Yet another Roland TR-909 repair. This here came in with a strange behaviour. When you turned it on, nothing happens over 3-5 minutes. Then suddenly she came to live, indroducing herself with a crash/cymbal sound and then turns off again. This procedure repeated like a loop. And everytime the loop got shorter, this sequence faster. After a while, let´s say 10 minutes, the TR-909 woke up and played normal. So first thing to check was the power supply including the reset line. The 5V rails just came slowly on. Looked like the charge curve of a capacitor.
And yes. The main 5V filter cap should be 2200uF but measured only 47uF and high ESR. I replaced that cap and she came back to life. Here’s the bad guy.
While I am in here I tought it’s a good idea to replace all the electrolytics in the psu. I also changed the input voltage to 240V. Like always: I’m documenting repairs for my reference and yours. I do synth restorations and repair with a specialty in vintage synthesizers, drum machines and other vintage gear, based in Düsseldorf, GERMANY.