Lonestar Classic dead?

[TheRumble]

So after days of using the search to no avail, and hoping that I don’t have to send this out to a tech(but probably will), I have a strange issue.

How it happened: Playing regularly low volume in my house, then all of a sudden a slow drop of sound and dimming of the jewel light until both sound and light stopped working. All channels, all settings.

What I’ve checked: V1,V4 and V5 preamp tubes aren’t lighting up. The fan or jewel light is not working. All wattage and channels settings make no difference. I’ve changed the power tubes and replaced the 1,4, and 5 preamp tubes. Not 100% sure the 12ax7 are good though, going to order some new ones..

Strange thing is with the loop in hard bypass the output knob adds more hiss the higher it goes. So I plugged into the effects RETURN and I have sound through the effects loop. So signal from my guitar playing through the speaker which is being controlled by the output knob.

So I’m guessing the preamp section died…or I’m curious about the screen resistors or the metal oxide varistor…I haven’t gotten in there to check any specs yet…

Any suggestions before I send it to a tech? I’m decently capable of working on amps, but there is so many things on the Lonestar classic and I don’t think I have the time to completely dive in there. Thanks in advance and sorry if this was covered somewhere.

 

[user 41014]

Odds of three tubes going bad together without taking out the fuse is pretty low I'm thinking.

I don't have a LS but I would check a schematic for where that jewel light sits in he circuit.

If it was me I would pull the chassis to see if there is anything obvious. If 3 tubes aren't lighting up I would check the heater circuit, and why V2 and V3 light up but not the others.

 

[Eevil]

Howdy..

Have you checkef the main power switch.. i had one lemon and it was a nightmare to figure out

 

[TheRumble]

Odds of three tubes going bad together without taking out the fuse is pretty low I'm thinking.

I don't have a LS but I would check a schematic for where that jewel light sits in he circuit.

If it was me I would pull the chassis to see if there is anything obvious. If 3 tubes aren't lighting up I would check the heater circuit, and why V2 and V3 light up but not the others.

Looking at the schematic now, can’t seem to find where the light is in the circuit.. I need to get better at reading these things.

And @Eevil good idea, excuse my ignorance but if the switch is turning on the amp just not the light, could it still be bad?

 

[Eevil]

Looking at the schematic now, can’t seem to find where the light is in the circuit.. I need to get better at reading these things.

And @Eevil good idea, excuse my ignorance but if the switch is turning on the amp just not the light, could it still be bad?

It can be intermittent and fade out randomly.. but amp was Mark III which did not have such an amount of tech stuffed into it.. just an out of box -idea

 

[Eevil]

Strange thing is with the loop in hard bypass the output knob adds more hiss the higher it goes. So I plugged into the effects RETURN and I have sound through the effects loop. So signal from my guitar playing through the speaker which is being controlled by the output knob.

But if this works all the time then it doesn’t fit

 

[bandit2013]

If the MOV blew out, it would not be a slow death, it would be a sudden smoke screen as they usually split when they give or shunt the current when the excess voltage is reached. You would know it if that was the issue.

The pilot light or jewel light is on the 12VDC supply. If you have the schematic, it is on the page where it shows the power transformer, it is the circle with the X on it right next to the fan (looks like a 4 leaf clover without a stem).

It is possible the 10,000uF cap took a nose dive or more than likely one of the diodes in the bridge circuit failed. There is a Voltage regulator LM7812 that maintains a 12V output that also supplies the V1 and V2 heaters. That same 12V circuit serves the relay gear for channel changing and also incorporates the strobe mute circuit that also supplies 12V to a few JFETS that are used to mute portions of the preamp when a channel change occurs. If the output regulator is not working or the gate voltage on the JFET is below 3-6V DC they will be in a shorting state. I believe what happened here is one or more of the 1N4007 diodes failed in the full wave bridge circuit resulting in low voltage output or no voltage output. IF the fan is not working or the jewel light is not on, it would be the the diodes. Replace all of them for the 12V circuit. It is possible this amp was made when Mesa had a bad batch of diodes. There are 4 more of them in the high voltage supply as well.

If you are not familiar with working on tube amps, they can store a high voltage in the main power supply capacitors. Some of the didoes may be located very close to the main power capacitors. I recommend you have a tech do any repairs as this is an involved process, The main board will have to be removed to replace the diodes if you want it done properly.

I would also send Mesa Customer Service an email stating everything you mentioned on your first post. That has plenty of information in it. Mesa will also recommend a suitable replacement diode to the 1N4007 diode. They may even provide you with the actual schematic and any associated information regarding the amp in particular for some issues.

 

[TheRumble]

If the MOV blew out, it would not be a slow death, it would be a sudden smoke screen as they usually split when they give or shunt the current when the excess voltage is reached. You would know it if that was the issue.

