New Express 5:50 for $450! Reverb needs FIX!!

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fredfunk

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Just picked up my first Tube Amp - a MESA Boogie Express 5:50 for $450!

Great tone, perfect cosmetic condition, but the reverb circuit is not working... I checked all the connections, and tried both channels. I tested the reverb tank and it looks good (Black = 250 ohms, White = 1.2 ohms). I rolled the tubes (no problem with #5). Soooo.... it looks like it must be the FETs on the mute circuit. If anyone has a schematic, or experience please send!
 
Express550_schematic.png
 
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Thanks
Rich Doty said:
There is a service bulletin
Click to expand...

Thanks Rich, this is incredible information! I have not been able to find this information anywhere on the internet. Unfortunately, I have the Express 5:50 (non-plus)... not sure if this fix will work for my amp as well. You don't happen to have access to a similar service bulletin for the 5:50 do you?

Thanks again!!
 
Last edited:
I looked at the schematic that you provided, and compared it to the reverb circuit in the “+”. Both are the same. The FET in question is on the input to V5b. To test it you can clip the “Drain” lead half way down. See: https://www.mouser.com/datasheet/2/676/jfet_j174_j175_interfet-2888180.pdf to locate which lead is the Drain. If that does not resolve the issue you can restore with a solder bridge on that lead. The FET is not that expensive and you can get them at mouser. Because this is on the driver side there should be not impact as the other FET on the reverb recovery at the input to V5a will mute the circuit (basically the one that needs to “mute” the circuit.
 

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