Friday, December 7, 2012
A Case of Mesa Laryngitis
I had a Mesa 5:50 Express here recently with a bad case of laryngitis. It could barely muster a croak, but otherwise seemed in good physical condition. After verifying that it was not under warranty and that I would not therefore void the owner's warranty I put my mind in Boogie mode.
What's that, you say? Well, when working on a Mesa you've got to get used to the idea that it may be a little crowded inside, but that there is a procedure to get to where you want to be. That means patience, quiet, no distractions, and technical information readily to hand. That's a Boogie frame of mind-you've simply got to get in that zone or you could be wasting a lot of time. When you're 64 like me, you become a little more conscious about time and using it efficiently.
One of the marvels of our digital information rich age is this: if you have a consumer product-doesn't matter what it is, either-you can be sure that some person somewhere has had the same problem and has written about it or posted about it on a chat board somewhere. That, after all, is how I fought my Maytag built Amana refrigerator to a standstill despite not knowing a thing about refrigerators.
So, after digging around I located a schematic for this amp, and an excerpt containing the master controls and muting circuit which you see above. It's patented by the way, and you can pull up a copy of patent number 6,621,907 which describes "A mute circuit for momentarily inhibiting signal travel in the signal path having an input terminal and an output terminal in response to a operation of a coil operated relay."
And that's exactly what it does. The circuit momentarily mutes the signal when a relay is switching-in this case a channel switching relay.
I also found out through further research that the J175 JFET transistor used in these things is a frequent source of problems leading to amp laryngitis.
After removing the chassis, testing the tubes, looking for cooked resistors, and measuring voltages throughout I figured the muting circuit JFET was a component of interest. I ordered half a dozen figuring I'd need them some day or other.
The disassembly process is a little involved. You have to get access to the underside of the circuit board and this is done by removing the pots and most of the switches on the front panel. Of course you have to number the pots with a sharpie marker as well as where they go on reassembly. Then you have to tackle the circuit board. It uses plastic standoffs with spring tabs that have to be squeezed while the board is being lifted a little. Patience, patience. You'll get there without breaking anything. The muting JFET was permanently marked as such on the circuit board. It's best to have a fine soldering tip and a solder sucker readily to hand after you've snipped the leads. Doing this and pushing the stubs through with your by now asbestos fingertip avoids damage to the pads on the underside of the board. Soldering in the JFET is just as easy after you've cleaned the holes out, but I recommend letting the JFET stand tall as there's less possibility for heat damage with long legs, Daddy.
After reassembly the Mesa sprang to life, ready to go out and make people happy. Then, and only then, is it time to police the wiring job and push the board all the way down on its standoffs.
UPDATE: A confidential informant has told me that when replacing JFETs under warranty that they are not allowed to lift up the circuit board but to pull the JFET out through the top. I guess the boys at Mesa know what they're talking about, but as the legs of the JFET are bent over a bit before soldering in place, you run the risk of damaging the traces and pads by this rough treatment. It would be my bad luck to have this happen to me. Just my opinion, mind you.