This post is not a video, just pics. It is a result of recent emails with a guitarist that bought an amp and wanted to switch out the AC off/on switch. Because of the lengthy exchange I decided to make a post of it and possibly help someone else.
In the meantime it’s a good example of a simple circuit. Check out the schematic and how using an ohm meter you could figure out how to replace the switch. The off brand switch had only 3 posts while the Fender had 4.
I’ll copy the original posts here:
Here is the schematic of the switch and wiring:
I will say this:
If I were you I would get access to a variac before replacing anything in the power section. Especially when changing switches on the AC that are not exact. Or if you could borrow another Ulitmate Chorus and drop the chasis and see exactly how it is wired. Without looking at the pics I would assume the wires have been hacked on since one was females and males and such.
These 2 points I listed are by far the easiest way to do it. The first you will still need to use an ohm meter and schematic.
Hope this helps.
Picture of old switch…Hopefully this was done by someone in a real big hurry.
Usually you can figure it out with a schematic if you know how to use an ohm meter and trace the wires back. Also you have to be able to know how the switch is working. It’s hardly ever straight forward with this type of thing. That’s why the tech that replaced the switch should have drawn on the chassis what wire and color goes where on the switch.
If you don’t have a variac though it could get a little scary. I tell you if you already have the switch and the chassis out, a shop should be able to do it for almost nothing I would say. Just tell them you want the switch replaced and you don’t have the wiring diagram or variac.
I think the secret will be just to the left of the switch drawing. See the little s/a s/b coming off Power On Off…that is showing (if I remember correctly) a double pole singe throw. If you look on or around the posts on the switch there might be the corresponding numbers. If not, you can figure it out with an ohm meter. If I have this correct from just looking at pics and a schematic, with the switch off, there should be no continuity (infinite ohms) from any post to any other post…
When the switch is turned on, there should be one post looking at another post and no more. In other words with the switch on, the feed would be through one post to another post but no more than that? Make sense? Your connecting 2 circuits at the same time but they are separated from each other. I don’t believe it will make a difference which side is which since I don’t believe there is a light in that switch.
If the numbers are in the plastic next to the post then you’re all set…now when you test the switch for infinite ohms is when it is not connected, connected it will still be high ohms but not infinite, again I’m guessing here. The main thing after you wire it up, is test the switch with the ohm meter with the power off. You should never see zero ohms across the poles, only on one side (circuit) of it. Remember you don’t want the 2 circuits to see each other with zero ohms or you will blow a house breaker.
The only other thing, without a switch in front of me be aware, that the incoming lines could be the 2 bottom posts or the two on the side, don’t assume anything from looks, use your ohm meter to find what is what.
Hope this helps,
If you’ve got time and can leave your chasis out, it is a good case study for you. By taking your ohm meter and look at the switch on the schematic, not the drawing of the switch, but the circuit representation on the left. See how one side goes the the AC cord? and the other to the switch? (i’m doing this from memory, I don’t have access to the scematic right now)
Have you used an ohm meter directly on your new switch, just to see what readings you get? This is basic stuff but all complex circuits are nothing more than several basic circuits. Since you’re having trouble seeing it, it’s kinda like flying on instruments when you can’t see where you are going, you have to rely on your readings.
(all this is done while the unit is unplugged from the wall of course) but once you see how you get zero ohms from the switch all the way to the blades on at the end of the AC cord where it plugs into the wall the schematic drawing will start making more sense. Then you can go across the switch and notice with it pushed one way you get zero ohms, pushed another you get infinity ohms.
Just food for thought,