The Cardmatic tube tester is an ingenious design. Various power
supplies,
a gm bridge circuit for measuring transconductance, a decade resistor
box,
and an accurate metering circuit are all connected together at the card
reader
so that anything can be connected to anything. Replace the card
reader
contacts with relays and you have an ideal platform for computer
control.
The tester is capable of at least 17 different test configurations,
ranging
from simple triode transconductance tests to high voltage rectifier
tests
to voltage regulator tests.
We'll start with the shorts test. One of the functions of the
power
supply is to supply a regulated positive and negative 150VDC.
Among
other things, these voltages are used for the shorts tests. They
are
applied to the tube elements in the opposite polarity from
normal. This
means no worry of cathode stripping, and any sort of grid emission will
cause
a short lamp to flicker. If there is enough leakage between any
elements
to allow enough current to flow to light a neon lamp, then the tube is
defective.
The sensitivity of the shorts test is adjustable and is part of the
calibration
procedure.
At the same time, the meter is placed in series with the heater and
cathode
to measure heater/cathode leakage. The meter can be programmed to
reject
tubes with as little as 10 uA of leakage current or as much as 170
uA.
It can also be disabled, which is done in the case of directly heated
filamentary
tubes.
For the quality test, the tester can be configured a number of ways
depending
on the tube to be tested. Here are some examples:
Transconductance Test:
Full scale meter reading can be programmed from 500 to 128,000
uMhos
in 100 uMho steps.
The tube can be in grid bias or self bias mode.
Negative grid bias, if used, can be set to -.1VDC to -99.9VDC in
.1
volt steps.
In self bias mode, the cathode bias resistor can be anywhere from
0 ohms to 70K ohms in 10 ohm steps.
In self bias mode, the cathode bias resistor can be bypassed by a
1000uf capacitor.
A regulated signal voltage of .222VAC is applied to the grid.
The plate voltage is regulated and can be from 10VDC to 260VDC in
10 volt steps.
Plate Current Test:
Full scale meter reading can be programmed from 100 uA to 500 mA
in
100uA steps.
The tube can be in grid bias or self bias mode, or can be a diode.
Negative grid bias, if used, can be set to -.1VDC to -99.9VDC in
.1
volt steps.
The plate circuit resistance can be set anywhere from 0 ohms to
70K
ohms in 10 ohm steps.
The plate voltage is regulated and can be set from 10VDC to 260VDC
in 10 volt steps, or an unregulated 360VDC, or up to 1200V P-P AC.
"Knee" Test for Sweep Tubes
High current test where the tube is having to charge/discharge a
large capacitance as its load. Most testers lack this test.
These are just a few of the more common tests, and we've already hit
quite
a few different combinations of test configurations. Since most
of
the more obscure test configurations are used only for tubes that are
already
programmed into the database we will not go into details here.
During the gas test, the tester simply measures grid current.
If
there are gas ions present in the tube, they will be attracted to the
negative
grid and grid current will flow, causing the meter to deflect. If
it
deflects above the green area for gas, then more than 3 uA of grid
current
is flowing and the tube should be rejected.
This tester also has:
A filament power supply that is programmable from .1 volts to
119.9
volts in .1 volt increments.
An auxiliary B+ supply that is regulated and continually
adjustable
from 30 to 300 volts (used mostly for voltage regulator tube tests).
A bias off supply that is used to keep unused tube sections biased
off during tests.
A test button that allows us to test two identical sections within
a tube with one test.
Protection circuits that automatically shut the tester off in case
of power supply overload or grossly off scale meter deflections.