My dad handed me another item to repair this weekend, a Roland GP-100 Guitar Processor, circa mid-1990’s. He reported that it had been acting strangely, so he replaced the backup battery in the unit, but after that, it wouldn’t boot (screen would light, but then it wouldn’t show anything, or respond to controls). To me, symptoms like this always scream power supply problems, and given the age of the unit, to me that meant capacitors. So I agreed to repair it, and brought it home. After confirming the problem myself (you never know how someone’s AC power can influence issues like this), I started looking quickly for the service manual, which I found quickly at this wonderful site! And quickly found the schematic for the power section of the board. Holy crap, I quite literally explained, they put 25V capacitors on the 21V rails. That’s a pretty small margin, and bound to lead to excessive heating1, and while the ones on the 7V rail are at least better (15V), it’s still a bit closer than one likes to see2. Throw in there’s no ventilation for the enclosure, and these things are bound to be bad… let’s take a look.
- Be warned, I’m actually going to include some pictures in this repair [↩]
- Derating capacitors according to the US Navy. TL;DR: at 75C operating temp for a 105°C Aluminum electrolytic capacitor, the max voltage should be 70% of it’s rated value. 21V across a 25V capacitor is 84%, and 7V across a 15V capacitor is ~46%, so while the 15V ones were okay, the 25V ones were WAY under rated. [↩]