Advertisement
150
T Y P E
6 0 3
E L E C T R O N I C M U L T I P L I E R
ing the leads of the indicator bulb between the
ground and the anode of the tube in question. If
a tube is conducting the bulb will not glow, since
the potential at the anode is considerably below
$90 volts. O n the other hand, the indicator bulb
will $ow if a tube is cut off, since the potential
at the anode will be essentially $150 volts.
In
using a neon indicator, it must be borne in mind
that a pulse passing through a small capacitor
(50
-
250 mmfd) will not indicate, because the
duration of the pulse is not sufficiently long to be
visible.
It is not necessary to have a neon indicator bulb
in the tool kit; one of the unused indicator bulbs
on the electronic unit may be used. For example,
bulb 5 in the indicator block in the A13 position
is not used; consequently this bulb can be used as
an indicator.
By plugging the cable extension
plug in socket A13, the indicator block can be
carried to the rear of the chassis for observation.
(An extension cable for the indicator blocks is
furnished with each machine.)
Since one side of
all the bulbs in the indicator block is already
grounded, it is necessary only to clip one test lead
to pin 5 of the sock& in A 13. T o prevent damage
to the bulb, at least 0.2 megohm should be wired
in series with the test lead. If the test lead with a
0.2 megohm resistor internally wired is clipped to
pin 5 of socket A l 3 , then the prod can be used
as a pointer and by observing bulb 5 in the indi-
cator block, the status of various tubes can be
determined.
A volt-ohmmeter is an indispensable tool in
servicing the electronic unit.
For most uses a
meter with as low a sensitivity as 1000 ohms per
volt can be used.
However, in order to check
potentials in very high resistance circuits, such as
grid circuits, it is necessary to have a high sensi-
tivity of around 20,000 ohms per volt. Since it is
not often necessary to determine grid
it is not necessary to have a meter with a sensitivity
as high as 20,000 ohms per volt for ordinary trou-
ble shooting
The anode potential of the 25L6's varies from
about $150 volts, when non-conducting to about
$20 to $30 volts when conducting
The anode
potential at either anode of a trigger changes from
approximately $40 volts to approximately $140
volts as the trigger goes
ON
and
OFF.
The poten-
tial at the anode of 6SK7's and triode sections of
12SN7's varies between approximately $50 volts
and $1 50 volts as the tube goes from conducting
-
.
state to non-conducting.
As in rhe case of the neon indicator, a meter
cannot follow a pulse through a capacitor.
The
fastest speed that a meter can be expected to fol-
low is about 2-3 pulses per second. When com-
puting at 5-cycle speed, it is possible to check most
operations by means of a voltmeter, since most
operations occur at speeds less than 3 per second.
Remember, though, that a meter cannot detect
pulses through a capacitor. Only an oscilloscope
will show pulses through a capacitor. Another very
important fact to remember is that a low sensitiv-
ity meter will draw sufficient current to cause a
trigger to operate when the m e e r lead is touched
to the anode of the non-conducting tube of a trig-
ger.
Therefore, care must be exercised when
checking the status of triggers with a meter; a false
impression may be obtained if the voltmeter causes
a trigger to trip.
Since grid current prevents grid potential from
rising above cathode potential, the actual potential
that a voltage divider produces cannot be checked
with a tube in place.
It is convenient to have a
dummy tube with all base pins clipped except the
heater pins for such purposes as checking positive
potentials at grids, or to stop certain operations.
The heater pins must be left in place because the
tube filaments are wired in strings, and tubes other
than the desired one may be rendered ineffective.
As already mentioned in connection with trouble
analysis, an ohmmeter is quite useful in checking
for shorted capacitors and resistors, or for shorted
tube elements.
Resistance values can also be de-
Advertisement
