Mark IV DEAD!

[steeldragonjovi]

So, I was playing some tunes tonight, and my Mark IV just started humming, got louder, then died. It won't turn back on. It's been having some issues with humming and crackling and popping lately, but I guess it just got pushed too far. Any thoughts? What exactly am I looking for if the fuse is shot?

 

[screamingdaisy]

Sounds like a power tube suffered a mechanical failure and shorted out.

If you remove the power tubes and give each a shake by your ear you should be able to hear some bits shaking around inside one of them.

 

[BlackBoxy]

Sounds like a power tube suffered a mechanical failure and shorted out.

If you remove the power tubes and give each a shake by your ear you should be able to hear some bits shaking around inside one of them.

+1

a power tube gone dead, change the fuse and a new tubes set!

 

[steeldragonjovi]

Took each power tube out and shook them, but there were no loose parts. Still yet to replace the fuse.

 

[lespaulguy32]

Sad to hear, man... I'm a n00b when it comes to things like this happening, as I just got my Boogie back in September. I do think it is a tube, however, as I did have a similar issue happen with my Fender. Keep us updated!

 

[screamingdaisy]

Took each power tube out and shook them, but there were no loose parts. Still yet to replace the fuse.

Sometimes that happens but the symptoms still point towards a power tube failure. I'd pick up a new set of tubes and fuse and start there.

Here's some reading if you're interested;

Mark IV User Manual

Page 18 has some info on diagnosing power tube failure.

 

[fretout]

It also may be worth it to check your preamp tubes if the power tubes and fuse are good. Definitely check your V1 preamp tube. If you've been getting some strange "shorting" or "popping" coming from the amp, this could be the culprit. I had the same issue on a used Mark IV I picked up years ago. Grab a pencil and tap the tube as it sits in the socket. Determine if the tube is faulty, try replacing the 12AX7/12AT7. What I typically do is pull the try switching the preamp tube the controls the Reverb with each V1, V2, ect., until you get sound.

Otherwise, I doubt anything serious is wrong. Mesa amps typically don't have fried boards because they install failsafes to prevent internal damage, so you probably just need some new tubes.

 

[dlpasco]

This happened with my Mark III about 12 (woah) years ago. Pull out the fuse and take a look at it. If the element isn't straight and intact, you probably blew it.

Usually cause: bad power tube. This troubleshooting guide is a big help http://mesaboogie.com/news/2010/11/mesa-amplifier-tube-troubleshooting/

 

[steeldragonjovi]

Thanks for all the input. That video Mesa posted about tube diagnosis is great, however, it's not so simple when it comes to my amp. I have done all those tests, and I can't identify any tube problems like they did. I had a problem with it once before where it would run hotter than it should because of a problem in the circuitry, but I had that fixed. I suppose it could be the same problem...

 

[bandit2013]

Check the speaker jack for shorts or corrosion on the speaker wires.

I had a simiar issue with my MKIV. However, tubes were still in good operating condition and the fuse never blew out. Had some popping noises then all of a suddent complete silence. Turns out the wires in the 90 degree speaker plug were corroded and fraying on the positive wire. There was also copper oxides forming on the wires at the insulation. When plugged in, and due to vibrations caused by the speaker, intermittent shorts would occur. I replaced the entire cable assembly and never had an issue since. My amp sat for some time (6 - 8 years without use) until my wife requested that I start playing again.

 

[steeldragonjovi]

Speaker jack on the amp's output? Is that what you are referring to?

 

[bgh]

How can an issue with the speaker wiring keep a Mark IV from powering up? (I am not trying to be sarcastic - this is a real question).

I know there are a lot of failsafes built into these amps. Is there one that tests for grounding and/or shorting issues with whatever speakers are connected to the amp?

Concerning the OP's issue, I would have guessed that a power tube issue could definitely cause the original problem, but in and of itself would not keep the amp from at least powering back up. However, if the tube issue took out the fuse, then there are (at least) 2 issues: The bad power tubes need to be replaced and the fuse needs to be replaced.

Concerning pre-amp tubes, I have always been under the impression that Mark IVs could power up without having all of the pre-amp tubes present. In fact, in some threads, I have read where some people swap them out 'hot'.

Again, this is not meant to be a sarcastic or implicating post. I have a Mark IV and want to keep learning everything about it that I can.

 

[bandit2013]

My bad, I guess I did not reall all the details. If nothing happens with the amp is plugged in and the power switch turned on, I would replace the fuse. Also check to see if there is power at the outlet (I know this is obvious but sometimes overlooked).

