June 13th, 2012, 11:15 PM
My Weber 5E3 was blowing fuses even with no tubes in it so I replaced the PT with a Magnetic Components DR PT. The amp works but it hums and the on/off switch positions somehow swapped so "off" is on and "on" is off. Any ideas on what the deal is?
June 14th, 2012, 12:07 AM
Re the hum is there a centre tap for the heaters on the MM? Did the Weber have one?
If there was previously a centre tap and now there isn't this will cause hum, you can create an artifical centre tap by adding a pair of 100k resistors to ground from the pilot light.
Re the switch either the circuit is closed or open so I guess what you mean is that when the switch is in the off position according to the text on the control panel it is on and vice versa. If that's the case then the easiest way is to rotate the switch so that it throws on the "right" direction for the text. Not sure why that would be - any chance you rotated the switch while fitting the new PT?
Good luck with it and keep us posted.
June 14th, 2012, 02:04 AM
No, the switch hasn't moved and only the lead from the PT was resoldered. I think the switch issue and the hum are probably related.
June 14th, 2012, 01:12 PM
Since I used the DR PT it has the blue bias tap which I taped off. Should I have grounded it?
June 14th, 2012, 01:14 PM
Get a Carling for the switch. Taping off the bias tap is fine.
June 14th, 2012, 01:58 PM
All of the above and see if the wires going to the plug are backwords. (shot in the dark)
June 14th, 2012, 04:12 PM
It is a Carling. Don't see how the leads could have been swapped since I only removed one lead when I changed the PT.
June 14th, 2012, 04:21 PM
Depends on the mode of failure. If, in dying, the weber PT had shorted to ground through rectifier failure, that could have destroyed one or more of the filter caps, and that could be a source of hum. Is it a 120Hz cycle hum? (just trying to brainstorm something here)
June 14th, 2012, 07:02 PM
Sounds like 120hz hum. It's not as bad as a shorted filter though. The amp still works but is not really useable with the hum.
June 15th, 2012, 02:17 AM
Have you got a scope you can put on it?
June 15th, 2012, 12:15 PM
I wish I had a scope. I've got some spare filters laying around so I'll go ahead and swap them in and replace the copper cap with a 5Y3.
June 16th, 2012, 04:01 AM
Can you post pictures? As you stated, seems like something is not right with the wiring of the switch. I know that I can not help you out but others here are really good when they can see pictures.