Mesa 5:50 Express, no sound

[Jazz P Bass]

It certainly appears that the 12 volt regulator is dead.
16 Vdc going in to it is plenty.
The question is 'why'. did it fail.
It could be a random failure or something downstream demanded too much current.

[Nirvana]

It certainly appears that the 12 volt regulator is dead.
16 Vdc going in to it is plenty.
The question is 'why'. did it fail.
It could be a random failure or something downstream demanded too much current.

So it looks like if I replace the 7812 voltage regulator that might be the fix?
Im curious how some of you narrowed down the potential problem to these 2 capacitors or the regulator? Also why does the pilot light come on if the output side of the regulator isn't putting out any voltage?

[Jazz P Bass]

Measure the voltage across the pilot lamp.

[g1]

Also it's possible the pilot light is not as shown on schematic. Mesa schematics aren't at the top of the 'trustworthy' list.

[Nirvana]

Measure the voltage across the pilot lamp.

I get 0 volts but the light is on. Can you look at the picture to see if Im doing anything wrong?

[Nirvana]

Measure the voltage across the pilot lamp.

If I take an AC voltage reading I get something

[Nirvana]

It certainly appears that the 12 volt regulator is dead.
16 Vdc going in to it is plenty.
The question is 'why'. did it fail.
It could be a random failure or something downstream demanded too much current.

I took this measurement of the voltage regulator, does this mean its not dead or did I not do it correctly?

[The Dude]

Here is a pinout of the regulator. It has an input voltage higher than 12V, Ground, and 12V output. If you have input and no output either the regulator is bad or there is a short/excessive load on the output. Check the output (to ground). Is there 12V there? If not, unsolder the output and check the output leg of the IC. If it comes up to 12V, check the resistance from the wire that soldered to that leg to ground to see if there is something shorted on the 12V line. There is also a possibility that the regulator works unloaded, but can't handle a load. If you measure no shorts on the 12V line, just try replacing the regulator if there is not 12V.

[Nirvana]

Here is a pinout of the regulator. It has an input voltage higher than 12V, Ground, and 12V output. If you have input and no output either the regulator is bad or there is a short/excessive load on the output. Check the output (to ground). Is there 12V there? If not, unsolder the output and check the output leg of the IC. If it comes up to 12V, check the resistance from the wire that soldered to that leg to ground to see if there is something shorted on the 12V line. There is also a possibility that the regulator works unloaded, but can't handle a load. If you measure no shorts on the 12V line, just try replacing the regulator if there is not 12V.

I tried what you said.....

I put the red lead from the meter on the input leg and the black lead from the meter on the ground leg and got 21v. Then I moved the red lead of the meter over to the 7812 output leg and got 0v, so then I disconnect the output leg and did the same thing again (image below) and got 0V.
Did I do this correctly and does this prove the regulator to be faulty?

[The Dude]

Yes. It is most likely a bad regulator. Replace it.

[g1]

If I take an AC voltage reading I get something

This means the pilot light is not wired as shown in the schematic. It is running off the 6.3VAC winding of the transformer, along with the power tube heaters.

[Nirvana]

My regulator says LM7812C on it, would a LM7812CT/NOPB found here work?
It also shows a J33 on it but it seems like that doesn't matter.

[The Dude]

Your link is for the homepage, but the LM7812CT/NOPB should work just fine.

[Nirvana]

Your link is for the homepage, but the LM7812CT/NOPB should work just fine.

Yea for some reason the "post link" tool reduced it to the homepage, not sure why, here was the link I posted:
https://www.mouser.com/ProductDetail...pFKHiRuQ%3D%3D

Thanks for the help The Dude. I'll order that now and get back to y'all when I get it in there. Do you have a preferred source for ordering parts? All the brick and mortar electronic shops that used to be around that would otherwise carry these (like Frys) are all out of business.

[Randall]

I get 90% of my electronics parts from Mouser, but also sometimes use Newark, or Digi-Key. For amp specific capacitors I generally use the wholesale end of Antique Electronic Supply.