View Full Version : 4270 Hum - AC Power Cap?

12-27-2009, 02:03 PM
First off, I'm a newbie and not sure if I'll fix this problem myself, but wanted to post in case its an easy fix.
Got a 4270 that I just de-oxed and replaced the lamps. Switches are clean and it looks great, but there is a hum with the volume all the way down. Doesn't matter which input source is selected. Have tried it in several outlets around the house.
Someone suggested that it's an AC cap on the power supply (its a musician friend of mine who said they used to have this problem with Fender guitar amps all the time).
Any thoughts/suggestions? If it's a simple solder job I can handle it. If it involves digging deep into this thing (its VERY tight in there), then I think I'll send it out.

12-27-2009, 11:17 PM
The 4270 should have two filter caps, the big ones next to the power transformer. Make sure you get the polarity right if you replace them. The filters can be a pain to desolder, they are in a tight spot, lots of wires you can melt with an unsteady soldering iron. While you are in there, check all the grounds in the receiver. All the boards have a little metal strap that is screwed to the chassis. Check all the screws on the speaker terminals and such too. I'm trying to find a hum in a 2220 right now, it can be pretty tough.

12-28-2009, 05:17 PM
Thanks, Jailtime, I was looking at those big boys thinking they might be the ones. I'll probably take a shot at replacing those, take your advice to check the grounds, and see if that fixes it. A lot of AKers seem to promote a full re-cap, but I think doing the whole unit is too much for me (in time, patience and skill).
Are you posting about the 2220? Wouldn't mind finding out what the hum turned out to be when you figure it out.