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35670a advanced troubleshooting question

Question asked by rumreich on Oct 27, 2009
Latest reply on Oct 28, 2011 by zzzzaudio
My four-channel 35670a dynamic signal analyzer has two bad channels, channels 1 and 2.  Even with the inputs grounded, The half-level and overload LEDs for channel 1 are always on and the half-level LED for channel 2 flickers.  These channels are on separate A2 analog input input cards, so there's damage to both A2 cards. 

I've gone through the service manual block diagram and traced the points where the signal goes bad: the output of buffer U5 (TP7).  I pulled U5 suspecting it was bad and found that the voltage at TP7 is still the same: 9VDC.  This should be an input node, so there's something broken pulling this node to 9 volts.  The service data shows this node driving the half-range det, differential overload det, analog switch and 25 khz AAF.  Unfortunately the service data gives no circuit details for these blocks, so I've hit a dead end.

Does anyone have an actual schematic for this section of the A2 card?  Or is anyone aware of a common part failure for the A1 or A2 card?  Any suggestions on debugging/fixing this problem would be greatly appreciated.

Mark R