32X Squeal Fix

Backstory

Everything I do has a little story to go with it.  If you just want to know how I fixed the Squeal issue, skip to "Resolution" below.

I bought my 32X brand new when Sega had given up on the platform and the stores were liquidating them.  I think I paid about $30 for it, then took the free copy of Virtual Fighter out of the box, went across the street to a used game shop that hadn't yet caught on and sold Virtual Fighter for $35.  So essentially I got paid $5 to take the 32X off of the store's hands.

A Tale of 2 32X's

It's probably not a wonder then that I didn't have very high expectations of the 32X, and simply dismissed the video distortion and audio squeal it introduced.  I legitimately thought it worked that way.  At the time I only had a Genesis Model 1, so I misplaced the spacer that it comes with for use with the Genesis Model 2.  About 3 years ago I went to a local used game shop and bought a whole 32X for $25 just to get the stupid spacer so I could use it with a Genesis Model 2.  The newly bought 32X was scratched up and didn't look as nice as my original, so I put the newly bought one away and continued using the one I had bought new.

You Mean It's Not Supposed To Sound Like That?

Connecting the 32X to an OSSC greatly amplified and magnified the video noise and audio squeal to the point where it was damn near unplayable, so I started searching around to see if anyone knew how to improve the situation.  I found a lot of articles and a lot of supposition, but almost nothing in the way of a solution.  One thing stood out, however.  Some users were reporting that their 32X systems didn't add any audio or video noise at all.  That got me thinking, so I pulled out the 32X I had recently bought used and swapped it out for my shiny original, and I couldn't believe it.  There was no discernible audio or video noise at all!

Conjecture

So I had one "good" 32X and one "bad" 32X.  In all of my searches for a solution, three suggestions kept coming up, but there never seemed to be any followup on a solution.

1. Power

It was suggested in a couple of places that insufficient power was a possible cause.  In my case I have been using a "Trio M2" power adapter from www.retrogamecave.com.  I would expect that if the power supply was the problem, the problem would exist with both 32X modules, but it only affected the one.  I did try switching back to the original power bricks for both the Genesis 2 and the 32X, and also alternating, but there was no detectable difference.

2. Capacitors

The next thing that came up in a few places was the fact that the 32X had one particular "Low ESR" capacitor.  ESR is the resistance a capacitor has to being charged.  High ESR can cause odd failures because a capacitor may not charge up until after a delay, or the power supply may not be able to overcome the ESR to charge it at all.  At the time I didn't have an ESR meter, so I just shotgunned the whole module, replacing all of the capacitors with brand new ones, and making sure to use a "Low ESR" capacitor in the appropriate place.  This had zero effect on the squeal or the video distortion.

3. The Inductance Coil "L7".

The lengthiest discussions I came across involved this part.  The service manual hosted at Console5.com describes L7 as "CHOKE COIL 330UH LHL13TB331K".  There were a number of users reporting that they could actually hear the coil physically squeal directly from the console even with the audio output turned off.  This was not the case with me - I only heard the noise and saw the distortion through the AV output.  After reading through several discussions, and at least one user who swapped it out without any improvement, I decided to give it a shot, but also to keep my options open.

32X main board. The original L7 inductance coil circled.

The Plan

In addition to a direct replacement, I decided to try a couple of others to see if the problem was just poor component choice.

Mouser actually sells the exact part described in the service manual.

https://www.mouser.com/ProductDetail/963-LHL13NB331K

In reading its description the thing that really bugged me was that it was described as "unshielded".  It seemed reasonable to me that a shielded inductance coil might yield better results, so I looked for coils with similar attributes - focusing mainly on inductance value, max current and maximum DC resistance.

This one was pretty close:

https://www.mouser.com/ProductDetail/994-RFS1317-334KL

This one was almost exact so I decided to try it even though it was the wrong package type (surface-mount rather than through-hole):

https://www.mouser.com/ProductDetail/81-MBH12282C-331MAP3

Today I finally got around to trying them out.

The first thing I did was to replace the original with the new LHL13TB331K.  It seemed like there may have been some small improvement, but the audio and video distortion were still terrible.

