# MB Dual Rectifier Roadster



## Paul Running (Apr 12, 2020)

What was the design philosophy for this model? If you have service experience with this model, I would be interested to learn of your opinion.


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## Ti-Ron (Mar 21, 2007)

All I can say is GOOD LUCK!
A friend of mine is an amp tech and refuse to do work on them. NOt saying that he is right but, he said they are nightmare to work on.


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## nonreverb (Sep 19, 2006)

Paul Running said:


> What was the design philosophy for this model? If you have service experience with this model, I would be interested to learn of your opinion.


J175 FET's are a typical failure in all Mesa's. They go from time to time....As a matter of fact, they also go in a few of the newer Marshall amps as well although those are usually N channel J113's.
I don't know why all the fear about Mesa's. I've been their warranty guy here for 10 years and have not had massive issues with them as a rule.


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## Paul Running (Apr 12, 2020)

nonreverb said:


> J175 FET's are a typical failure in all Mesa's


Rich, do you know if MB has investigated these failures? I have not familiarized myself with their specific application...are they using them as amplifiers or switching? I'm wondering about the failure mode for this device. I will admit that authentic schematics for some of their models are not easy to obtain.


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## nonreverb (Sep 19, 2006)

Paul Running said:


> Rich, do you know if MB has investigated these failures? I have not familiarized myself with their specific application...are they using them as amplifiers or switching? I'm wondering about the failure mode for this device. I will admit that authentic schematics for some of their models are not easy to obtain.


They J175's are almost exclusively used as clamping FET in the switch matrix of their amps. Hence why the signal drops out when they fail. They did away with the old-style LDR's some time ago. I guess it's a cost-effective way of doing things and it works fairly well as they usually make it through warranty before they fail.


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## Paul Running (Apr 12, 2020)

nonreverb said:


> They J175's are almost exclusively used as clamping FET in the switch matrix of their amps. Hence why the signal drops out when they fail. They did away with the old-style LDR's some time ago. I guess it's a cost-effective way of doing things and it works fairly well as they usually make it through warranty before they fail.


Economics has never been a strong topic with me. I remember when I was working at Leigh, I came across a MOSFET device that failed regularly, in a switching operation. I made a suggestion to product engineering of a solution to reduce field-failures. Basically they weren't interested, as long as it surpassed warranty, all was fine with them. I played their game however, it can get to you if you allow it to rent space in your head...a question of ethics.


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## Latole (Aug 11, 2020)

I do some work on two MB few years ago. I hate them.


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## nonreverb (Sep 19, 2006)

Paul Running said:


> Economics has never been a strong topic with me. I remember when I was working at Leigh, I came across a MOSFET device that failed regularly, in a switching operation. I made a suggestion to product engineering of a solution to reduce field-failures. Basically they weren't interested, as long as it surpassed warranty, all was fine with them. I played their game however, it can get to you if you allow it to rent space in your head...a question of ethics.


I remember in ancient times when I was a computer monitor tech (when monitors were still CRT). We'd get certain models in where the switching power supplies would always go south....and with the same failure every time. Placing electrolytic caps near anything that generates sufficient heat and you can guaranty failure every time....after the warranty runs out of course. Economics at work.....


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## jb welder (Sep 14, 2010)

Fet's seem to be fairly common failures in muting and switching circuits across all brands. Not sure the exact reason but they seem to be more delicate than bi-polar devices. The oldest units I can think of where these circuits had fairly common failures would be Traynor 6400 mixers (mute Fet's). So that is going back a fair bit of time already.

The big deal with Mesa is not so much the complexity as the lack of serviceability, in my opinion.
Kind of like having to 'pull the engine to change the spark-plugs' in the automotive world.
There are mesas with relays stacked on top of eachother 3 or 4 high. And so many connections hard-wired so you have to unsolder everything to pull a board (or detach each and every pot).
A lot of techs (I for one) prefer the troubleshooting to the actual physical process of the repair. So I'm not interested in fixing them as it is too time consuming, relative to other brands. If I had a helper where I could find the fault, and let them do the part replacement, I'd be all in. For those who work on them, it is definitely rewarding financially.


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## nonreverb (Sep 19, 2006)

jb welder said:


> Fet's seem to be fairly common failures in muting and switching circuits across all brands. Not sure the exact reason but they seem to be more delicate than bi-polar devices. The oldest units I can think of where these circuits had fairly common failures would be Traynor 6400 mixers (mute Fet's). So that is going back a fair bit of time already.
> 
> The big deal with Mesa is not so much the complexity as the lack of serviceability, in my opinion.
> Kind of like having to 'pull the engine to change the spark-plugs' in the automotive world.
> ...


I agree the older LDR type switches were a PITA to replace. That said, the newer versions rarely have relay problems, it's the muting FETS which are easy to replace. Mesa's boards are robust enough (probably for this reason) to remove components from the top of the board without having to pull everything apart.
PS - Correction on my part: It's a muting FET not a clamping FET. Not proof reading my posts....my bad.


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