HP 400E, Operating And Service Manual

Page: 35 / 57

Models 400E/400EL
SYMPTOM
1. No response to input signal
2. Low B+ voltage at TPI or Low
B- voltage at TP2
3. Low gain at high frequencies
4. High gain at high frequencies
5. Low full scale readings
6. I nstrument will not range
above 1 volt but works OK at 1
vol t and below
Section V
Table 5- 6. Troubleshooting Tips
PROBABLE TROUBLE
I. Fuse A2FI open. Check power supply voltages. Check AC signal
according to Paragraphs 5-53 through 5- 55 to isolate the area of
trouble.
2. Disconnect jumper wire. Measure resistance to ground at both
jumper terminals. If 10 ohms on the meter side, CI6 or CI9 is
shorted , if 100 ohms, C35 or C29 is shorted; if zero, C8 or C9 is
shorted. Disconnect R20 and R28 to isolate the Impedance
Converter. If low resistance is on power supply side refer to
Paragraph 5-51 and Table 5-7.
3. Check A2C22 for open if 10% low.
Lift
A2C39 and check for
oscillations; if no oscillations check A2QI5 and 1 6.
4. Check A2C30 for an open.
5. Check A2CRI5 and A2CRI6.
6. Relay Al KI stuck in closed position.
7. I nstrument will not range
7. Relay K2 stuck in closed position.
below 3 volts but works OK on
3 volt range and above
8. TP3 voltage can not be
adjusted properly
9. TP5 voltage can not be
adjusted properly
10. TP4 voltage can not be
adjusted properly
11. TP4 voltage varies and meter
needle wobbles
12. Low line transients
13. Transients on range change
(l
V
to 3V)
14. Peaking at 5MHz (10%)
15. Voltage slightly low on 1 mV
range
16. Low voltage (10 and 20Hz)
near full scale
5-48.
TROUBLESHOOTING.
8. Extreme condition: check A2Q5, Q 6 and Q7. Small variation:
change value of A2RI8*. (Refer t o Table 5-11).
9. AC output circuit. Check A2QI5 and QI6. Refer to Paragraph
5-58 and Table 5-11 .
10. Meter Amplifier Circuit. Check A2Q8 thru Q 13.
11. Isolate by shorting A2C 17 to ground. If voltage at TP4 still varies
the trouble is in the Meter Amplifier. Refer to Paragraph 5-54 and
Table 5-9. If voltage is constant the trouble is in the Impedance
Converter. Ref er to Paragraph 5-55 and Table 5-8.
12. Check A2Q3 and Q4. (If a n old instrument change A2Q4, A2R73
and R74 to current part number). Check A2CR20 and CR2I.
13. Match reverse resistance of A2CR9 and CRIO. Check S2CRI and
S2CR2. Check relays.
14. Isolate by disconnecting orange wire to switch. Voltage at pin 21
should be same as input. Refer to Paragraph 5-55 or 5-56.
15. Change value of A2R72* (refer to Table 5-11).
16. Check A2Q13. Change value of A2C31* (Table 5-11). Check
A2C I
0
and C20.
549. These procedures should only be performed
when the 400E/EL can not be calibrated according to
procedures in Paragraphs 5-32 through 547. I f the
400E/EL is slightly out of specifications and can not
be corrected by the proper adjustment, refer to Table
5-11 Factory Selected Values. If the 400E/EL
is
inoperative or completely out of specifications
proceed
as
follows:
5-13