Mesa 5:50 Express, no sound

[Nirvana]

That's still very low. Try checking the resistance of that cap now with the one leg lifted. It will probably charge to a high resistance.
At the output of the regulator (with those preamp tubes removed), I would expect maybe 1K or higher ballpark resistance. At least you have a very obvious low resistance to track down. Unfortunately, there are a lot of things running off of that 12V line, and it could be any one of them. Lifting board connectors might help identify a board that may contain the culprit (for example the FX loop board).

The cap resistance actually didn't change....
With the capacitor leg removed and preamp tubes removed I get 0.2Ω on the cap and 250Ω on the output of the regulator
With the capacitor leg removed and preamp tubes connected get 0.2Ω on the cap and 4.2Ω on the output of the regulator

Is that difference in regulator resistance mean anything?

[Helmholtz]

A cap that measures 0.2R between its terminals is shorted.

4.2R with tubes in makes sense, as 3 times 4.2R makes 12.6R, which is the typical heater resistance of a 12AX7 between pin 4 and 5 when cold.

[g1]

Cap must be replaced, it's shorted like Helmholtz said. Try it with a new cap and see if the 12V shows up at the regulator output.

[Nirvana]

Cap must be replaced, it's shorted like Helmholtz said. Try it with a new cap and see if the 12V shows up at the regulator output.

Alright, I shall replace the cap then. Can you guys confirm any of these will do? I just filtered 15uf & 12VDC but they look different than the one that's in there.
https://www.mouser.com/Passive-Compo...z0z819Z1yzs91g


The one in there right now has a beveled side with a indented circle like this:


Does the circle mean anything... like the "+" side or something?
I see there are film, ceramic, Al, electrolytic, etc and I am too much of a noob to know what difference these various styles make....

[Helmholtz]

12VDC rating is not enough.

Original cap looks like a molded solid tantalum cap (as made by Kemet, Vishay and others). Typically bevelled end is positive.
What is written on the case?

This said, a 15µ/20V Al ecap will probably work fine. I would go for longest lifetime/ highest temp. rating available.
Shape doesn't matter. Also consider radials.

[Nirvana]

12VDC rating is not enough.

Original cap looks like a molded solid tantalum cap (as made by Kemet, Vishay and others). Typically bevelled end is positive.
What is written on the case?

This said, a 15µ/20V AL ecap will probably work fine. I would go for longest lifetime/ highest temp. rating available.
Shape doesn't matter. Also consider radials.

It says 156k on one side and 1103 on the other side

[Nirvana]

12VDC rating is not enough.

Original cap looks like a molded solid tantalum cap (as made by Kemet, Vishay and others). Typically bevelled end is positive.
What is written on the case?

This said, a 15µ/20V Al ecap will probably work fine. I would go for longest lifetime/ highest temp. rating available.
Shape doesn't matter. Also consider radials.

What is an ecap?
I've narrowed it down to 20V 15µ and the remaining factor looks like the tolerance....do I want 5%, 10%, or 20%?
Does the schematic in this thread indicate it'll need to be polarized?

[Helmholtz]

What is an ecap?
I've narrowed it down to 20V 15µ and the remaining factor looks like the tolerance....do I want 5%, 10%, or 20%?
Does the schematic in this thread indicate it'll need to be polarized?

- Ecap is a short form of electrolytic capacitor - as opposed to foil capacitor. Ecaps are typically polarized.
- Tolerance is not critical here.
- Schematic shows a + sign at the cap end which connects to the regulator output.

[Nirvana]

- Ecap is a short form of electrolytic capacitor - as opposed to foil capacitor. Ecaps are typically polarized.
- Tolerance is not critical here.
- Schematic shows a + sign at the cap end which connects to the regulator output.

Got it.
Are either of these 2 a good choice? is one a better than the other?
https://www.mouser.com/ProductDetail...6DAcHQzA%3D%3D
https://www.mouser.com/ProductDetail...UpV5MVPg%3D%3D

[Helmholtz]

Got it.
Are either of these 2 a good choice? is one a better than the other?
https://www.mouser.com/ProductDetail...6DAcHQzA%3D%3D
https://www.mouser.com/ProductDetail...UpV5MVPg%3D%3D

Both are fine.

[Nirvana]

I just got the capacitor in the mail today, put it in, and now it works!
Thank you Helmholtz and g1 for sticking with me on this, you guys were a big help, saved me $500, and I also feel more confident around electronics now.
Much appreciated.
Im curious what causes such a small capacitor to crap out like this. The tubes and preamps are all 10 years old...maybe its time to change them

[BlackSquirrel]

Im curious what causes such a small capacitor to crap out like this. The tubes and preamps are all 10 years old...maybe its time to change them

Just gotten two Mesa M-Pulse bass amps in, a 360 and a 600, with the identical problem! Curious indeed! The owner said the amps both worked fine when he put them up a few years ago.

[g1]

Just gotten two Mesa M-Pulse bass amps in, a 360 and a 600, with the identical problem! Curious indeed! The owner said the amps both worked fine when he put them up a few years ago.

Did you find the same cap shorted at the output of the 12V regulator? I guess a tantalum cap at this voltage rating is not a good fit in this part of the circuit.