Mesa Boogie Mark IV problem

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Mesa Boogie Mark IV problem

Postby Stevenrb718 » 22 Nov 2019, 04:53

I have a Mesa Boogie Mark IV that doesn't fully power up or produce sound. When powered the tubes glow, but nothing else shows power. The amp doesn't make any sound but a decent hum that does change messing around with a couple dials. Where do I start?
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Re: Mesa Boogie Mark IV problem

Postby plush » 22 Nov 2019, 12:52

Stevenrb718 wrote: Where do I start?


You need to start looking for a decent amp technician to fix it.
Otherwise you risk with hurting yourself or damaging your amp thus raising total repair costs.

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Re: Mesa Boogie Mark IV problem

Postby FiveseveN » 22 Nov 2019, 13:04

Let me put it to you another way: ever shocked yourself with ~300 V DC? I have and would not recommend it.
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Re: Mesa Boogie Mark IV problem

Postby alexradium » 22 Nov 2019, 13:55

Stevenrb718 wrote:I have a Mesa Boogie Mark IV that doesn't fully power up or produce sound. When powered the tubes glow, but nothing else shows power. The amp doesn't make any sound but a decent hum that does change messing around with a couple dials. Where do I start?

Apart from the good advice from other forumers, you must provide basic voltage measurement from the main transformer.
At least,did you try changing preamp tubes?
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Re: Mesa Boogie Mark IV problem

Postby Manfred » 22 Nov 2019, 17:40

Here the Schematic:
Mark IV schematics.pdf

First check if the audio signal comes out of the recording output if yes then the preamp should be in order the error could be in the output stage.
Oxidized contacts of the switches of the FX-jacks are also a frequent fault, to check the switch "FX FULLTIME ON" off on the back panel.
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Re: Mesa Boogie Mark IV problem

Postby bmxguitarsbmx » 22 Nov 2019, 20:18

There is more than 300v in a MK4. If you want to do it yourself, start by learning the basics of electronics and then read a book on troubleshooting. Keep in mind that if there is a problem with the bias supply, you can nuke your power tubes and both transformers just by turning it on. -Vbias does come on with standby, so the first thing I always do is make sure I have a -Vbias on the output control grids and that it is within reason for the power tubes that are being used. Then I measure the current each power tube is drawing. If that checks out, you are good to start troubleshooting without destroying the expensive parts of the amp. Use alligator clips to connect your meter to ground and put your left hand behind your back(right hand behind your back if you are left handed). Keeping one hand behind your back will eliminate the possibility of dying. Having your other hand on the strings of a guitar that is plugged into the amp is great way to kill yourself.

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Re: Mesa Boogie Mark IV problem

Postby deltafred » 22 Nov 2019, 21:09

First try swapping preamp tubes (with known good ones) as it sounds as though the output stage may be working. If that fails find a good tech as it will be cheaper than trying to fix it when you have no idea what you are doing.

There is nothing a tech hates more than having to fix an amp (or anything) that a well intentioned amateur has been poking about in with a soldering iron. First they have to find and correct all the faults that said amateur has introduced before they can fix the original fault.

All this costs time to the tech and additional money to the owner.

If you want to learn about tubes a Mesa Boogie is definitely not the place to start. Start by building a preamp, then add low power push pull output stage and make yourself something like a Blues Junior.
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Re: Mesa Boogie Mark IV problem

Postby phatt » 24 Nov 2019, 15:13

You can at least try simple basic (And Safe) test.

With the amp unplugged from mains simply remove and reinsert all the valves.
Valve pins/Sockets do get corroded over time and if the amp has been sitting for a long time or stored in high humid conditions then yes wacky things can stop an amp from working. Even just wiggling the valves can bring it back to life.
Many times I've resolved problems like this just by the above.
If the amp still hums then yes you may need pro help to fault find.
Phil.
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Re: Mesa Boogie Mark IV problem

Postby bmxguitarsbmx » 25 Nov 2019, 23:24

phatt wrote:You can at least try simple basic (And Safe) test.

With the amp unplugged from mains simply remove and reinsert all the valves.
Valve pins/Sockets do get corroded over time and if the amp has been sitting for a long time or stored in high humid conditions then yes wacky things can stop an amp from working. Even just wiggling the valves can bring it back to life.
Many times I've resolved problems like this just by the above.
If the amp still hums then yes you may need pro help to fault find.
Phil.


If you aren't checking that all power tubes have -VBias and what current they are at, it's not a safe test. Take it to a tech.

