Sunn Concert Bass "death cap trouble": pilot light on constantly now

[Gioanpj]

Hi,

I recently removed the "death caps" on a Sunn Concert Bass amp. The amp powers up and functions; however, the pilot light on the AC switch stays on all the time, even in the off position.

I'm wondering what I did wrong, or if there is a way to get the light in the power switch to turn off when the switch is off.

Here is a description of the wiring:

The three wires go into the accessory outlet and from there:

green --> to ground on chassis.

Black --> circuit breaker --> thermal cutoff (located on the heatsink) --> to middle tab of AC switch
Black --> to the first side of the AC switch --> (second side of switch) primary wire to transformer

White --> Primary wire to transformer



Thanks.

[Enzo]

Since you said middle tab on switch, may I assume there are three terminals on it and not just two?


Right now you have two black wires to the switch, one from power, the other to transformer. Is ther a third wire to the switch? Maybe a white one?

The short answer is probably just swap the two black wires.

Inside the switch, you have the switch itself, and two terminals, then usually there is a neon bulb (and probably an internal resistor) wired to one of those terminals, with its other end having a terminal on the switch. The cold side of the mains is wired to that bulb-only terminal. Now when all wired, the 10v on the switch is across the bulb. If you have the mains feed wired to the terminal that is also internally wired to the bulb, then the light stays on. If you wire the mains to the other side of the switch, then the 120v only gets to the bulb when the switch is closed.


If I have misunderstood your switch, the answer is still that you have the light wired to the wrong side of the switch.

[Jazz P Bass]

Agree, agree, agree.
The black wires are inverted at the switch.

[loudthud]

I dug out my Concert Bass and I think he has it wired correctly. But, my amp is an older unit and I don't know if they used the same switch in newer amps. The old amps have a Carling switch that has three terminals, two on one side and one on the other. On a Sunn Beta amp a narrower Carling switch is used that has three Fast-On terminals all in a row, you have to look at little numbers on the side of the switch (and go to Carlings website) to figure out which terminal is which.

[Gioanpj]

So, I switched the primary and hot wires. Same result. I did some more messing, then the amp worked, but no light in either position.

I got the multimeter out to check the resistance of the switch. There is no continuity between the two outermost poles no matter the position (on or off). There is continuity between the two poles closest together (middle, and one of the outermost poles) in the on position.

I'm assuming the switch is bad.

However, I'm confused about how to wire the thermal cutoff sensor located on the heatsink (if that's even what it is). Any tips?

By the way, I want to say that originally, the switch was wired into the neutral side of the circuit, but following a diagram for taking the death caps out of a fender amp, I moved it to the hot side.

[Jazz P Bass]

Please post a pic of the switch.

The neon lamp will not show continuity.

Here is the schematic.

sunn-concert-bass-amp-power switch.pdf

Black. from the power cord goes through the circuit breaker, then through the thermal switch to the power switch.
The transformer winding goes to the 'switched' contact of the switch.

The other transformer winding goes to the power cord White wire.
You will also need a white to the last switch lug, which will power up the neon lamp.

[Jazz P Bass]

It appears that Sunn changed the positioning of the Thermal Switch.
In this drawing (which is a lot clearer) the TS is after the power switch.

Sunn Concert Bass Schematics and Service Info.pdf

[Enzo]

If you have three terminals, two of them will measure off and on, the third will have no continuity ever, because it is the other end of the neon bulb.

[Gioanpj]

Hello,

Yes, I will take a picture of the switch when I get home. Looking at the schematic: what is the purpose of the ".0047uf 1000v" capacitor in the power supply circuit? This schematic, as opposed to the one I have, has a three prong plug; mine just had the two.

So, if I understand correctly, a neutral wire needs to run to the one leg of the neon bulb, the other leg must be wired to the hot side?

[Jazz P Bass]

So, if I understand correctly, a neutral wire needs to run to the one leg of the neon bulb, the other leg must be wired to the hot side?

Yes.

That is what the schematic shows.

[Gioanpj]

I figured it out thanks to everyone's input. The problem was that I wasn't running the neutral wire to the one end of the lamp. Another successful fix. The amp sounds sweet. Now, if I could just figure out the acoustic 270 I'm still working on. That's a real puzzle!