Preamplifier; Power Supply; Meter Bridge; Etched Circuit Board Repair - HP 400F Operating And Service Manual

Section V
Paragraphs 5-43 to 5-51 and Tables 5-4 to 5-8
MALFUNCTION INDICA TION
Instrument will not operate on line voltage,
UNE ON lamp will not light.
Instrument will not uprange above 0. 3 volt,
works on O. 3 volt range and below .
Instrument will not downrange below 1 volt,
works on 1 volt range
voltage at A2R8 cannot
No voltage at A2Ll.
Power supply output unregulated.
No ae output.
Instrument operates improperly with inputs above
KHz,
100 but works with inputs
quencies.
Instrwnent will not operate properly on
range.
Meter deflection on all ranges with no input.
Meter remains at zero with any input on any range.
5-43. Refer to Table 5-4 for a Ust
functions and their probable causes.
NOTE
All the voltage measurements in this
section should be made with the
400F /FL input shorted and the RANGE
switch set to 1 volt.

5-44. POWER SUPPLY.

5-45. Measure the power supply outputs at Jumper wires
{f1
and N2 for +26 v and -26 v respectively.
outputs are incorrect. first check the components in
the +26 v section of the power supply. because the
control transistor in the -26 v supply is referenced
to the +26 v output. Consequently,
comes uru-egulated, the -26
Refer to Table 5-5 for a list of check point voltages
in the power supply.
Table 5-5.

Power Supply Voltages

CHECK POlNT
Emitter Q4
Collector Q4
Collector Q6
Emitter Q8
Emitter Q9
5-46. AMPLIFIERS.
5-47. Both the preamplifier and the meter amplifier
are internally dc coupled.
in the amplifiers are incorrect, the amplifiers will
not operate properly. Measure the dc voltages in the
amplifiers at the check points listed in "Tables 5-6
and 5-7.
5-6
Table 5-4.
and
above.
be properly adjusted.
of
lower fre-
O.
1 mv
of
possible mal­
If
both
if
the +26 v be­
v
wUl also be unregulated.
VOLTAGE
+26. 0 v ±l v
+41. 5 v ±5 v
+27. 5 v ±1 v
-26. 0 v ±1 v
-43. 5 v ±5 v
If
the dc voltages anywhere

Troubleshooting Guide

PROBABLE TROUBLE
and
Fuse FI open.
Relay A2Kl stuck
but
but Relay A2Kl stuck open,
Impedance Converter Circuit
A2Q3).
Jumper wire Itl broken.
A2Q6, A2Q7 or Zener diode A2CR12.
A2R33 shorted.
Filter switch 82.
Range switch SI, wafer D.
A2Q15. A2Q16. A2Q17.
A2C38. A2C39, A2C40.
Diode A2CRl shorted (400F
A2Cl shorted (400FL only).
Table 5-6
CHECK POlNT
Source Q l
Drain Q l
Collector Q2
Collector Q3
Table 5-7
CHECK POINT
Emitter QI0
Collector QI0
Collector Q l l
Collector Q12
Collector Q13
Collector Q15

5-48. METER BRIDGE.

5-49. Measure the dc voltages on the transistors in
the meter bridge and compare the readings with those
in
given Table 5-8.
meter terminals.
approximately -9 volts ±1 volt with respect to circuit
ground.
Table 5-8
CHECK PO lNT
Collector Q16
Base Q17
5 - 5 0 .
E T C H E D C I R C U I T B O A R D R E P A I R .
5-51. TheModel 400F/FLuses plated-lhrough. double­
sided, etched circuit I:x>ards.
closed. or A2K2 stuck open.
or A2K2 stuck closed.
(A2Ql. A2Q2 and
only). or capacitor
Preamplifier Voltages
VOLTAGE
±Q.
2. 3 v 5
-
-17. 0 v :t2. 0 v
- 7. 5 v ±Q. 5 v
-21. 4 v :t1 . 0 v

Meter Amplifier Voltages

VOLTAGE
:to.
- 0. 64 v
1 v
9. 20 v ± 1 . 0 v
+
0. 97 v :to. 2 v
+
+22.00 v :tL O v
+11. 00 v :to. 5 v
±C .
+ 2. 30 v 5 v
Also measure the voltages at the
The meter should he floating at
Meter Bridge Voltages
VOLTAGE
- 9 v ±1 v
-17 v ±l v
To prevent damage to
400F/FL
v
01795-1