1) Don't nuke the Output transformer, Power tubes, or Power transformer. That's more expensive than taking it to a tech.
2) Don't turn on an amplifier without tubes in it unless you have it on a Variac and you are keeping voltages below the Max voltage all power supply caps are rated at. While Mesa Boogie is pretty good about using 500V Power supply caps throughout, Un-loaded B+ can be higher than 510V on amps that normally only see 470V.

So, maybe those things have worked for you in the past. If you were an amp tech you would eventually learn the hard way. You may even be faced with the moral dilemma of telling the customer whatever expensive part you blew up was blown up before you got your hands on it. Sorry for being harsh. The best advice is to take it to a tech or get a thorough education for yourself.
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Re: Mesa Boogie Mark IV problem

Postby phatt » 26 Nov 2019, 12:38

bmxguitarsbmx wrote:
phatt wrote:You can at least try simple basic (And Safe) test.

With the amp unplugged from mains simply remove and reinsert all the valves.
Valve pins/Sockets do get corroded over time and if the amp has been sitting for a long time or stored in high humid conditions then yes wacky things can stop an amp from working. Even just wiggling the valves can bring it back to life.
Many times I've resolved problems like this just by the above.
If the amp still hums then yes you may need pro help to fault find.
Phil.


If you aren't checking that all power tubes have -VBias and what current they are at, it's not a safe test. Take it to a tech.

1) Don't nuke the Output transformer, Power tubes, or Power transformer. That's more expensive than taking it to a tech.
2) Don't turn on an amplifier without tubes in it unless you have it on a Variac and you are keeping voltages below the Max voltage all power supply caps are rated at. While Mesa Boogie is pretty good about using 500V Power supply caps throughout, Un-loaded B+ can be higher than 510V on amps that normally only see 470V.

So, maybe those things have worked for you in the past. If you were an amp tech you would eventually learn the hard way. You may even be faced with the moral dilemma of telling the customer whatever expensive part you blew up was blown up before you got your hands on it. Sorry for being harsh. The best advice is to take it to a tech or get a thorough education for yourself.


Sorry if my comment upset you but maybe read it again as I Never said to power the Amp with no Valves.
Kind regards, Phil.
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Re: Mesa Boogie Mark IV problem

Postby okgb » 26 Nov 2019, 16:42

Often passive fx loops use a simple switching jack [ 1/4" ] that can get corroded , try cleaning the two jacks and use a 1/4" cable from the send to the return
if this doesn't work at least you may have prevented a future problem , doesn't hurt to clean the jacks & pots
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Re: Mesa Boogie Mark IV problem

Postby bmxguitarsbmx » 26 Nov 2019, 18:51

My apologies Phil. You are right, you didn't say to turn it on. From the wording of the OP, I'm assuming not much understanding of electronics, so I think the best advice is to take it to a tech.

As an amp tech, I could write several paragraphs on the stupid things people do tho their amps. But, dude! Just take it in :slap:
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Re: Mesa Boogie Mark IV problem

Postby phatt » 28 Nov 2019, 00:26

Hi BMX,
All good mate, yeah if in doubt with little knowledge one should take it to a shop.
It's always a problem when folks post a Q about these types of repairs as we have no way of knowing how well they can follow instructions.
Some Q's on forums I just don't answer if I feel there is a chance they might send themselves into the next life.

Maybe the forum could have a quick reference list written by qualified folks covering the basic safety checks, a list of thing to watch out for when Amps give trouble.
A *Read this First list* before asking for help with hi voltage amplifiers.
Cheers, Phil.
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Re: Mesa Boogie Mark IV problem

Postby bmxguitarsbmx » 30 Nov 2019, 01:54

Hey Phil. This forum does seem to be for stomp boxes, which are relatively safe to play with both in terms of nuking inexpensive parts and low risk of shock. The Music Electronics Forum has a lot of Professional repair tech members. I should have posted this link sooner.

https://music-electronics-forum.com/for ... y.php?f=22
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Re: Mesa Boogie Mark IV problem

Postby okgb » 30 Nov 2019, 20:37

Could give the op, some easy things to try, that don't require opening the amp, like do you get anything from the preamp out ? or can you run a preamp into the pwr amp in ?
does the speaker hum or hiss ? this would give the tech some heads up, and may save some time & cost. the fets doing switching in mesa is not uncommon to be the problem
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Re: Mesa Boogie Mark IV problem

Postby bmxguitarsbmx » 01 Dec 2019, 01:57

The obvious things to do are to send a signal in the input, see if it comes out the fx send. send a signal in the fx return, see if it comes out the speaker.

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