2:90 Not working on B channel

Triaxis, Quad Preamp, Stereo Power series, Recto Preamp

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Mark II
Posts: 61
Joined: Tue Sep 15, 2015 4:49 pm
Location: Gold Coast, Australia

2:90 Not working on B channel

Post by MesaMad1378 » Sat Oct 17, 2015 10:49 pm

Hi there all, well I had a problem with my 2:90 blue light, when I received it (before the voltage mod), thought I'd share what I did to repair it.
I was playing for about 3 or so minutes then I heard a pop and one of the cabs cut out. I checked the output fuses straight away and found the b channel fuse blown. I went and got some fuses, 1amp 3ag, and put it in, turned it on, waited for a minute for tubes to warm up, then flicked it from standby. I watched the tubes and immediately the top 2 on the b channel started to glow. I suspected the tubes were gone. I drove down to the guitar store, got 2 matched sets of Mesa 6L6 tubes (yellow, it had blue in it). I drove back and fitted them to the outer ( a class) positions, turned it on, let it warm up, flicked it off standby, watched tubes, same thing.
I got hold of a good, legible, schematic and proceeded to go through it trying to nut out what the problem was. I did some tests etc all over the board comparing readings and found the grid no.2 voltages, pin 4, on the top 2 tubes were not at around the -50vdc like the rest. I pulled all the tubes out and looked at all the sockets, some were stretched, maybe not connecting to tube pins properly. I closed them up so they were all tighter, refitted tubes, still no good. I tested resistors all around and couldn't get a good reading on R363(equivalent of R362, I think) which was 82k. So I sourced anothe resistor and changed it out, still no go. Tested the resistor, it tested fine. I went through the schematic again, did more tests. I came to the conclusion that the orange drop C20 (equivalent to C22, I think) was the culprit. I removed it and tested it. Orange drop shagged. I sourced some new orange drops, fitted and tested. Here goes, connected, tested all good, turned it up and away she went.
So what I think happened was during travels, the tube pins had stretched the sockets, therefore not connecting to the tubes properly, causing the capacitor to blow.
Any thoughts, please feel free to add

Cheers Brett
Mesa Dual Rectifier, TriAxis, Recto Recording,
2:90, 2:100, SUS-4 16ru Case,
Flux Five, Throttle Box EQ, 5 Band EQ,
Straight Traditional 4x12 Cabs.
And other stuff!!

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Mark II
Posts: 94
Joined: Fri Oct 24, 2008 8:28 am
Location: N.E. Pa.

Re: 2:90 Not working on B channel

Post by TOOLGUY » Mon Nov 02, 2015 4:35 pm

I had a bad A channel due to a shorted C16 orange drop.
I was told not to go chasing orange drops because they "usually don't go bad"...Low and behold after a long time searching...that's what it was.
Glad you fixed it.
Main rack
Stuido Pre
crate gs-4125 4x12 /
Randall RT412CX
Digitech gsp2101 dual cpu
Mesa 2:90
Fender hm-412a 4x12

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