HP 5501B Operating And Service Manual page 103

i. Place LIQ CRYS jumper to NRM. TP16 will alternate between about 0 V to about -5 V.
Record these voltages (the 11 V function of the HP 5005A/B is useful here). Measure
voltage atTP13. It should be the same 0 V reading as at TP16, within 20 mV. If OK, go to
step k. If not, go to step j.
j. Measure the voltage on C13. During and just after the switch (U12A) is closed (DELAYED
ACTIVE signal on Test IC U9 (Pin 3)), the voltage should equal the approximate zero
voltage at TP16. If it is not, or if the voltage rapidly goes to zero, then C13 is shorted or
the switch U12A does not work. Also check that U12B is open and U11B is not shorting
this node.
k. Is the voltage at TP14 within 20 m V of the approximate -5V voltage at TP16. If yes, go to
step m. If not, go to step I.
l. Measure the voltage on C14. During and just after the switch (U12D) is closed
(DELAYED PASSIVE signal on Test IC U9 (Pin 2)), the voltage should equal the
approximate -5V at TP16. If it is not, or the voltage rapidly goes to zero, then C14 is
shorted or U12D is faulty. Also check that U12C is open and U11C is not shorting this
node.
m. Set PHOTO DIODE jumper to TEST, LIQ CRYS to PASS, POWER AMP to UP. Verify
TP16 voltage is approximately -5V. Measure voltage at TP14 and record as V14.
Measure voltage at U11A (Pin 3) and record as V3. Measure voltage atTP12. Is it equal to
[2 X V3] - V14 within 30 mV? If yes, go to step o. If no, go to step n.
n. Verify U11A (Pin 2) is at the same voltage as U11A pin 3.lf not, U11A or R15 bad. Also
check R14, R10, and R11.
o. This procedure thus far has shown that the Subtracting Sample and Hold circuitry works
during optical mode. Next the operation during warmup is checked. Remove power from
HP5501B. Move all A3 jumpers to NRM except ~REF ON to HI. Connect an ohmmeter
between Pins 6 and 7 of U12B. Turn HP5501B on. Meter should read open-circuit when
the DISABLE signal [Test IC U9 (Pin 5)] is low. Meter should read less than 250 Ohms
when DISABLE is high. (The HP 5005A/B measures Ohms to ground, therefore Pin 6 or
Pin 7 of U12B will have to be grounded for this test). If the result is satisfactory, go to
step p. If not, U12B is defective.
p. Connect an ohmmeter between U12 (Pin 10) and ground. Ohmmeter should read less
than 250 Ohms while OPTICAL (U9, pin 12) is low. If it does, go to step q. If not, U12C is
faulty.
q. Set ~REF ON to LO, HTR OK to HI and wait for ready LED to stay on. Ohmmeter should
read open. If not, U12C is bad or the node is shorted. If it reads open, the Sample and
Hold is OK.
HP5501B
Service
8-43