Arfa

Useless bloody thunderbird, I remember reading the original of this story
but cannot now find it or could you at least repeat the model number.
What was the failure in the board, could you tell whether over-current or
over-voltage, any deliberate track thinnings for tell-tales.


It's a Supersonic 100 head. The board failed to produce any 'controlled'
bias to the valves, and all four grids were at -89 volts. It looks as though
it works by measuring the voltage across four low value Rs in the cathodes
by using A-D inputs on the microcontroller on the board. The bias is then
produced by feeding PWM signals from four port pins to driver transistors
followed by filter caps. In the case of this board, the supply volts, reset
pin etc were all at correct levels, but the micro did not produce any
output. You could not even measure any clock at the xtal pins. No evidence
of any cause was visible. Board was pristine. Double sided surface mount,
and all examined under strong magnification, but no signs of any damaged
components.

Arfa