Mark III+ blowing fuses

Mark I's, II's, III's and the almight IV

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Ratou
Bottle Rocket
Posts: 16
Joined: Tue Apr 01, 2014 9:34 am

Mark III+ blowing fuses

Post by Ratou » Tue Dec 18, 2018 11:49 am

Hi all,

Thanks in advance for your help on this as I've been trying to figure this out for a while.

I have a Mk III that has been serviced (new caps) and modded to III+ specs by Mike B a few years ago (2016 I believe, was done by previous owner). While it sounds very good, I have had problems with it blowing fuses consistently after being played for any slightly extended period of time (ie. more than 30 mins). I have tried to troubleshoot the cause to the best of my ability but have not been able to find the issue. I even brought the amp to a very good Mesa/Boogie tech in London (John Kelly) who said it's one the cleanest and most silent Mk IIIs he's seen and wasn't able to replicate the blowing fuse even after leaving the amp on for prolonged duration on his bench. I brought it back home and blew a fuse an hour later...

Symptoms:

-Power transformer gets very hot when amp is played (too hot to touch). Called Mesa on this and the guy I talked to on the phone said it's normal and that I shouldn't worry. Also, the local tech I brought it too said the trannys where fine, he completely checked the amp with a scope and didn't find anything. The reason I mention this is that the fuse holder above the PT (because of PT heat?) gets also very hot (again, can barely touch it) so was thinking maybe the high temps where causing the fuse to be unstable or something?

-Amp blows fuses even with power tubes unplugged. I've left the amp with only the power chord and speaker plugged in (with and without power tubes) and it still blew a fuse after a while.

-It's a 110V amp so I use a step down transformer. I use a very high quality one (rated for 1000+ watts) that has been working very well with other 110V Mesa's (Mark II and Mark IV). They never blew a fuse even after being left on for multiple hours.

Again, thanks a lot for any help on this. I really like this amp and would like to gig with it but unfortunately I've been struggling with this mysterious fuse issue...

lovetoboogie
Mark II
Posts: 119
Joined: Tue Mar 25, 2014 10:14 pm
Location: New York City

Re: Mark III+ blowing fuses

Post by lovetoboogie » Tue Dec 18, 2018 2:46 pm

Are you using a 3a Slo-Blow fuse?

Couple of things...

A Mark III power section is super efficient and that power tranny should NOT be getting that hot. I certainly don't mean to discount Mesa's advice or your tech that it's fine. There is a difference between a PT getting hot and one that is too hot to touch. If the amp is biased conservatively which it should be then the circuit is pulling too much current elsewhere....

You might want to take an AC voltage measurement off of the stepdown transformer and see what it's actually feeding the amp.

Ratou
Bottle Rocket
Posts: 16
Joined: Tue Apr 01, 2014 9:34 am

Re: Mark III+ blowing fuses

Post by Ratou » Wed Dec 19, 2018 4:25 am

Thanks for your reply and advice

Yes I'm using 3A slow-blow (same ones as I use in other Mark amps).

Agreed on the PT, it doesn't feel right that it gets that hot. I've been suspecting this is related to the blowing fuses as they blow only after a while and not directly when I switch the standby off or something. To me this could mean that it happens once the amp reaches a certain temperature.

Will measure the output of the step down and see what it says

jrb32
Mark III
Posts: 201
Joined: Wed Jan 31, 2018 8:32 am

Re: Mark III+ blowing fuses

Post by jrb32 » Wed Dec 19, 2018 5:20 am

Grab a multimeter and check the 2W resistors on the output valves (they're the big 4 colour band ones, easy to spot).

Should be 4x/2x 470 Ohms for 100W/60W and 2 x 470 Ohms + 2 x 2.7K Ohms for Simul Class.

These are hard to spot sometimes when they blow and can look normal. A lot of people miss this when checking the amps. Check their values match up and aren't blown or way out of tolerance. Every single Mesa I've ever had, has had these blown or out of tolerance. If one comes in that hasn't I would replace them with high W wirewound as a precaution

Did John Kelly use your step down transformer or one of his own? Just saying that could be the one thing that hasn't been ruled out yet. Weird it's fine on a Mark II though so I don't think it's that...

The other common things are a 680 Ohm 2W resistor on the board (surrounded by 4x smaller electrolytic axial capacitors) that always blows and should have been replaced with a higher W wirewound resistor as maintenance.