The next one I tried was the 994-RFS1317-334KL.  This was the shielded coil with a slightly lower DC resistance value, but was otherwise rated the same as the original part.  The footprint was too narrow to fit directly so I had to very carefully bend the legs of the coil to fit onto the board.  There was a definite improvement with this one.  The volume of the distortion was cut about in half, but it was definitely higher-pitched.  It was better but still awful.

Resolution

By the time I got to the 81-MBH12282C-331MAP3, my hopes were not very high.  Fortunately the vias had solder pads on the top of the board as well as the bottom so I didn't have to do anything too crazy to attach it.  There's an exposed ground point directly under the spot where the coil sits. This isn't a problem for the through-hole inductors, but would have caused a short on the SMD inductor, so I covered it with a tiny corner of black electrical tape before placing the new coil on top of it. There wasn't a lot of room to maneuver but I managed to get some good solid connections.  When I fired up the 32X for the first time I couldn't believe my eyes and ears.  It was dead quiet - not even a hint of the squeal or visual distortion.  For a moment I thought I may have blown out the audio circuit, but then the game started up clear as a bell.

32X with the 81-MBH12282C-331MAP3 inductance coil installed as a replacement for L7 to fix audio squeal and video distortion.

Now I can obey Admiral Akbar, storm the imperial Death Star, and wipe out enemy fighters without distortion!

But Why?

Someone on the shmups forum challenged me to try to explain why this worked when the parts were technically the same.  My initial thought was that the new part was just "better", but the question made me consider other possibilities.  Before going into this I thought shielding might be the answer, but, then, of course 994-RFS1317-334KL did not eliminate the squeal.  Since it reduced the volume of the squeal, I'm inclined to think the shielding played a part, but it was definitely not the "smoking gun".

Possibility 1: Vibration
In my research I read that physical vibration is a factor with inductance coils.  At least one example corroborating this is the Turbo Express where the coils are covered in gray epoxy - presumably to dampen vibrations.

The shell of the LHL13TB331K and  994-RFS1317-334KL have a kind of skirt at the bottom that makes contact with the board in a circle all the way around the coil. When I first tried replacing the coil with the new LHL13TB331K, I tried to make sure to get full contact with the surface of the PCB, but in fairness, I was just eyeballing it. While it didn't appear to do much good it's possible it wasn't completely flush with the board and was allowing for vibration. With the 994-RFS1317-334KL, the legs were too far apart to fit through the vias for the original coil and putting right-angle bends in them prevented the skirt of the coil from making contact with the board at all. With the 81-MBH12282C-331MAP3 (SMD), basically the entire bottom of the coil is a conductor. I had to put a little triangle of electrical tape over an exposed ground point under the coil to keep it from shorting.  So the entire underside of the coil sits flush against the board with a piece of electrical tape serving as a cushion- it's possible that full contact and the additional cushion is allowing the PCB to absorb all of the the vibration and that's the real difference.

Possibility 2: Inductance Value
The other thing is the inductance variation between the parts. When I measured the original, and the LHL13TB331K replacement, they both registered 0.3mH. It isn't exact, but it's at the low end of tolerance. The 994-RFS1317-334KL measured 0.33mH - exactly as rated, and it _did_ seem to make an improvement. Since my component analyzer doesn't have an easy way to measure SMD parts, I used DMM probes to measure the MBH12282C-331MAP3 and got 0.36mH. I assumed the DMM probes were going to introduce a certain amount of inductance by themselves and that might be throwing the reading off so I dismissed the difference. But since the question was asked, I went back and measured the LHL13TB331K using the same probes and got 0.3mH - meaning that the probe wires weren't throwing the measurement off as I had assumed, and the SMD coil does have a significantly higher inductance value.

The LHL13TB331K is rated at 330uH with a tolerance of 10%, meaning anything from 297uH to 363uH _should_ technically be acceptable, and the MBH12282C-331MAP3 is still within this margin, but in practice at the opposite (high) end compared to the original part.  From what I've been reading about coils, this shouldn't make that big of a difference, but who knows?

So, to recap, although the MBH12282C-331MAP3 has the exact same ratings as the original coil, it's also shielded, has nearly 20% more inductance, and is almost definitely making better physical contact with the PCB.

I have no intention of fussing with that 32X any further now that it's working perfectly, but logically if you wanted to try fixing it without replacing the coil, you could try doing something to dampen the coil by fixing it to the board, like using a soft epoxy to fix the coil to the board.