APPJ PA0901A Repair

[atnaat]

Unfortunately a shorted valve has sent an SMD component up in smoke in my APPJ PA0901A.

Currently the amplifier is

• Distorting on one channel driving speakers, however it drives small loads fine.
• Oscillating on one channel, which sounds like a beating heart, but only when the amplifier is turned off and back on after the valves have warmed up.
• When turned off the left channel fades away in a few seconds as normal and the right stays at mostly the same volume and plays with less distortion for several seconds.

I've attached a picture of the blown SMD component, the opposite component
has '431' printed on the top of it, Is is safe to assume this is an LM431? Also from what I've described above does it sound like the shorted valve has taken anything else out, should I be replacing other components as a matter of course when repairing it?

 

[famousmockingbird]

Maybe used as a current source at the cathode of the EL84? Could you trace where the leads go and draw up a schematic? Hard to tell what it does by the pictures alone.

 

[HollowState]

I suspect that you can simply replace the LM431 and the tube too of course. This is an adjustable precision zener shunt regulator. (Data sheet) Be sure to order the same SOT-23 case style. The other chip resistors near it look ok, but check them carefully anyway.