

[monouchi]

Super thanks MKL.

 

[slugger_dan]

great post. My AES is not 3-4 but I always use the headphones and a buzz like that would be extremely irritating if I had to put up with it. will keep a tab on this incase I ever get another AES..

 

[werejag]

3-5 system

where do i solder on the missing cap?

 

[MKL]

3-5 system

where do i solder on the missing cap?

To pin 1 (VCC) and 2 (GND) of the YM3016:

http://img707.imageshack.us/img707/282/63104368.jpg

 

[sonofx51]

I can't believe how much neo info you have stored in your head MLK. Thank you for always sharing it, I've learned quite a bit from your posts in searching the forums for different mods.

 

[doggydodomi]

Ahhh...for audiophile neo owners...perhap, they should upgrade all the caps to Audience Auricap and the electrolytic to Muse or blackgate or whatever $40/cap they can find..ha.

 

[Xian Xi]

It may be worth mentioning here that the headphone out on revision 3-6 has the R and L channels reversed. I had noticed this years ago and wasn't the only one:

http://www.neo-geo.com/forums/showthread.php?t=94456

Since then I have found why. It's another mistake of good ol' SNK designers. Apparently they realized it and made a correction on later boards by moving a couple of resistors:

Hey MKL,

So is it only the 3-6 that is reversed and not the 3-5 as well?

 

[MKL]

3-5 is ok

 

[DaytimeDreamer]

I could swear i could hear some buzzing through the headphones jack of my AES when i connected it to my stereo speakers. Gonna open it up and see what reveision it is. Its a jap system near the 50K serial number.

However I aint too brave to do all this myself. Can I send you the board if some work needs to be done MKL?

 

[Xian Xi]

3-5 is ok

Cool. I think I modded one that was reversed, in fact I think it was the one that I made the tutorial with which is why the channels were switched. Played mind games with me.

 

[MKL]

I could swear i could hear some buzzing through the headphones jack of my AES when i connected it to my stereo speakers. Gonna open it up and see what reveision it is. Its a jap system near the 50K serial number.

However I aint too brave to do all this myself. Can I send you the board if some work needs to be done MKL?

Yes. 50K can be expected to be either 3-3 or 3-4 and both are supposed to have the buzz issue.

Cool. I think I modded one that was reversed, in fact I think it was the one that I made the tutorial with which is why the channels were switched. Played mind games with me.

I've posted pics of 3-5 and 3-6 so you can follow the traces from the DAC through to the headphone jack and see that they don't go to the same contacts on the jack: 3-5 is correct, 3-6 is reversed. Since this is due to a PCB layout mistake, I don't see how 3-5 could be reversed.

 

[Xian Xi]

I was referring to a 3-6 that was reversed not the 3-5.

 

[MKL]

I was referring to a 3-6 that was reversed not the 3-5.

Ah, OK. Well, a corrected 3-6 is easy to spot as there are a couple of resistors that are visibly messed with. If it looks stock it's reversed.

 

[WolfHound]

To pin 1 (VCC) and 2 (GND) of the YM3016:

http://img707.imageshack.us/img707/282/63104368.jpg

got a Nichicon 10uF 16V capacitor soldered to positive+ on pin 1, negative- pin2 and it worksss removing the high pitch buzzing sound on my AES3-5.

Thanks to MKL for the sound improvement

 

[Martyr]

I know this is an older thread, but it is worth a try.

My Neo Geo AES has the 3-4 revision board and makes a buzzing noise, but not always.
Could this be fixed with adding the caps or is it a ground issue?

I use a RGB cable with the headphone jack for stereo sound.

 

[Katsujinken]

I just got an AES from a vintage game store downtown. I had them test it, it worked fine there and I was really careful with it while getting it home but now that I've finally had a chance to fire it up, it makes this buzzing noise and it's SO loud, it's awful! I tried the headphone jack and it's a little more bearable but it's not a problem with the TV because even completely disconnecting the composite cable from the back of the AES doesn't make it stop while I continue to listen through headphones.

This is terrible, I was so excited to finally own one and I don't know anything about soldering or circuit boards, that's why I got the AES and if it was fine at the store I don't think they'd take it back and even if they did, I don't know when or ever I'd see another as I hadn't before. What do I do? It seems like a grounding problem to me so is there anything simple to stop or at least reduce the problem? I don't like the idea of sinking another 100 bucks for a professional just to get it working after all this.

 

[Xian Xi]

I just got an AES from a vintage game store downtown. I had them test it, it worked fine there and I was really careful with it while getting it home but now that I've finally had a chance to fire it up, it makes this buzzing noise and it's SO loud, it's awful! I tried the headphone jack and it's a little more bearable but it's not a problem with the TV because even completely disconnecting the composite cable from the back of the AES doesn't make it stop while I continue to listen through headphones.

This is terrible, I was so excited to finally own one and I don't know anything about soldering or circuit boards, that's why I got the AES and if it was fine at the store I don't think they'd take it back and even if they did, I don't know when or ever I'd see another as I hadn't before. What do I do? It seems like a grounding problem to me so is there anything simple to stop or at least reduce the problem? I don't like the idea of sinking another 100 bucks for a professional just to get it working after all this.

Upgrade the video output and have the person who does it fix the audio, too.

 

[Katsujinken]

Yeah that's as good a suggestion as any, thanks. It just kills my buzz that I can't invite people to play a new centerpiece of my gaming collection because the music is barely audible overtop until I deal with this added hassle. So can anyone who knows about electronic repair do this fairly easily? It's a US model AES with a serial number just below 29 000. Which design does that mean this one uses so I can tell them what parts they may need?