Where can I find fuses for a Mesa Boogie Mark IVB?

[playingitloud]

Yesterday, I had noticed a plastic burning type of smell while playing my MKIV then today a power tube went red.

Once I had noticed that a power tube started glowing then I turned off the amp.

I had let it cool down long enough so that I could readjust the power tube positions (I had recently moved the MKIV from one rack to another rack case and I figured maybe the tubes might have moved out of the socket a little bit).

I had turned on the amp again and waited a few minutes then turned standby off and the amp shut down (probably blew a fuse).

Does anyone know where to order replacement fuses for the MKIV?

The manual says:

The FUSE should be a 3 amp Slo-Blo for use with 100 or 117 volt A.C. Mains.

However, the back of the amp says 4 AMP S.B. so I am a little more than confused here.

Thanks ahead of time for any help!!

 

[user 41014]

I would pull that chassis and look for evidence of burning. I just checked a Mark IV-A I have and it says 3A for the fuse.

Is the Mark IV-B different?

The IV is notorious for 3 brown caps that stand up fairly large on the board to leak and damage the preamp board they sit on. If they are 10,000uF sprauge they should preemptively changed. If they are 8200uF they are not known to leak. Mesa has turned away repairs if the leaks are particularly damaging.

https://store.mesaboogie.com/products/fuses-250-volt-slo-blo-1-4-x-1-1-4-6-3-mm-x-32-mm.html

 

[bandit2013]

I saw your other post. I thought it was a 4 amp fuse as I would have had the 3A in my stash of stuff.
You can order fuses from digikey. I use them for work all the time. There is also Mouser electronics.

Fuse size is 3AB/3AG rated for 250VAC, 3A slo-blow. I am not sure what the melting point should be most have a 200 I2T rating.

https://www.digikey.com/en/products/detail/bel-fuse-inc/3SB-3-R/1009736
https://www.digikey.com/en/products/detail/littelfuse-inc/0313003-HXP/777261
The pricing for the Mesa fuse is not bad for 5 fuses. Shipping may cost you more than that.

 

[playingitloud]

I would pull that chassis and look for evidence of burning. I just checked a Mark IV-A I have and it says 3A for the fuse.

Is the Mark IV-B different?

The IV is notorious for 3 brown caps that stand up fairly large on the board to leak and damage the preamp board they sit on. If they are 10,000uF sprauge they should preemptively changed. If they are 8200uF they are not known to leak. Mesa has turned away repairs if the leaks are particularly damaging.

https://store.mesaboogie.com/products/fuses-250-volt-slo-blo-1-4-x-1-1-4-6-3-mm-x-32-mm.html

Thanks for the reply.

The 4A will be the correct one for my Mark IVB (I had taken out the fuse last night and read the small inscription).

It looks like Mesa Boogie has used 3A on some amps and 4A on others.

I am pretty concerned about the burning smell so I will need to take the chassis out later this week and check to see if there is anything that was obviously burnt/melting inside before calling a local Mesa Boogie repair guy (about 30 minutes away).

I hope the amp is fine since it is an exceptionally mint condition amp :-(

 

[playingitloud]

I saw your other post. I thought it was a 4 amp fuse as I would have had the 3A in my stash of stuff.
You can order fuses from digikey. I use them for work all the time. There is also Mouser electronics.

Fuse size is 3AB/3AG rated for 250VAC, 3A slo-blow. I am not sure what the melting point should be most have a 200 I2T rating.

https://www.digikey.com/en/products/detail/bel-fuse-inc/3SB-3-R/1009736
https://www.digikey.com/en/products/detail/littelfuse-inc/0313003-HXP/777261
The pricing for the Mesa fuse is not bad for 5 fuses. Shipping may cost you more than that.

Yes, it a 4 amp fuse and I'll order a batch of them so I won't need to look around for them at a later time

Thanks again as well.

