Month: November 2017

PCB – Printed Circuit Board heat damage

In electronics heat is often The enemy.  In valve amplifiers there is a tension between this basic rule and the valves themselves which get mighty hot and also the established design methods for amplifiers.

The problem is that heat damage caused by power resistors and valves eventually changes the circuit board material from a good insulator to a partial conductor. This damage is not reversible.  

The burns do have to be quite 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.

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.

Steps

  • 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…

Amploft