Heathkit IM-18 - Troubleshooting Chart; Completely Inoperative; Inability to Obtain DC Balance; AC Balance Issues

HEATHEIT
Page 31
TROUBLESHOOTING CHART
COMPLETELY INOPERATIVE
1. Make sure that power is being appliedto the
instrument, This may be measured across
the primary winding of the power trans-
former (black lead to black-red lead),
2, If pilot lamp and tube filaments do not light,
check voltage between the yellow leads of
power transformer (5-6 volts AC),
8, Check voltage between each end of electro-
lytie capacitor and ground, Correct voltages
are shown on the Schematic.
Check the 12AU7 tube,
INABILITY TO OBTAIN DC BALANCE
1, Check the 12AU7 tube for an unbalanced con-
dition (Substitution),
2, Check the two .005 capacitors C5 and C6
(Pins 2 and 7 of the 12AU7),
3, Check the components in the cathode circuits
of the 12AU7 tube (Pins 3 and 6), These
circuits include the ZERO ADJ control, Rl,
R33, R34, and R35,
4, Check Range switch assembly carefully,
AC INOPERATIVE
1, Check the 6ALS5 tube,
2, Check C2, .047 pfd 1600 volt, and the two
.02 u.fd capacitors, C3 and C4,
3, Cheek Function switch assembly carefully,
AC BALANCE
1. Disconnect test leads from instrument be-
fore adjusting the AC Baiance control as
directed in the manual,
2, It is imperative that DC balance be obtained
before this adjustment is made,
INACCURATE AC READINGS (The inability to
obtain AC calibration),
1, Check capacitors C2, C3, and C4,
2, Check the 6AL5 tube,
3, Check the AC Calibrate control, R3, NOTE:
With the test lead plug inserted, there may be
a residual reading. This is due to stray AC
pickup in the test leads, Readings on the two
lower AC ranges will normally be low,
INACCURATE DC READINGS
1, Check the DC calibrate control, R4,
2. Check resistor in the test probe, Make
sure that it is not being grounded,
OHMS INOPERATIVE
1. Check the OHMS ADJ control, R2, for
correct value,
2, Check Range switch for proper assembly,
3, Check the battery,
OHMS INACCURATE
1, Check the battery (Substitution),
2, Check the value of allresistors onthe range
switch which have a value beginning with the
number "9", (The 9,1 Q R31 should receive
special attention.) NOTE: The ohms section
of the VTVM is not intended for use asa
standard, Where a great degree of accuracy
is required, a bridge should be used,