MTH Fire Station 30-9102 sound improvement modification

All, first happy to now be a part of the forum and back into the hobby. Yesterday I picked up an MTH fire station 30-9102 from my local dealer and brought it home and was very sad about 2 things, the truck motor to move the fire truck in and out did not work at all and the sound was terrible from the sound module. The motor was an easy fix, bad solder joints on the wiring at the motor, but the sound I did a little reverse engineering on the MTH mainboard and figured out a massive and relatively easy fix for those with this station. This makes a dramatic night and day difference to the sound quality.

The issue is, the sound signal source trace and circuit from the microcontroller has 2 capacitors shunting and distorting the signal to the amplifier before it's even amplified, meaning the amp just takes this now distorted signal and makes it even that much worse. The fix is amazingly simple, remove these 2 surface mount capacitors and enjoy instantly better sound. I'm really struggling why the designer put them there in the first place. I mean I understand filtering, and stray voltage protection (any capacitor to ground on a signal circuit basically shunts high frequency or voltage transients to ground)- however, in an audio circuit, let alone the signal feed into the amp, this was a bad thing.

Anyway, all you need is any soldering iron to just heat both sides of the tiny surface mount cap and since we are removing it, most irons have a long enough tip to touch both ends and then just slide it to the side off the pads and that's it.

A picture says 1k words- so here is my first diagram and post.

MTH Fire station capacitor removal

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Don't get me wrong, the sound is not perfect, but I think it's many times over improved from what you might hear stock on any one of these. Watching other posted videos of this station- you can hear the difference between mine and theirs with everyone really needing to do this mod and MTH take notes on this at the factory. This makes me wonder if the other stations (gas?) use this board with different sounds and have the same issue. It still has that low quality encoded audio- that's just the sound and the chip limitations as implemented, but now without added distortion it's just night and day better. For comparison, listen to a few other unmodded posted videos and see if you can hear the difference. https://www.youtube.com/watch?v=2qxjUHYfTmk and https://www.youtube.com/watch?v=vNJAt9WAEUM 

Yes, I think it makes it slightly less loud volume wise, but to my ear, that was also huge distortion that rattled the building as well, just making it horrible to the point I was really unhappy. Hoping my first post here helps some folks out

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Fire station after mod

Interesting stuff. I notice that the board date is July 2015, so this is a much newer version than the original release, which must have been nearly 20 years ago. I wonder if the original had similar capacitors, though they would have been lead type, not surface mount I imagine. You are pretty clever to have noticed this glitcsh in the board design. Obviously mth's vendor rushed this into production before adequate board testing. 

grj, if you are reading this thread, what's your take on this?

Rod

We are never too old to learn something stupid....

It's hard to tell from the videos as the volume levels were way different, but I filed it away to see for myself, I have one of these that I will probably stick on the layout.

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