R&E » Keithley 2001 repair

DAC U531 is programmed to generate -4.21 VDC at PRECOMP+. That signal is then
applied to ACF through R560 and U532 (DAC line pulled low). ACF is routed to AMP
IN via U526, Q516, and the AC input buffer.

AMP IN is tied to the inverting and non-inverting paths of the variable gain ampliÞer
(VGA). NETOUT (output of U519) is routed to the Zero-Crossing AmpliÞer which,
based on the polarity, generates the appropriate COMP- signal that is applied to comparator
U507. The comparator selects the path that the AMP IN signal will follow
through the VGA by closing the appropriate analog switches of U509.

The negative (inverting) AMP IN path is through R530, U515, U509, Q501, and U516 to
pin 12 of the multiplexer (U511). The output (OUT) of the multiplexer is routed
through buffer U342 to ACV/A. The signal on ACV/A is switched through U320 (/AC
pulled low) to the A/D buffer (U322) which is conÞgured for x1 gain. Measure +4.21V
at A/D IN.