Apart from that check bias supply voltages and all diodes. Also do overall sweep of the board and check for anything that looks black or burnt or traces of anything blowing.

Check all the above and then try new V5 and power tubes, but I'm sure this has been done already.

Good luck - should be fine for an amp tech to check these things or yourself with a simple multimeter if you've used one before? With amp OFF and UNPLUGGED, filter caps discharged (turn both standby and power on switches on after amp is disconnected from mains, wait about 20 minutes to be absolutely safe. Double check voltage with multimeter (see below) to check voltage stored on filter caps before continuing. If in doubt you can use a wire to short the caps to ground (chassis) permanently before continuing)

Regarding the voltage checks:

I'd recommend not doing any voltage checks on the amp unless you know what you're doing, but it is easy to do. I normally stick one of the mounting screws back screwed to the chassis, clamp the negative black multi-meter probe lead to that, set multimeter to DC voltage mode, turn amp on (both standby and power) and with one hand behind my back carefully probe the positive side of the filter caps with the red probe. The voltages are about 350-470V DC so BE CAREFUL as these can kill you! Don't wear a necklace or put your whole arm or head in or anything... You can get an idea of what voltage is being fed to the amp at least that way. Again if in doubt get an amp tech to check this for you and report back.

If ALL of the above has been ruled out, then it could be a bad power transformer...

Not sure which part of the UK you're in but give me a shout if you're stuck and in the South East.

Thanks,

Jon

Ratou
Bottle Rocket
Posts: 16
Joined: Tue Apr 01, 2014 9:34 am

Re: Mark III+ blowing fuses

Post by Ratou » Wed Dec 19, 2018 6:40 am

Thanks a lot your help and insight!

I'll perform all these checks and will revert

PS. I'm based in London

Ratou
Bottle Rocket
Posts: 16
Joined: Tue Apr 01, 2014 9:34 am

Re: Mark III+ blowing fuses

Post by Ratou » Thu Dec 20, 2018 4:43 am

So done some measurements last night:

-Output of the transformer is 125V - 50Hz
-All the resistors test very close to their value. One the 470 Ohms on the power tubes tests 505 but I wouldn't think that it's far enough to cause issues?

I replaced all the the tubes with ones I know work properly and left the amp on out of it's chassis with only power and speaker plugged in. What happened is that the temperature of the PT rose steadily until it got extremely hot (maybe could fry an egg on it at one point, stayed hot even a couple after amp was off) and after a 1 and a half the fuse popped. At this point I'm pretty sure that the issue comes from transformer overheating.

Could the 125V - 50Hz be too much for the transformer? Read somewhere that the 50Hz could cause more strain on a PT designed for 60Hz. The amp tech that looked at the amp had his own transformer (not sure what he used) so that would one difference between his set up and mine (he used the same tubes, my fuses etc), maybe he had one where he could control the voltages exactly?

The Mk II that I run through this transformer has a much bigger PT (105) so maybe its tougher and can handle a wider range of inputs?

Other possibility is that the transformer is damaged somehow but again, the tech fully tested it and said it was ok

jrb32
Mark III
Posts: 201
Joined: Wed Jan 31, 2018 8:32 am

Re: Mark III+ blowing fuses

Post by jrb32 » Thu Dec 20, 2018 4:51 am

If you live in London, go back to John Kelly and take your stepdown transformer with you. Get him to try with both and check the temperature of the transformer with both stepdown transformers.

That way if it gets really hot with one and blows a fuse, but it doesn't with the other, then obviously there is some kind of weird incompatibility going on with that stepdown transformer.

If not then it'll be the transformer.

Cheers,

Jon

Ratou
Bottle Rocket
Posts: 16
Joined: Tue Apr 01, 2014 9:34 am

Re: Mark III+ blowing fuses

Post by Ratou » Thu Dec 20, 2018 5:59 am

Yeah I might have to do that, will try to to arrange something when I have some free time. Thanks for your help in any case!

lovetoboogie
Mark II
Posts: 119
Joined: Tue Mar 25, 2014 10:14 pm
Location: New York City

Re: Mark III+ blowing fuses

Post by lovetoboogie » Thu Dec 20, 2018 10:20 am

You will need to get that AC down...particularly at 50hz. Just not enough iron in the Mark III PT core to handle it as it seems. The heat and the fuse problem are your clue...

You might want to consider getting an export PT in there. Reach out to Mesa and check the cost.