The pilot light or jewel light is on the 12VDC supply. If you have the schematic, it is on the page where it shows the power transformer, it is the circle with the X on it right next to the fan (looks like a 4 leaf clover without a stem).

It is possible the 10,000uF cap took a nose dive or more than likely one of the diodes in the bridge circuit failed. There is a Voltage regulator LM7812 that maintains a 12V output that also supplies the V1 and V2 heaters. That same 12V circuit serves the relay gear for channel changing and also incorporates the strobe mute circuit that also supplies 12V to a few JFETS that are used to mute portions of the preamp when a channel change occurs. If the output regulator is not working or the gate voltage on the JFET is below 3-6V DC they will be in a shorting state. I believe what happened here is one or more of the 1N4007 diodes failed in the full wave bridge circuit resulting in low voltage output or no voltage output. IF the fan is not working or the jewel light is not on, it would be the the diodes. Replace all of them for the 12V circuit. It is possible this amp was made when Mesa had a bad batch of diodes. There are 4 more of them in the high voltage supply as well.

View attachment 2541

If you are not familiar with working on tube amps, they can store a high voltage in the main power supply capacitors. Some of the didoes may be located very close to the main power capacitors. I recommend you have a tech do any repairs as this is an involved process, The main board will have to be removed to replace the diodes if you want it done properly.

I would also send Mesa Customer Service an email stating everything you mentioned on your first post. That has plenty of information in it. Mesa will also recommend a suitable replacement diode to the 1N4007 diode. They may even provide you with the actual schematic and any associated information regarding the amp in particular for some issues.

WOW, thank you so much for this explanation. I will definitely send Mesa an email and try and find a Mesa certified tech to do the work. Hopefully my information on how it happened and your potential diagnosis can get this thing working in no time. Your the man @bandit2013

 

[bandit2013]

Just trying to be helpful. I would suspect the linear voltage regulator may have also seen some stress too. Hopefully you can get your amp up and running again. No fun when things do not work.

Your description reminded me of what others had with their Mark V90's. Since the Jewel light is part of the 12V circuit with your amp, that was a giveaway what the issue is. Not all Mesa amps use the same method for the pilot light, Rectifiers run them on the 120VAC power, at least the big bottle models.
Mark V90, that is just and LED so it is not line voltage powered, it runs off of the 5V DC supply which is also fed by the 12V supply.

Wait and see what Mesa responds with. They may have some other details that I am not privy too. I am not a Mesa Tech or work for Mesa, but I can read a schematic and understand most of the basic design what Mesa has been using. What I can find on-line may not be the current design as Mesa does make changes periodically to their amps if any issues arise.

 

[TheRumble]

Just trying to be helpful. I would suspect the linear voltage regulator may have also seen some stress too. Hopefully you can get your amp up and running again. No fun when things do not work.

Your description reminded me of what others had with their Mark V90's. Since the Jewel light is part of the 12V circuit with your amp, that was a giveaway what the issue is. Not all Mesa amps use the same method for the pilot light, Rectifiers run them on the 120VAC power, at least the big bottle models.
Mark V90, that is just and LED so it is not line voltage powered, it runs off of the 5V DC supply which is also fed by the 12V supply.

Wait and see what Mesa responds with. They may have some other details that I am not privy too. I am not a Mesa Tech or work for Mesa, but I can read a schematic and understand most of the basic design what Mesa has been using. What I can find on-line may not be the current design as Mesa does make changes periodically to their amps if any issues arise.

Thanks @bandit2013, unfortunately I got a pretty stock response from Mesa, “could be the issue, but take it to your local Mesa authorized tech…” Obviously i was hoping that it would something that was common enough to them to point me in the right direction and was fairly easy to address and fix, but at this point I think I’ll take what information I’ve graciously gotten here and pass it on to whoever the tech may be. Hopefully it’s not a huge bill…anyone know what they charge?

 

[bandit2013]

If it has no warranty, I would do the work myself. If it was under warranty, it goes to Petaluma or certified Mesa tech. You can ask for a quote before you bring in the amp for service. They usually charge by the hour. Have no clue what that cost would be. Sorry on that part.

 

[bandit2013]

Just note that I only referenced a potential issue. This part of the circuit may be just fine. Only way to confirm it is to place an oscilloscope on the circuit and power it up and look at the waveform. Sure if you know where to probe it, Depending on the circuit, testing parts while they are still in the amp with a digital multimeter may or may not work out. Some do have a diode test. If I were to find one bad, I would probably replace all of them. I am not all that familiar with the Lonestar so if this is a common issue or not common, I would say depends on the age of the amp. I do know the Mark V90 had some issues. I know a few that had to replace the diodes. So far that has not been an issue with the V I have yet. It has other gremlins for now.