HP 5501B, Operating And Service Manual, Page: 108 / 120

HP5501B
Service
8-48
If no activity is found, check U17 (Pin 2). If Pin 2 is at a TTL high level or changing states,
then either the laser is not tuned properly, the Reference Receiver is failing or not receiving
adequate signal. Verify laser tuning according to step c before continuing with step d. If U17
Pin 2 is at a TTL low level and there is no output activity as described above, then U17 may
be bad or its output may be shorted. Go to step d.
c. Align an HP10780A Receiver so that the laser beam is incident on the photodiode lens.
If the laser is properly tuned, the LED on top of the HP10780A should light and the
HP10780A should output a square wave at the REFerence frequency. This test can be
performed with the laser installed using one of the axes' receivers. In this case, if the axis
interferometer is installed, vibration (etc.) will modulate the REFerence frequency but a
frequency in the range of 1.5 to 2.0 MHz should still be present. If a steady REFerence
frequency signal is not detected by the
HP10780A Receiver, then a laser tuning failure is
indicated which is independent of the Reference Receiver. See HP5501B Trouble
Isolation Procedure, paragraph 8-72. Otherwise proceed with step d.
d. Measure and record the DC voltages at A3U17 (Pin 10) and the ACR5-R46 node. With
~REF ON jumper still set to la, place a piece of paper between the laser assembly and
the sampler assembly to block the laser beam. The voltage at U17 (Pin 10) should drop
by 250 mV or more, indicating adequate signal, and the DC voltages listed in Table 8-6
should be measured on the pins of A3U17 with the beam blocked:
Table 8-6. A3U17 Reference Receiver Test Voltages
PIN DC VOLTAGE POSSIBLE TROUBLE
9 , 12, 14
Vcc (+5 Vdc)
+15V External Power Source, +5 Volt Regulator
(A3U7).
1, 15 GRD Ground
11 TTL HIGH A3U17, A3U1F, A3U3B, or Digital Circuits
8 +0.7 Vdc ±0.1V A3CR5, A3U17, or associated passive components.
7 +4.8 Vdc ±0.1V A3C22 or A3U17
6
+0.7 Vdc ±0.1V
[2 mVp-p noise
(nominal)]
A3U17 or associated passive components (A3R43,
A3R44, A3C23 etc.).
5 +2.3 Vdc ±0.2V A3C24 or A3U17
4 +0.8 Vdc ±0.1V A3C23 or A3U17
10 +1.6 Vdc ±0.2V A3C28 or A3U17
3 Vcc +0/-0.1V A3U17
2 V cc +0/ -0.1V A3U17
13 <80 mVdc A3U17
16 +0.7 Vdc ±0.1V A3Q6 or A3U17
If any voltage is incorrect, check or replace the part listed.
If the voltage at Pin 10 does not drop according to the above, measure the DC voltage at the
CR5-R46 node with the beam blocked. It should be approximately 15 Volts and should have
increased at least 6 mV above the value measured and recorded previously when the laser
beam was unblocked. If not, check R46, C27, or CR5. CR5 can be checked by removing power
from the HP5501B and checking for diode action of CR5 using a DC ohmmeter. Resistance
measured in the reverse direction should be much larger than the resistance measured in the
forward direction. If CR5 is OK replace U17.