Ratou
Bottle Rocket
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Joined: Tue Apr 01, 2014 9:34 am

Re: Mark III+ blowing fuses

Post by Ratou » Fri Dec 21, 2018 10:03 am

Yeah agreed, is there any device I could use to control the voltage coming out of the transformer? What AC voltage would be ideal for an amp like this?

Not sure if I should be running the Mark II using this transformer now, even if it's runs fine I'm worried it could damage the PT over time

lovetoboogie
Mark II
Posts: 119
Joined: Tue Mar 25, 2014 10:14 pm
Location: New York City

Re: Mark III+ blowing fuses

Post by lovetoboogie » Fri Dec 21, 2018 12:30 pm

Just to play it safe you would like to see 110-117vac max with a high quality step-down.

Tripp-Lite and Seven Star are good brands in the US for tube gear and other sensitive equip.


https://www.bhphotovideo.com/c/product/ ... _down.html

woodbutcher65
Mark III
Posts: 197
Joined: Fri May 04, 2018 10:36 pm

Re: Mark III+ blowing fuses

Post by woodbutcher65 » Wed Jan 23, 2019 7:21 am

I don't think that voltage difference is enough to account for the amp running THAT hot.


Try this: Pull out all four power tubes. And then run the amp. Of course you'll get no sound. If the fuse blows with no power tubes installed, then that tends to indicate that you have a power transformer that has an internal short in it and that will have to be replaced.

Is the output transformer staying cool?

Ratou
Bottle Rocket
Posts: 16
Joined: Tue Apr 01, 2014 9:34 am

Re: Mark III+ blowing fuses

Post by Ratou » Wed Jan 23, 2019 12:13 pm

woodbutcher65 wrote:I don't think that voltage difference is enough to account for the amp running THAT hot.


Try this: Pull out all four power tubes. And then run the amp. Of course you'll get no sound. If the fuse blows with no power tubes installed, then that tends to indicate that you have a power transformer that has an internal short in it and that will have to be replaced.

Is the output transformer staying cool?
I had already tried that and the amp behaved in the same manner: got hotter and hotter until the fuse popped.

The OT doesn't get nearly as hot as the PT but to be honest haven't looked at it too much as was focusing on the PT.

I bought a 'brownbox' that allows you to fine tune the voltage (plugs between the wall and the amp, between wall and step down in this case), I've been using it with the IIC+ with voltage set a 117V (instead of 125V out of the wall/step down) and PT seems to be running cooler at first glance. Haven't tried it yet with the III but will try tonight.

But as you say I'm a bit worried that it won't do the trick and that there could be damage to the PT...

woodbutcher65
Mark III
Posts: 197
Joined: Fri May 04, 2018 10:36 pm

Re: Mark III+ blowing fuses

Post by woodbutcher65 » Wed Jan 23, 2019 6:24 pm

That doesn't sound good for the power transformer. Do you by chance have a clamp-on ammeter or know someone who does? Ideally it'll be the kind that can read DC amps, too.

I have an Extech that does this. I'd measure the current from the PT to the power supplies on each lead. If they're all in spec then it'd have to be the transformer at fault.

If current draw on any lead is excessive, there's something shorting in the power supply or down stream from it.

With the amp unplugged and caps bled down, check the coupling caps and see if you have any shorted ones. I'd particularly want to take a look at the output coupling caps coming off the phase inverter, which should be a pair of 0.1 uF orange drops. I've seen some orange drops fail and when they do they always short. In that part of the circuit it results in red plating and high bias current draw.

lovetoboogie
Mark II
Posts: 119
Joined: Tue Mar 25, 2014 10:14 pm
Location: New York City

Re: Mark III+ blowing fuses

Post by lovetoboogie » Thu Jan 24, 2019 3:16 am

On the heels of Mesa doing the upgrade, all info provided points to PT core saturation with that elevated AC particularly at 50 cycles...It's beyond my pay grade but it has to do with the magnetic material in the PT and flux lines. Some power transformers are just marginally rated to begin with. It happens often. The heat, blowing fuses, even with secondary unloaded. The difference between 110v, 117v and 125v AC is significant at 50 cycles. Get the AC down to 110 and you should get some relief if the damage to the PT core hasn't been done. I can only "assume" you are getting adverse results at home because your techs location has a lower line voltage.

Mesa Boogie has specifically advised against Step Down transformers and there is probably a good reason. You still might want to consider an export PT. It would be cheaper than another Step Down x-former or the brown box, etc...

But please let us know how you make out. This will be helpful info for the forum!

-J

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