HP 400E, Operating And Service Manual

Page: 36 / 57

Section V
a. Check the instrument for any obvious
evidence of trouble, such as loose or broken
wires or broken con nectors. Check for burnt
or loose components or separations or cracks
i n the printed circuit boards. Ensure that all
p i n s are clean .
b. Isolate the trouble to a particular circuit
using the block diagram Figure 4- 1 and the
schematic Figure 7-1 . Then refer to the
troubleshooting steps for that circui t . Table
5-6 gives some probable troubles for s peci l1c
symptoms.
NOTE
Test voltages
in
this section ana 011 the schema­
tic arc
nominal.
A tolerance 0/
±
1 0%
is allow­
able
aJ/a
l/1 ore
wheJ/
sta tea.
5·50. POWER SUPPL Y.
5-5 1 . Check with a dc vol tmeter ( 3450B) at TPI and
TP2 for + 26 volts and - 26 vol ts respectively. If the
TP voltages are improper , check the voltages
I
isted in
Table 5 - 7 . If the volt age for a given componen t is
wrong, t he t roub le is probably in that component or
its associated circui
1 .
Table 5 -7 . Power Supply Voltages
COMPONENT VOLTAGE
Collector Q I
+39V
Collector Q2 +26 .5V
Emitter Q2 +6.98V
Base Q3
-O .6V
Collector Q3
-23 .5V
Collector
Q4
-39V
5·52. AIVIPL I F I E RS.
5 -53 . Set t h e 400E/EL to the I volt range, and
connect a full scale in put . With a sensitive ac
vol t meter, monitor the ac ampl i fier output at t he
negative s ide of A2 C34 or A2C3 6 . The output s hould
be 150m V.
I
f it is not 1 50m V , measure the ac vol tage
at A2 pin 22. The voltage at pin 22 should be 3 mV . lf
these two voltage readings are correct , the meter
ampli l1er and met e r bridge are operating properly.
5-54. I f the voltage at pin 22 i s low, pull the
wht/orn/yel wire from pin 22 , and measure the ac
signal at the wire . It should be 3 mY. If the voltage on
the wire is proper , the t rouble is i n the meter
5-1 4
Models 400E/400EL
ampl ifier. If i t isn't correct, the trouble is either in
the Post At tenuator or the Impedance Converter.
5-5 5 . To check t he Impedance Converter, measure
the ac voltage at its output (A2 p i n 21 ) . The output
voltage should be very close to the in put voltage since
t he Impedance Converter is a unity gain ampli l1er .
With a I volt i n p ut , the output should be 0.98 volts
±0.02 volts .
5-5 6. Both the Impedance Converter and the meter
amplifier are int ernall y dc coupled . If the dc voltages
anywhere in the amplil1er are incorrect , the amplifier
won't operate properly . Consequently a check of the
dc voltages is a good check of the amplifiers.
5-5 7 . Tables 5-8 and 5-9 contain the dc voltages on
all of the transistors in the meter amplifier and the
Impedance Converter. I f the measured voltage on a
given t ransistor is wrong, the trouble is probably in
that t r ansistor or its associated circui t .
NOTE
Measure these ac voltages with the
iJ/put
shortea. A ac voltmeter with low iJ/put
capacitance ana very high inpu t resistance JllUst
be usea. Ihe -hp- ,",Joad
3450B
is recolll­
menaea. A ll ac voltages arc
±
10%
ex­
cept where otllelwise stalea.
Table 5-8. Impedance Converter Voltages
I
TRANSISTOR
E B C
Q5
(S) -6V
i(G)* I
(D)
- 1 4 .6V
Q6
-15 .3V -14 .6V -7.4V
Q7
-6.7V -7.4V -2 1 . 5V
*Cannot be measured.
Table 5-9 . Meter Amplifier Voltages
TRANSISTOR E
Q8 +19V ±20%
Q9
+0.02V
Q I O + 8.2V
Q I I +0.9V
Q I 2
+9V
Q I 3
+0.7V
Q 1 4 * +3V
*
In bridge circui
1.
B
1 9.5V ±20%
+0.57V
+8 . 5V
+1.9 V
+8.5V
0
+3 .5 V
C
+2 5 . 5V
+8 . 5V
+ 1 .8 V
+8 .5V
+O .7V
-4 .6V
+26V