Keithley 151 Instruction Manual page 10

SECTION V -- MAINTENANCE
A. GENERAL
The only periodic maintenance the Model 151 requires is replacement OC
EKDl and BAXZ, the zero set batteries, at inter-&Is of about two or three
years. If the zeroing controls become inoperative, the batteries need
replacement.
The modulator and other components have an indefinite life and should
not be tampered with unless there is a failure.
B.
TROUBLE-SHOOTING PROCEDLIRE
If the instrument is inoperative but replacement of tubes doe:: not cure
the trouble, the following step-by-step procedure is indicated.
1. Check the S-plus and B-minus voltages at the junctions of R170,
C120-B and R171,
C119B.
These should agree within about
of the voltages indirated on the schematic.
If these arelOg
markedly different, check the fuse and 'hen the ac voltage frcm
the transformer (about '23i.~ v to ground). Then check the selenium
rectifiers and the filter capacitors.
2. If the proper voltages are present in the power supply
a.
Remove V-h, the 12AT7, from its socket, sh.crt the clltcut
terminals and the input termi.nals.
5. Connec' an oscillosope to the plate of W-B (pin
6).
Rotate the SHORT CIRCLTIT ZERO to one end of its travels.
If the ac amplifier is functioning and the bias batteries
BAl and BA2 are providing voltage, a 60 cps carrier signal,
proportional Ian amplitudeto the rotati.on of the potentic-
meter (until the amplifier saturates), should be seen.
By returning the short circcit zero con'~.rcl tr midymsition,
it should be possible to reduce the signal ir i.he point
where it disappears 51; the fi~rst stage tube n, i~se. If
th4.s occurs, the ac amplifier is functioning. Proceed
:n the instructions in paragraph D.
C.
If there is an ac signal present at the output which is
not affected by the short-circuit zero control, short
pin
9
of V-l to ground.
You will have to remove the in-
put stage shield to do this.
Replace it when 0bserv:r.g
the output with the oscilloscope cn pin 6 of V-2.
If the
signal is no longer present and just background tube noise
is seen, either the modulator is defective yr the input cir-
cuit is open somewhere between Cl05 and the input terminals.
V-l