There is a sticker on the underside that covers two screws. This label can be heated and easily removed. I used a reflow station set to to its lowest setting.
There are also some snaps that must be pried apart somehow. I was able to use a guitar pick.
Once inside, there's a nicely labeled PCB with a "BD" section.
The board does not lend itself to being reverse engineered, despite the labeling. The solder resist makes it difficult to see the traces. It also makes it near impossible to shine light through the board. Additionally the solder resist covers the vias, making some signals harder to follow.
Despite this, it wasn't too hard to figure out. It is extremely similar to the TR-606 schematic, including component values.
I redrew the TT's kick similarly, until I found the difference. One of the two oscillators is mostly the same, the other has drastically different values.
It turns out that they roughly doubled the values for the leftmost oscillator's resistors. This increases the Q-factor, affecting the decay.
They also put a digital pot in parallel with R27 to give a kind of tuning control. The digital pot is a 10K that, when in parallel with R27's 6.8K, gives an equivalent 4K down to 500Ω. This is in series with R25, yielding a total equivalent range of 1.18K to 4.68K. Our original TR value of 3.3k falls inside this range, so it doesn't strictly need to be changed.
R28 and R90 sum to 1.24M instead of 680K. Something must be changed here. There are multiple ways to come up with a value around 680k. 1.5M in parallel would work, but I opted to change R28 to a 470K. This gives 710K and I find it suits my taste.
Here we can see a before and after of the kick waveform. Also included is a real TR-606 at the bottom. The TT is much tighter now and the transient is slightly louder than before.
