HP 87114B, Service Manual

Page: 109 / 174

Trace at 6200 dB
A at trace at 6200 dB indicates that the DSP is getting no data (or zero data.) Make sure that
the receiver and CPU boards are making proper connections to the backplane . The problem is
most likely either in the ADCs on the receiver board, or the DSP circuitry on the CPU board.
T o determine the exact cause , test the DSP circuitry .
T esting the DSP circuitry:
Locate J32, an 8x3 jumper just above the DSP shield on the A2 CPU board assembly .
T emporarily place the jumper in the \TEST" position as shown on the board. This replaces the
receiver's ADC clock and data signals with a clock signal and fake data generated on the CPU
board. The instrument should power up normally . Observe the CRT .
If random noise data trace around 0 dB is observed, the CPU is functioning
properly . Replace
the A5 receiver assembly or go to \Detailed Receiver Troubleshooting" below .
If no random noise data trace is observed around 0 dB: replace the A2 CPU .
Detailed Receiver Troubleshooting
T o Conrm an ADC Problem when the Trace is at 6200 dB:
Check for activity at the following pins on the receiver's backplane connector .
Pin Number Signal
C27 ADC D A T1
A27 ADC CLK1
C26 ADC D A T2
A26 ADC CLK2
If these signals are not present, replace the receiver assembly .
4-20 T roubleshooting and Block Diagrams