Marshall JCM2000 TSL/DSL Bias Drift disease – TSLitis

Amp repairs seem to go in waves.  The latest one being a string of TSL with what I call TSLitis.  This increasingly common condition causes these amps to overheat and do serious damage the amp.

The problem is that the amps circuit board material is not doing its job of insulating the connections properly and what happens is that valves bias drifts which in turn drives the valves harder overheating the valves  which causes destructive feedback loop.  All of this places a  strain on the power supply,  valves and supporting circuitry; all of the most expensive components in an amplifier.

The ones I have seen seem to be from around 2000 to 2002 and are quite easily identified.

Things to look for:

  • Amp misbehaving after an hour or two and excessive heat
  • Discolouration of the valve bases due to overheating.  This is usually visible only on the reddish brown bases where they go distinctly dark brown as they meet the glass envelope.
    A unique  electrical burnt Bakelite smell – see photo
  • The colour of the valve PCB (only visible by opening the amp and inspecting. Effected boards seem to be a light green colour and dark green boards
  • Red plating of the power tubes (that is the flat grey piece of metal you can see inside the glass envelope).  It is supposed to be grey but when over heated turns a “cherry”  red colour.
  • Amp transformer failure if left on for a prolonged period.
DSCF6071
Valve overheating – evidence on the base. Top good. Lower overheated.

 

DSL / TSL Power Amp boards - dark green good
DSL / TSL Power Amp boards – dark green good

 

 

 

 

 

 

 

So what is the cure?

Some folks are offering kits to remedy this   by separating the sensitive bias circuit from the main board.  This is quite a big change and I suspect will have reliability issues in the future.  It is best,  in my view at least, to replace the PCB all together.  It’s not as expensive as you may think and renews many other components effected  by age like filter capacitors, valve sockets etc