 

[bandit2013]

That burning smell may have been the screen resistors getting hot. It could also be the OT transformer heat soaking. Once a tube goes into runaway condition many parts will begin to overheat. That is the typical smell you get with the Red Plate takes over. The glues that secure the tube into the plastic base also starts to reflow and the plastic base will take a thermal hit. It more or less was the tube materials outgassing due to excess heat in the tube. If it smelled really bad to the point you saw some smoke coming out of the amp, that would be the screen resistors failing. Had that happen with the Mark V90. If you know what a phenolic plastic burning smell is you would recognize it right away, if not, it just smells really bad, worse than hot plastic smell. Has a sweet sic order to it, almost like burning fiberglass but worse.

 

[playingitloud]

That burning smell may have been the screen resistors getting hot. It could also be the OT transformer heat soaking. Once a tube goes into runaway condition many parts will begin to overheat. That is the typical smell you get with the Red Plate takes over. The glues that secure the tube into the plastic base also starts to reflow and the plastic base will take a thermal hit. It more or less was the tube materials outgassing due to excess heat in the tube. If it smelled really bad to the point you saw some smoke coming out of the amp, that would be the screen resistors failing. Had that happen with the Mark V90. If you know what a phenolic plastic burning smell is you would recognize it right away, if not, it just smells really bad, worse than hot plastic smell. Has a sweet sic order to it, almost like burning fiberglass but worse.

I had received the fuses today (thanks again to each of you for the links), and I have replaced the power tube pair where one had red plated.

The amp sounds great again with no red plating or blowing fuses or a burning smell so I am assuming the issue was related to the red plating power tube.

Also, I did open up the head chassis and looked at the inside.

I did find there is two 10,000uF brown caps but they don't say Sprauge on them, however, they do look like the ones everyone says to replace.

I had checked around the base and they were clean so it doesn't look like any leaking has started to happen yet.

I will be getting them replaced so that I know it should have many years left if I decide to keep the amp or sell it later.

Thanks again @electric mayhem and @bandit2013 for the help and advice!!

Happy Holidays!!

 

[user 41014]

I had received the fuses today (thanks again to each of you for the links), and I have replaced the power tube pair where one had red plated.

The amp sounds great again with no red plating or blowing fuses or a burning smell so I am assuming the issue was related to the red plating power tube.

Also, I did open up the head chassis and looked at the inside.

I did find there is two 10,000uF brown caps but they don't say Sprauge on them, however, they do look like the ones everyone says to replace.

I had checked around the base and they were clean so it doesn't look like any leaking has started to happen yet.

I will be getting them replaced so that I know it should have many years left if I decide to keep the amp or sell it later.

Thanks again @electric mayhem and @bandit2013 for the help and advice!!

Happy Holidays!!

You're welcome. There are some 10k brown IC ones in addition to the brown sprauge ones. I think you are good if it is those and definitely good news on no leaking.

Do you have a meter or continuity tester? If so and you had redplating, I would check the 2 orange drop capacitors by V5 near the power tubes for shorts. Those are the coupling caps for the bias circuit.

Enjoy the holidays as well!

 

[DrKev]

I would not simply replace the fuse and continue troubleshooting yourself. Fuses should never blow on their own accord. Always assume a blown fuse is indicative of fault and something somewhere needs to be diagnosed and replaced. An experienced amp tech can do that without risking further damage to the amp or expense in repairs.

 

[user 41014]

I am finding varistors bad at a high rate. With the fuse having blown I would check and replace if need be. It is about the size of a dime and can be red blue or black and will either be in the area of the fuse or on the power supply board with the big capacitors. Varristors protect the amp from transient voltage spikes and having lost a fuse is/are worth checking.

By all means have someone look at it if you are uncomfortable doing it. Opening up the amp and looking is half the battle many times. I have found problems well over 50% of the time visually.

If you think you were redplating you might want to poke the screen resistors right over top the power tubes just to make sure they don't crumble. They are the fat brown carbon composition resistors and a couple of them will have red-purple-red-(silver) stripes. There will be a couple smaller brown resistors for the heaters. I'd poke those too.