Mesa Boogie 0.50 Caliber (EL84)

This amp, unusual with its master volume control had become unreliable. Occasional red plating, bit of noise.

The cause is heat damaged circuit board. Just look at the pictures below showing the damaged sections – the dark areas indicating heat damage.

The problem is that heat damage eventually changes the circuit board material from a good insulator to a partial conductor. The burns do have to be severe mind you. You will usually see inner layers of fibreglass exposed, carbon particles. The only real cure is removal of the effected area. For small areas of damage the effected area can be drilled out but for larger areas a section may need to be cut out.

In this boogies case I took inspiration from a post on the boogie board where the owner replaced the section of pcb with hard wired chassis mounting sockets.


  • Add metal plate to chassis
  • Cut holes for B9A sockets and fit them
  • Remove section of PCB
  • Wire up new valve  sockets for connections to output transformer, screen supply, grid drive, cathodes and heaters.
  • Wire up PCB and compensate  for any circuit breaks following removal of damaged section.

During testing…

Mesa Boogie Dual Rectifier Problems

If you are having trouble with any of the following boogie symptoms then I may be able to help you out as I have spent more than my fair share of time on these metallic monsters.

Mesa Boogie amps are reveered by players for their unique sound. But for us Amp techs they can be a pain.  Not only do they use unique to Mesa Boogie components but they also choose to hide  circuit board markings so that diagnosis can be tricky they are helpful with schematics but they schematics are not easy to connect with the PCB in the amp as there are no component references.

Special Boogie Symptoms I have come across include:

Intermittent output, or rather occasional fading output.  This is a common problem in amps usually caused by an unreliable connection somewhere in the signal chain.  Usually such a fault will be either permanent or intermittent and respond to thump on the cabinet.  With Boogie it is extra annoying as the intermittent nature is temperature / time related and of course it will let you down when you really need it to be reliable.

Bias LED flicker

Bias light flicker – check the little red or green light on the rear when the output fades.  Sometimes the output fading and the light flicker coincide together and sometimes not.

Channel Switching Problems

Channel switching strangeness including flicker,  dim lights, no lights etc.  Seems more noticable on footswitch but that actually it is usually the same with the rear switch.