HP 435B Operating And Service Manual page 80

S
ERVICE
S
HEET 2 (Cont' d)
An increase d noise
l
evel may be ca u sed by C1, C6 or C30 line
no i se filters.
Ran ge-to- range i
naccur ac y between the 100
m
W range and
anoth e r r ange may be due to α s
haping cir c uit def ect.
R
ange Switch
R ange-to-range inaccur acy which i s cau se d by the RANGE
s witc h attenuators can easily b
e iso lated by p erforming one ofthe
In strumentation Accur acy Performance
T ests (refer to Section
IV) .
T h ird Amplifier
Adjust the CAL ADJ control from its pr esent settin g to the εεω
stop. Then adju st t h e co ntrol to the εω stop. The meter r ea ding
will norm ally c
hange by ±2 dB (>4 dB from stop to stop). The ac
voltage at ΤΡ 4 will c h ange from the nominal setting to approxi-
mately -35%ο (ccw stop) and +70% (εω stop).
Sync hronous Detector
The phase chan ge ofthe 220 Hz s
ignal betw ee n the p
ower sensor's
sampling gate and the Synchr onous Detector cannot be mea-
sured directly b ecause the d
etector output
i
s do rather t
han ac.
H
owever, the phase difference at ΤΡ4 (the inpu t to the detector
c
ircuit) can
b
e measur ed. Because the phase change b etween ΤΡ4
and the detecto r is kn own, the phase relationship between the
drive signal (ΤΡ7)' and t h e ΤΡ4 signal indicates th e total phase
shift through the ac amplifi ers. This is the step-by-ste p pr ocedure
f
or
c
hecking phase shift.
α. Set the
P
ower Meter and (if use d) the range ca libr ator con-
trols as shown in th e general troubleshooting inf ormation above.
b. Connect th e osc ill osco pe's vertical inputs to the 220 Hz
drive (ΤΡ7) through α divid e-by-ten probe (C hannel Β) and to ΤΡ 4
through α
o
ne-to-o ne pro b e (C hannel Α).
ε. Set the oscill osco pe con trols as f
ollows: Channel Α sensitiv-
ity to 0.05V/division
w
ith ac coupling, Channel Β sensitivity to
0.2V/divi sion, horizon ta l s w eep to 0.5 ms/division and the dis-
play mode to Ch a nnel Α and Β, c
hopped with triggering from Β.
d. Adjust the v erti ca l p osition co ntrols until both traces are
symetrical with resp ect to the horizontal center line (refer to the
typical waveform b e l ow).
e. Set the tim e base magnifier control to Χ10. The horizon tal
scale
i
s now 50 As/division (refer to the expanded wavefo rm
below).
Troubleshooting
B
lock Diagram ,
S
ERVICE
S
HEET