Mesa 5:50 Express, no sound

[vintagekiki]

Stories from the master workshop
(nothing personal)

The crisis of rock 'n' roll arose when musicians instead of the instruments they were playing, accepted soldering irons, and instead of technical (professional) books for personal education use unverified stories from various portals and media presentations.

The only thing that matters for the components is that they are within the standards and limits prescribed by the amp manufacturer.
There are relatively few manufacturers of the components, and the names we see on them are commercial.

[Nirvana]

Hey everyone,
So I replaced the regulator and still same situation. I get 16.2V going in and 0.07V on the output. any other ideas?
Could it have something to do with the mute circuit?

[Jazz P Bass]

I would have measured the output pin pad to ground before reinstalling the new regulator.
You could still try it with it in.
Something downstream could be shorted to ground.

[Nirvana]

I would have measured the output pin pad to ground before reinstalling the new regulator.
You could still try it with it in.
Something downstream could be shorted to ground.

I see, do you have any suggestions on maybe how to identify any possible shorts downstream? Any chance it could be the capacitor that is right after the output?

[Jazz P Bass]

First see if you 'have' a short.

You can remove the cap if you think that may be an issue.

[Nirvana]

First see if you 'have' a short.

You can remove the cap if you think that may be an issue.

How do I check for shorts?

[g1]

How do I check for shorts?

Measure resistance between regulator output pin and ground.

[Nirvana]

Measure resistance between regulator output pin and ground.

If I put the red probe of the multimeter onto the output pin of the regulator and the black probe onto the middle pin the reading starts at 282Ω and slowly increases to and tops out at 286.

[g1]

So you had wondered about the capacitor right after the output pin. Disconnect one leg of the cap and see if that resistance reading changes.

[Nirvana]

So you had wondered about the capacitor right after the output pin. Disconnect one leg of the cap and see if that resistance reading changes.

Yea I wondered about that but I don't really know what I'm talking about, does it seem like that could likely be the issue? And if I disconnect one of its legs will that cause any problems with the circuit board when I power it up?
Also, after seeing those resistance numbers I mentioned earlier, how are you interpreting them and what does this tell you about what is going on exactly? Sorry for all the questions, I really just want to learn more about this stuff.

[g1]

And if I disconnect one of its legs will that cause any problems with the circuit board when I power it up?

It won't cause any problems worse than it is now. And no damage. But you don't need to power it up, just resistance measurements.
However, the tube heaters are fairly low resistance, and they are across the output of the regulator. So remove V1,2, &5, and do your resistance measurement from regulator output (pin3) to ground. If you still measure low resistance, lift one leg of that cap and check again.

[Nirvana]

It won't cause any problems worse than it is now. And no damage. But you don't need to power it up, just resistance measurements.
However, the tube heaters are fairly low resistance, and they are across the output of the regulator. So remove V1,2, &5, and do your resistance measurement from regulator output (pin3) to ground. If you still measure low resistance, lift one leg of that cap and check again.

I removed the V1 V2 & V5 pre-amp tubes and I get a resistance that starts out at 200 and very quickly goes down to 0.2. Same thing happened before I removed them. This was all done without any power and the tubes are simply removed, not desoldered, hopefully this is what you meant by remove. Also the meter beeps as if Im taking a continuity reading, it didn't beep when measuring with the power on.
Does it sound like I did this correctly? If so I can move on to your suggestion about lifting one leg of the cap.

[g1]

Yes you did it correctly. Try lifting one of the cap leads and see if the resistance comes up.

On a side note, you mentioned the meter on 'continuity' did not beep when amp was powered up. You should not measure using resistance or continuity range when any voltage is present (unit powered up). It can damage your meter and usually won't give a valid measurement anyway.
In very rare cases it is done (in a powered unit) after verifying there is no voltage on the component being measured; but as a general rule, resistance and continuity checks are done with unit turned off and power supply caps discharged.

[Nirvana]

Yes you did it correctly. Try lifting one of the cap leads and see if the resistance comes up.

On a side note, you mentioned the meter on 'continuity' did not beep when amp was powered up. You should not measure using resistance or continuity range when any voltage is present (unit powered up). It can damage your meter and usually won't give a valid measurement anyway.
In very rare cases it is done (in a powered unit) after verifying there is no voltage on the component being measured; but as a general rule, resistance and continuity checks are done with unit turned off and power supply caps discharged.

Got it, thanks for that advice. I'm glad you pointed that out this early in my experience. The resistance DID go up to 4.2Ω after removing one of the legs, what does this tell us about what is going on and are we looking for a specific range to rule out the cap?
Attached image to show what is going on, the capacitor in question circled in red.

[g1]

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).