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Mesa Big Block 750 - Problem

pstone_foo

pstone_foo

Apr 13, 2002
23
5
63
Ann Arbor, MI
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#1
Yesterday when I was playing my Mesa Big Block 750 amp, it started to cut in and out, and five minutes later it stopped putting out a signal out altogether. I have checked all the tubes, the one external and two internal fuses, and they are all fine. One observation is that the heat sink on the left side of the amp is VERY hot, and the one on the right side is cool to the touch. Anyone have any ideas? I have a bad feeling it needs to go back to Mesa for warranty repair. Thanks for your help.
 
#4
Here is what repair shop came back with as far as what is wrong with my amp:

"We looked at your unit and the output module is compromised, also there is a problem with the fuse #1 tang on the power supply board. We called Mesa Boogie to order parts/advisement. RD advised us they would like to have the unit sent back for warranty repair. You will have to pay to get the unit sent back to Mesa Boogie. They will perform the warranty repairs and return the unit at no charge."

The good news is that this happened (only a couple weeks!) before the five year warranty happened. I have to say that even though Mesa is fixing at their cost (I have to pay freight there), I am still a bit disappointed that such an expensive piece of gear would break down with so few hours on it. I love the tone and the flexibility of this unit, especially the distortion / tuner pedal, so hopefully the fix will keep it going for many years to come.

Hope this info helps other Mesa Big Block users!
 
#5
You're dissapointed that an amp would break down that's over 4 years old, and the manufacturer is still fixing it??? I don't get that, to be honest... After all, you've had it for 4 years, and all you've gotta do is pay shipping one way - I guess I don't understand the complaint...


- georgestrings
 
#6
I understand the disappointment the OP has. It seems to me that there have been way to many complaints about the Big Block 750/V12 amps cutting out or overheating. With the V12, there is an upgrade from Mesa that adds a vented top cover and another fan. So, obviously, they are trying to correct this issue. That's good customer service, but still disappointing. I've never had an issue with my V12, but should I still send it in as a precaution, or wait for the, apparently inevitable to happen? For such pricey gear, it is inconvenient and rather disappointing, IMHO.
 
#7
My recent experience:

I purchased a used BB750 because the marketing fluff claimed it would run at 2 Ohms.

It did not run at 2 Ohms. Mosfet "protection circut" kept engaging. (The unit ran fine and kicked booty at 8 and 4 Ohms).

Local Mesa-certified tech replaced the ENTIRE mosfet output stage at Mesa's request.

Amp would still not run at 2 ohms. Mosfet protection circuit.

I called Mesa again, and spoke to warranty repair. They laughed out loud over the phone when I told them I was trying to run the amp at 2 Ohms. They said the service dept. has other opinions than the marketing dept.

The amp is still under warranty, so It's currently in Petaluma. They are checking it over, and they said they can make it 2 Ohm stable by slightly reducing its output at 2 Ohms. (so it will not output the full 750. More like 620 and change...).

If you're still under warranty, I'd recommend sending it back to Mesa in CA. It will save you some time and BS unless you have a killer, killer Mesa amp tech near you. (I do not.)
 
#8
There is lots to read here on TB about Mesa customer service. On the plus side, they offer 5 year warranties on all their products and the warranty is fully transferrable. That alone is a very good warranty.

On the negative side, they do not respond to emails, you are responsible for shipping costs, the turn around time is not great and the folks at Mesa who answer the phone are civil but not exactly friendly.

A good solution shipping is to go through the dealer if they are willing to assist. My dealer is a large chain store and they covered shipping both ways for the recent overheating upgrade that was done to my M9 Carbine. My amp had never failed but I arranged to send it in as there are known issues with some of the first production runs of current models. Any Carbine made now comes with all the upgrades but i wanted mine taken care of before it failed as it was getting very hot during use.

I think Mesa customer service is decent and basically fair but they could earn a lot more fans by taking things up a notch.
 
#9
Crabby said:
I think Mesa customer service is decent and basically fair but they could earn a lot more fans by taking things up a notch.
Click to expand...

I think you're right. Once you get it to the service dept. in Petaluma they treat you right... but getting it there is the rub.

I too had to pay ($30) for them to ship me a box w/ form fitting styrofoam to ship my unit back for repair (since I bought used and didn't have original box). Problem was they sent the wrong-sized box so I had to re-pack anyway... whatever... no big whoop.

The transferrable warranty is key: I'd have been boned without it.
 
#10
fu22ba55 said:
I think you're right. Once you get it to the service dept. in Petaluma they treat you right... but getting it there is the rub.

I too had to pay ($30) for them to ship me a box w/ form fitting styrofoam to ship my unit back for repair (since I bought used and didn't have original box). Problem was they sent the wrong-sized box so I had to re-pack anyway... whatever... no big whoop.

