HP 3465B - Page 51

Section VII Model 3465B
7-17. If the 300 ms segment is extended a permanent
"overrange" condition exists. Check the Auto Zero and
Reference Supply circuits. With 1 V dc still applied to the
input, check the waveform at the COMP/CP terminal. The
waveform should look like the following:
OV —
-2V-
_7V —
■0«2«- -O.Zt-
The width of the A segment will vary proportionally as the
input amplitude is varied. If this signal and stipulations are
not met, the fault is in the Analog-to-Digital Converter.
(Remember, the logic section has already been cleared or
the Control State Counter could also cause problems here.)
The noise that appears in the comparator off state is just
that and can be ignored.
7-18. Power Supply Faults.
NOTE
In protecting batteries and circuitry, the low
battery voltage detection circuit may shut
down the instrument if the power switch is
momentarily turned off then back on. To
restore normal operation, the instrument must
be turned off by the front panel switch for a
minimum of 3 seconds.
7-19. Verify the supply voltages in the following sequence:
+ Vb,+ 10 V and-7 V.
a. The + Vg supply should indicate between -t 3 V dc
and -t 6 V dc. The voltage should be present in both the ON
and OFF positions of the front panel power switch. The
"on" reading may be slightly lower than the "off reading.
b. If at least -t 3 V is not indicated in the "on" posi
tion, the problem is in the primary power source. Check the
A3 board, batteries and power line.
c. If both the + 10 Vand - 7 V supplies are also inoper
ative, proceed as follows.
1. Check C18, CR19 and CR20 for shorts.
2. Check the voltage across R99 (with a floating
meter). 300 mV is typical. 500 mV or more indi-
3.
cates that the - 7 V supply is excessively loaded.
If this is the case, lift the various - 7 V jumpers,
including the collector of Q22 shown on the
accompanying layout. Lift the jumpers one at a
time, while monitoring the supply, until all
jumpers are up. This will isolate the fault if it is
in the loading.
Check the waveform at the base of Q33. The
waveform should be similar to the illustration
below.
»0,7(+Vb)
Reference level should be -t Vb - (^ 0.7 V). The
collector of Q33 should be a non-symmetrical
square wave of at least 15 volts peak-to-peak
with the approximate 10 ps period.
4. If the waveforms are not present, continue to
monitor and proceed through the following. If
at any step a fault is detected and corrected and
the waveforms appear, go to Step 5.
(a)Lift R112 at the junction of U17, pin 6, and
connect it to -t Vb. If waveforms appear, the
fault is with U17 or CR36.
(b)Lift Q38. If waveforms appear, the fault is in
the low battery voltage detection circuit.
(c)Lift Q35. Waveforms? Yes? Q35 is at fault.
If the above procedures do not restore the
required waveforms, the fault is with Q33, Q34
orTl.
5. Lift R112 at the junction of U17, pin 6, and con
nect it to an external supply. Adjust the supply
(not to exceed -t .6 V dc) until -t 11 volts is mea
sured across C26.
(a)Momentarily short emitter to collector of
Q27. If the voltage at the -t 10 V test point
comes up, the fault is in the -t 10 V regulator.
Check R75, U16, Q27, Q26 and CR17 in that
order. If the voltage does not come up, isolate
the troubled area by lifting the -t 10 V
jumpers, one at a time, and proceed to the
relevant circuit if the -t 10 V is restored.
7-2