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Non ADA Gear => Amps => Topic started by: Soloist on February 07, 2018, 02:48:01 PM

Title: Mesa 50/50 lo power issue
Post by: Soloist on February 07, 2018, 02:48:01 PM
Hey all,
since I re-configured my rig I thought I would give the 50/50 another shot. First off it sounds great, way better than before. I simplified my rack a bit so the gain staging is much easier. The issue I am having is on lo power it sounds like a blown speaker on one side. I swapped the speakers leads and it moved to the other cab. Ok the speaker isn't the issue. It doesn't do it on hi power, so what do I need to do to fix the lo power setting?
Title: Re: Mesa 50/50 lo power issue
Post by: Systematic Chaos on February 07, 2018, 05:03:23 PM
I’ll send you the orig Boogie service schematics when I get home. I had the same issue with my old 50/50 in 2009 and the culprit was a faulty resistor in the fixed bias circuit.

*edit* here’s the doc: https://www.dropbox.com/s/tieq9t4izk4dz3c/5050%2BMod.pdf?dl=0 (https://www.dropbox.com/s/tieq9t4izk4dz3c/5050%2BMod.pdf?dl=0)
Title: Re: Mesa 50/50 lo power issue
Post by: Soloist on February 08, 2018, 11:16:26 AM
Thanks SC!  I figured you or MJMP would know. Had a feeling it was a resistor I just didn't know where. :bow:
Title: Re: Mesa 50/50 lo power issue
Post by: Soloist on February 08, 2018, 11:58:05 AM
Ok SC, after looking over the schematic I see 4 resistors in the fixed bias circuit.
R322
R201 10k
R241 47k
R242 68k
Any idea which one I need to replace? I was hoping that you remembered which you replaced.
Title: Re: Mesa 50/50 lo power issue
Post by: Systematic Chaos on February 08, 2018, 05:17:08 PM
IIRC it was R242 on mine. If I were you I take a MultiMeter and measure them all. Also measure R229 (220k, parallel to the Hi/Lo Pwr Switch).
Since I had my PCB out back then I ended up replacing all component with new and higher qual/spec ones ;-)
Title: Re: Mesa 50/50 lo power issue
Post by: MarshallJMP on February 09, 2018, 03:44:57 AM
From what I can see, the hi/lo switch doesn't change anything to the bias circuit, or am I missing something? It just seems to lower the voltage to the phase inverter.
Title: Re: Mesa 50/50 lo power issue
Post by: Systematic Chaos on February 09, 2018, 04:57:53 AM
Mine made a crackling sound on one channel when switched to LoPwr. Everything was fine on HiPwr. After I replaced R242 (which was failing) everything was sweet again....
Title: Re: Mesa 50/50 lo power issue
Post by: MarshallJMP on February 10, 2018, 02:30:56 AM
Mmm strange  :o
Title: Re: Mesa 50/50 lo power issue
Post by: Systematic Chaos on February 10, 2018, 05:24:10 PM
Here's a doc I compiled with some more how-to info including pics on the modification (see attached
Title: Re: Mesa 50/50 lo power issue
Post by: Soloist on February 25, 2018, 08:26:57 AM
Ok, just got my 50/50 back from the shop here was the problem:
Blown grid resistor in channel A, which caused the 3 remaining grid resistors values to stretch out of spec to 500 ohms.
Replaced all 4 grid resistors with 470 ohm , 5w carbon composite resistors.
Now that it is properly fixed, need to tweak all my patches,  since they were made with the mesa being f#&$ed up. Thanks for the assistance guys :thumb-up:
Title: Re: Mesa 50/50 lo power issue
Post by: Soloist on February 27, 2018, 06:15:29 AM
....and a big thanks to SC. I had the deep mod installed while it was in the shop. :headbanger:
Title: Re: Mesa 50/50 lo power issue
Post by: Systematic Chaos on February 27, 2018, 05:31:05 PM
Yeah, the old 50/50 with the Mesa Deep Mod sounds boss :thumb-up: