St 400 transformer hum is excessive. Fix or problem?


Sep 17, 2013
Picked up a couple st 400 units. Replaced all electrolytics including 2 large 10,000 mfd units. Replaced the 10,000 mfd with 18,000 mfd new caps I got from eBay. Both units have very good reproduction, excellent bass, but have excessive transformer hum. With no signal you can hear the transformer humming. May or may not be in speaker depending on speaker type, but is very annoying. Have I created a problem or just have one not understood. Old tube tech.
It is not clear in your question whether the hum is mechanical and coming from the amps. If so then maybe the transformers has become delaminated and are vibrating. You could try repotting but it may not work.
If the hum is coming from the speakers then you may have a ground loop. If you get the hum without any input cables connected to the amp then try using a 3 to 2 prong ac adapter on the ac cord (dont remember if it has 2 or 3 prong AC). If 2 prong then try grounding the chasis
You replaced the power supply caps but you may need to replace signal caps on the audio amp board.