Troubleshooting the Bass Pad

I purchased this Drumtraks for a few hundred dollars below the usual price because of its condition. Upon powering on and plugging in my headphones I noticed that the bass drum pad (channel 1) was not responsive. 

I wanted to make sure that it wasn't just a bad switch so I shorted the pads hoping for a sound, no luck. I started from the output (FINAL MIX). 

I started probing that very circuit (shown to the right). I traced from audio out following the signal path backwards to CH1-D. No luck picking up the signal here, so it must be lost somewhere in schematic D. 

Chasing the Problem

After ruling out a bad pad switch and the circuit above leading the the audio output jack, I moved to the isolated CH1 output. Starting from the beginning now I followed the signal upstream until I lost it just after the CEM3320 (a famous VCF chip).

The Temporary Fix

The CEM3320 is bad. I verified this by comparing its outputs to an identical chip used for the Toms on CH3. I've found that bypassing the VCF entirely produces the proper bass sound although it sounds unfiltered. The new CEM3320 IC was installed and the pad works beautifully.