Hi All
I just burned out the 90.9 r resistor r1 on the d side of the board in a moment eerily like H bonilla's below.
I tested the a side and it was functioning fine . When I went to do the d side I shook a little on the test probe and shorted the IA wire to the b terminal on the MJE 5731 . I am guessing that this was the cause because the smoke escaped shortly after that . Cannot find any errors in the diode string . Does a shorted transistor cause a burnout of r1 ? In which case I need a new c4s board I guess ......