Problem with dual recto: cannot find a solution

[tomprs]

Hello,

I have a 2001 three-channel dual rectifier and a few weeks ago it suddenly lost 90 % of its volume on all channels. I brought the amp to my tech that told me the following:

"It is a very complicated issue … ;-) The muting buss mutes the signal for a fraction of a second in 3 different areas of the preamp at the moment you switch channels, the muting is done with Fet transistors. The system is only partially described on the schematic. I cannot find any documentation on the net regarding problems with the “muting buss” either. The problem with your amp was that the signal of all preamp channels stayed muted all the time. I did suspect the “muting fets” themselves (= often the problem) but it is not the case here, it is the signal that drives the fets that is posing the problem. As a last resort I finally did bypass the system, the amp then works again and the switching of the channels works fine … that’s all I can say.

Could you help us on this one? I don't know what to do and don't really want to rip the amp appart and spend 500 Euro in repairs.

Thanks a lot for your help!

Thomas.

 

[afu]

1) Call or email Mesa.
2) Does it still sound muted when the loop is bypassed?
3) Does it still do it when the loop is active and the channel is selected from the rear panel?
4) Does the Solo function work?
5) Is the Send knob up around Noon?

The main mute bus is divided into sub-circuits for the channel volumes, the output control, and fx send. The only other muting that would be on all channels is all inside the main fx loop circuit (send and return, solo). If you have some sound, the dry signal may be what you are hearing.

Just as a cheap chance at a solution, unplug the amp, get some contact cleaner or rubbing alcohol, wet a spare plug, and run it in and out of the send and return jacks a few times. It won't hurt to see if the jack thinks something's there when it isn't, because it's dirty.

 

[tomprs]

Thanks for the tips! I'll try the contact cleaner trick. I emailed Mesa a few weeks ago but received no answer so far ... it still sounds muted when the loop is bypassed.

 

[afu]

Hey, Tom. The jacks won't be the issue. I threw it out there in case everything worked with the loop off. The components around the jfet for the channel volumes would be likely if it's a muting issue. Alternatively, all of the jfets for muting are chained together to the ground point and all could be active for some reason. If they're getting a steady voltage, instead of a pulse, that would keep them active.