Does the fan at least turn on when the amp is turned on? A fuse may look good but may fail a resistance test. Replace the fuse if you have not done so already. If you can get the the lights on the amp, are the cathodes hot in standby? If not, you could start pulling tubes one at a time and see if anything changes, leave it in stand by only. With any luck it might just be a preamp tube dropping the voltage on the cathode circuit.

My first thoughts on what happened with my MKIV was blown power tubes. I was playing and, then the amp began to pop a bit, that did not last long however as I continued to play the amp gradually faded to complete silence. Sounded like turning the power switch off without using the bypass switch. Looked for hot plates when I powered it back up. Everthing was working fine the next day. However when it happened again it went completely dead. Fuse was fine. I suspected power tube failure and ordered new one's. Same thing happend again. It did power back up but no sound at all. That lead me to check the 90degree spearker plug. Hence the suggestion to check the wires in the speaker plug. Just a suggestion. Also My original tubes and fuse were still good.

Now for the speaker short circuit: If the seconday of the ouput transformer is shorted (in my case, the speaker plug shorted internally - intermmitently), the current will increase on the primary and overload the tubes, and everything else. I was quick enought to power down the amp every time I had the issue. Every symptom pointed to the power tubes failing. Only exception the fuse did not blow out and the plates were normal (not red plating).

 

[bgh]

Hey, thanks for the explanation. I have not seen that happen before, but your description makes perfect sense.

For the OP, does you Mark IV power on at all? Or is it completely dead? If the latter, step 1 has already been given - replace the fuse. If that works, you then know why it would not power up. The next issue to address is your tubes - to see if they are the culprit as to why the fust blew (they typically have a reason when they take the plunge).

 

[bandit2013]

Checking the wires in the speaker plug only takes less than a minute.

It is possible there may be a short in any of the preamp tubes too. If you start popping fuses, you can get them at Radio Shack in quantity. You could either start pulling one tube out at a time and power up for a brief time (stand by only), if the amp lights up you will find the bad tube.

 

[steeldragonjovi]

Hey, thanks for the explanation. I have not seen that happen before, but your description makes perfect sense.

For the OP, does you Mark IV power on at all? Or is it completely dead? If the latter, step 1 has already been given - replace the fuse. If that works, you then know why it would not power up. The next issue to address is your tubes - to see if they are the culprit as to why the fust blew (they typically have a reason when they take the plunge).

Yeah, I have it up and running again now that I've replaced the fuse. However, I can only use it in half power. Also, it takes about ten minutes for the amp to get to the set volume (without me tweaking an knobs and such).

I've done several checks on the tubes and can't find anything wrong with them. I'm really at a loss for this. I did have an issue with the amp a few years ago involving running at too hot of a temp, but I had it completely fixed. I wonder if it's a similar issue.

 

[bandit2013]

What power tubes are in your amp? I know the mark IV can use all 6L6 or 5881, two EL34 on the outside with 6L6 or 5881 in the inside, or if you are brave, 6v6 but must be run in tweed mode only.

By re-reading your original post, definately sounds like one of the tubes red plated on you. Sometimes they will work again for a short while until it happens again. If the plates are black (assuming you are not using RCS black plate tubes) chances are the tube suffered from self oscillation which is typical cause for red plate issues. Also the age of the tube and how much use, does not matter what volume or power level is used, if the tube is on the way out it wil either go quietly or not.

 

[bgh]

Yeah, I have it up and running again now that I've replaced the fuse. However, I can only use it in half power. Also, it takes about ten minutes for the amp to get to the set volume (without me tweaking an knobs and such).

I've done several checks on the tubes and can't find anything wrong with them. I'm really at a loss for this. I did have an issue with the amp a few years ago involving running at too hot of a temp, but I had it completely fixed. I wonder if it's a similar issue.

Could it be that there is a loose or slightly cracked solder joint somewhere? It may be taking a few minutes to get hot enough to work correctly.

 

[daviedog]

Simple trouble shoot, pull ALL tubes & fire up. Does fuse blow? If not the problem is tube related. Dispose all tubes that have red plated. The plates have deformed & tube will not reliably function.
Replace preamp tubes. Fuse blow? No, power tubes next.

Now, if fuse blows under minimal load ( tubes removed) you have a short in the power supply.
10 minute warm up? Leads me to believe filter cap, maybe one, they are wired in parallel.
Lesser chance, power resistor or rectifier diode. Very remote, P/T.
Easy fix for a tech...