HP 85662A Troubleshooting And Repair Manual page 245

A3A9 Track and Hold, Troubleshooting
Display problems which involve only the video are often caused by malfunctions on either
A3A8 Analog-Digital Converter or the A3A9 Track and Hold. The blue
(SHIFT)
key functions
and the Digital Storage Test Patterns can be used to isolate the failure.
Figure 4 shows the Digital Storage Test Pattern. Refer to "A3 Digital Storage
Troubleshooting" to generate the test pattern. The three vertical lines represent the detector
outputs on A3A9. The left one is the negative peak detector, the center one is the sampling
mode and the right one is the positive peak detector. These should move up and down when
a VIDEO signal is present at A3A9J1, indicating normal operation. If one is stuck, the blue
(SHIFT) key can be used to verify this.
, / I
Figure 4. Typical Digital Storage Test Pattern
Slight stair-stepping of the diagonal line could be caused by a low order bit failure on A3A8
or a noisy component on A3A9. To isolate the assemblies remove A3A9, jumper A3A8TP1 to
A3A8TP6 and push Instrument Preset. The Display is a diagonal line. If the diagonal line is
not stair-stepped or noisy, the problem is on A3A9.
Noise on the peaks of the displayed signals is most often caused by leakage in Q6 or Q14. This
is very noticeable in 0 Hz frequency spans. To determine whether Q6 or Q14 is defective,
sampling detection is used. (SHIFT) TRACE A
(BLANK)
(KSe) puts the instrument in the
sampling mode. A noisy signal when using the sampling mode generally indicates a defective
Q14. A defective Q6 produces noise in the normal detection mode and in the positive peak
detection mode,
(SHIFT)
TRACE A
(MAX HOLD)
(KSb).
4 A3A9