Hi guys. I'm hopeful you will find the following information useful to fix the distorted audio problems. I have repaired 2 DCS boards (both from TOM machines) with the same symptoms: Crackling and distortion.
Specifically, what I discovered on both of them was a severe imbalance in the +/- voltage rails. You can check the DC voltages at C18 (+) and C19 (-) and the magnitudes should be relatively close to each other (within 0.5v). Note that when measuring the negative supply rail you will be measuring between ground and the negative side of the capacitor as the positive side of the capacitor is tied to ground. Typically what I have found with a misbehaving card is a significant difference in the rail voltages. As you can see in the image below, the positive rail voltage is ~13.5v with the negative rail at ~-5.6v. They should be balanced.
These rail voltages supply the power amplifier ICs directly. This can cause one half of the waveform to "clip" prematurely (in this case, the negative side) which results in very distorted audio on loud passages. However, the rail voltages are also regulated to +/- 5v at U25 and U26 to supply the TL084 op amps. The 78L05 (+) and 79L05 (-) regulators don't do well when the input voltages get close to the regulated value (+/- 5v in this case). In fact, these regulators will drop out altogether when the input voltages get at or below +/- 5v (which is not uncommon in these boards from my experience). An imbalanced supply to the op amps can cause significant distortion.
In the case of the board in this example, I was powering it via a test setup, using 17v AC from a transformer I had laying around.
The fix, fortunately, is a simple one. Adding a 470 ohm, 1/2 watt bleeder resistor to the terminals across C20 and C21 (the large filter capacitors), provides a continuous load (~40mA) to keep the rails balanced.
As you can see in the image below, the magnitude of the rail voltages after the modification are within 0.4v. This modification removed the distortion in both cases.
One other thing to check while you have the board out. I have seen on one occasion, where electrolyte from a leaky capacitor had somehow worked it's way to the bottom side of the board and started arcing between an adjacent PC board trace and the terminal of C19 (circled in the picture below in red). This caused a lot of popping and cracking as the arcing occurred, and extremely erratic rail supply voltages (even after the resistor modification). Simply cleaning the electrolyte and the area impacted by the arcing fixed the remaining problem on that board.
I suspect that there are a number of components (capacitors, resistors, op amps, etc.) that, over time, drift slightly from their nominal values and eventually magnify the impact of the imbalance. I have had excellent success with this simple modification.
I sincerely hope this information is useful and you are able to fix your sound board and get back to a fully working machine.
Best regards,
Jeff