The transferrable warranty is key: I'd have been boned without it.
Click to expand...

I know this is an old thread but for those who need it - The problem with the BigBlock 750 overheating on the left amp only is caused by bad send and return connectors for the effects. Replace the connectors (or short them out if you don't use them) and the issue is solved. It is caused by an open connection so near to the power amp speaker outputs and a phase relationship with the effects loop.
 
#11
Brian Street said:
I know this is an old thread but for those who need it - The problem with the BigBlock 750 overheating on the left amp only is caused by bad send and return connectors for the effects. Replace the connectors (or short them out if you don't use them) and the issue is solved. It is caused by an open connection so near to the power amp speaker outputs and a phase relationship with the effects loop.
Click to expand...
I think you might want to check with the service department on this, I don't think this is accurate advice.

It might solve an intermittent signal problem, but this has nothing to do with overheating.
 
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#13
TMARK said:
It was one of my favorite amps. The overheating was killing me. I replaced a bunch of Mosfets but ended up running it at 4 Ohms. I sold it but still miss it.
Click to expand...
This amp has a very basic fault. The jacks for the effects are right next to the audio output. When the effects jacks get dirty, they open their connections and the speaker output feeds into the effects in and overheats the left most heatsink (ultrasonic oscillation). Simple fix. Short out the connections on the effects jacks if you do not use them (Or use a shorting plug). A proper repair is to replace the old jacks with new ones. 100% solution. Fixes every one. A quick test is to plug a shorted 1/4" connector into the effects return. The overheating goes away immedeately.
 
#14
agedhorse said:
I think you might want to check with the service department on this, I don't think this is accurate advice.

It might solve an intermittent signal problem, but this has nothing to do with overheating.
Click to expand...
I am an Master Electronics Technician with 35+ years in the field and running my own store. I have seen the oscillation with my oscilloscope, affected the oscillation with my hand proximity to the effects jacks and corrected the problem with a shorting plug in the effects return. I have also shorted out the effects return and replaced the effects return jack, as per customer preference. It has fixed them all.
 
#15
@Brian Street You are very new here and possibly have no idea who agedhorse is. Your credentials are excellent but pale in comparison with this gentleman’s.

i agree that Effects Loops can cause a lot of spurious problems but overheating a power amp module is not one of them. I feel that when you “fixed” the problems in other amplifiers you actually fixed some other thing that was generating your overheating.
 
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#16
Brian Street said:
I am an Master Electronics Technician with 35+ years in the field and running my own store. I have seen the oscillation with my oscilloscope, affected the oscillation with my hand proximity to the effects jacks and corrected the problem with a shorting plug in the effects return. I have also shorted out the effects return and replaced the effects return jack, as per customer preference. It has fixed them all.
Click to expand...
I notice that the oscillation is at roughly 40kHz, way above your hearing (and mine). The amp is a bridged amp arrangement. The negative amp (Left most) is affected the most although some of the oscillation is present on the positive amp. (Right most) The heating issue is not affected by the volume (Bypasses the input amp) and is unaffected by control positions (Bypasses the tone control). The simple test is to insert a shorted 1/4" connector into the effects return. It never heats. Actually connecting equipment to the effects jacks also corrects the heating issue.
 
#17
BassmanPaul said:
@Brian Street You are very new here and possibly have no idea who agedhorse is. Your credentials are excellent but pale in comparison with this gentleman’s.

i agree that Effects Loops can cause a lot of spurious problems but overheating a power amp module is not one of them. I feel that when you “fixed” the problems in other amplifiers you actually fixed some other thing that was generating your overheating.
Click to expand...
No disrespect to agedhorse or to you Bassman Paul. I worked on Bigblock750 specifically. I have not seen this issue with any other guitar or bass amp. It is specific to the design of the Bigblock750. Yes, I am giving away a repair shop secret.
 
#18
Brian Street said:
No disrespect to agedhorse or to you Bassman Paul. I worked on Bigblock750 specifically. I have not seen this issue with any other guitar or bass amp. It is specific to the design of the Bigblock750. Yes, I am giving away a repair shop secret.
Click to expand...
I understand you are an engineer with Mesa and I thank Mesa for all their help and assistance over the years. Please confirm my information by testing the open connection by inserting a piece of paper or plastic into the short on the effects loop input jack. Oscillation begins immediately on the Bigblock750. I have confirmed on several units.
 
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#19
Brian Street said:
I am a Master Electronics Technician with 35+ years in the field and running my own store. I have seen the oscillation with my oscilloscope, affected the oscillation with my hand proximity to the effects jacks and corrected the problem with a shorting plug in the effects return. I have also shorted out the effects return and replaced the effects return jack, as per customer preference. It has fixed them all.
Click to expand...
Why do you need to short it out instead of just replacing it?
 

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