HP 400D Operating And Servicing Manual page 30

IV
Sect. Page 12
Symptom
Microphonics (Cont' d. )
Response at 10 cps is low or high.
Response at 4 mc low.
Motorboating.
High plate or screen voltage for V2,
or VS.
Low plate o r screen voltage for V2,
or VS.
Voltmeter reads high Or low on two voltage ranges
that are multiples of 1000 apart.
TABLE 4-2.
TROUBLE SHOOTING CHART (CONT'D)
V3, V4, and/
V3, V4, and/
Possible Cause and/or Remedy
If
instrument seems to be equally microphonic on
the 0. 3 volt range with 0 . 3 volt input, change
If
instrument is less microphonic on the
V2.
0. 3 volt range, replace VI.
Poor electrical contact in range switch.
inspect switch contacts.
that will not leave a deposit.
Check all solder connections.
jOints will appear as microphonics.
V3, V4, and VS may also cause microphonicB.
One or more weak tubes (try V3 and V4 first), de-
fective coupling capaCitor (CIB, Cl9, etc. ),
defective cathode by-pass capacitor (C20,
Weak tubes (VI through VS).
Motorboating may be caused by low voltage from
the regulated power supply, Or from low ac line
conditions.
VS (see paragraph 4-10
Weak V2, V3, V4,
cate trouble).
Trouble in power supply (see paragraph 4-9).
Weak or defective tube.
Open cathode resistor.
Defective grid coupling, cathode , or screen by-
pass capacitor. Refer to paragraph 4-10.
Defective tube.
Defective range switch component(s).
replacing entire range switch assembly.
Clean and
Use a cleaning solvent
Do not disturb
Loose or imperfect
or
etc).
to lo-
Refer to paragraph 4-10.
Correct by