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 Post subject: Mark V:90 - GEQ Stuck On
PostPosted: Tue Jan 16, 2018 3:08 am 
Bottle Rocket

Joined: Fri Jan 12, 2018 3:52 am
Posts: 2
Hi folks!

I play my Mark V:90 channel 3 with GEQ on 95% of the time, but I recently realized that, when switching to other channels, the GEQ was definitely on despite being set to off for the other channels.

Whether switching GEQ off on the face of the amp, using the footswitch, or using an external switch, it always seems like the GEQ is activating/deactivating- lights changing etc- but it never actually turns off, since the sliders always affect the sound.

After looking over schematics, I'm pretty certain this is a stuck JFET in the GEQ circuit.

Obviously I'm not working on this bad boy myself- replaced a screen grid resistor and it was harrowing enough.

Silly question, but anyone found any success in "un-sticking" something in this situation? I'm waiting on the local repair place, but due to my location in the world, it's pretty slow...


Last edited by The Casual Doctor on Wed Jan 17, 2018 6:58 am, edited 1 time in total.

PostPosted: Wed Jan 17, 2018 3:10 am 
Mark III

Joined: Tue Nov 08, 2011 10:48 am
Posts: 270
Hey there, some time ago I had a talk with a mesa service tech and he mentioned that topic for the V. It seems those disfunctional JFETs in the GEQ area are known. I think you are straight up to the cause of your problem - remember those JFETs are self-conducting. Good luck!

PostPosted: Mon Feb 05, 2018 7:07 am 
Triple Recto

Joined: Fri Feb 22, 2013 7:31 am
Posts: 3066
Location: North Carolina
The J175 FETs are P-channel type. Zero volts on the gate will drive the FET into conduction. Mesa uses a 12V supply to hold the gait and turn off the FET. There is one JFET that is used to turn on and off the GEQ, labeled as J175EQ. If you can find it, best to confirm the gate voltage (amp needs to be turn on but can be left on standy) with the GEQ switches set to off, there should be 12V on the JFET gate. If not, it could be somewhere else causing the issue. What some techs seem to do is replace all of the JFETS even if they are good. More work than needed. May just be a loose ribbon cable or solder joint on the pull-up resistor.

Current amps: TC-100, TC-50, JP-2C, MK V, Roadster, RA100

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