Embedded Files
March 2025
At first glance
The pads for this Drumtraks were also contaminated more than my last repair. After cleaning the pads and powering on, each pad carried a static sound when hit and some component sounded digitized.
Bass drum pad sounds more like a cowbell. Likely because of an issue in the filters or DAC.
Snare and Rim pad both sound very "digital" and could share the same issue as the Bass pad.
Tom drum pads do not work. When hit a very faint static is heard.
Individual channel volume adjustments do not work. When the volume knob is low the pad's sound is slightly altered. At the same time, the seven segment display indicates that the volume is changing.
Percussion 1 & 2 pads carry a static sound when hit. They also sound "digital". This may be an issue with EPROMs since swapping Percussion 1 & 2 fixed this.
Troubleshooting Channel Volume Control
So I did spend some hours trying to figure out why the Bass drum pad sounded the way it did but I didn't see any more clues other than excessive noisiness in the signal chain. I took a step back and began with the volume control issue.
The Drumtraks has a master volume control and a per-instrument volume control. The per-instrument control operates nearly identical as the per-instrument tuning which aided in the debugging efforts since the tuning control works. By holding down any single pad and turning either knob, the sound can be tuned up/down or its volume can be controlled.
I began by opening the schematics and found the instrument volume and tuning portion shown below just after the O84 buffers. I picked the Bass drum pad (channel 1) and measured both outputs on my scope. I only knew that this was a control voltage which should not look like the square wave I was seeing on the volume output of channel 1.
For comparison the tuning CV at the same place for channel 1 was a voltage from 1.4V to 3.6V that would increase or decreased as I turned the tune knob. To be extra sure the square wave was a fluke I swapped IC U232 for U228 and saw that now the volume control voltage looks like what the tuning voltage was! Before this swap I did follow the signal back to make sure the other side of U232 & U228 were getting the same signals which they were.
So that's it, now the volume control works and the tuning hardly does anything. I immediately ordered a replacement CD4051BE and wrapped it up.
Google Sites
Report abuse