Tektronix 1A2 Instruction Manual page 25

Maintenance—Type 1A2
and diode tester. The VOM DC sensitivity should be at
least 20,000 ohms per volt; DC voltage accuracy for either
the VOM or VTVM should be within 3%.
CAUTION
Be sure the test prods are well insulated (except
for the very tip) to prevent accidental shorts when
reaching a test point. If you use the VOM or
VTVM as an ohmmeter to measure resistances when
semiconductors are in the circuit, know and use
ranges (usually RX1K and higher) that deliver a
current of less than 2 mA at full deflection.
3. Milliammeter to determine full-scale current delivered
by the VOM or VTVM on ohmmeter ranges used for semi-
conductor testing; range 0 to 2mA.
4. Test oscilloscope to signal trace the switching circuits,
a wide-bandwidth oscilloscope (DC to 10MHz or better)
with calibrated vertical deflection factors down to 0.1 V/cm
without a 10X probe (1 V/cm with a 10X probe). To low-
frequency signal trace and check DC levels in each ampli-
fier stage, a low-bandwidth oscilloscope (DC to about 300
kHz or better) with calibrated vertical deflection factors down
to 50 mV/cm without a 10X probe (500 mV/cm with a 10
probe). An Ext Trig Input connector on the test oscilloscope is
desirable. The wide-bandwidth oscilloscope can be used if
the vertical deflection factor is 50mV/cm without a 10X
probe; the low-bandwidth oscilloscope can be used to signal
trace the switching circuits if the bandwidth limitation is
considered.
5. Flexible cable plug-in extension to permit operation
of the Type 1A2 out of the plug-in compartment so that all
sides of the unit are accessible for servicing.
NOTE
The plug-in extension must be used with care since,
under certain conditions, it can cause the instru-
ment to oscillate and/or have poor high-frequency
response.
6. Adapter for use, if the test oscilloscope Ext Trig {item
4) connector is a UHF type of connector, in a low-frequency
signal-tracing setup to check phase relationship of the cali-
brator signal at output of each amplifier stage in the Type
1A2. (Signal tracing setup includes items 4 through 8.) Tek-
tronix Part No. 103-0015-00 recommended.
7. BNC T connector to use in a low-frequency signal-
tracing setup for connecting to the two BNC coaxial cables
{item 8) and to the Cal Out connector on the oscilloscope
used with the Type 1A2. Tektronix Part No. 103-0030-00
recommended.
8. Coaxial Cables (two required) for use in low-frequency
signal-tracing setup to apply the calibrator signal to the
Type 1A2 and to the test oscilloscope Ext Trig Input con-
nector; equipped with BNC plug connectors on each end.
Tektronix Part No. 012-0057-00 recommended.
9. Miscellaneous: Replacement tubes, transistors and diodes.
In-Circuit Diode Checks
In-circuit checks of diodes can be made quite easily by
using a voltmeter to find out if the diode is functioning pro-
5-4
perly in the circuit. Measure the voltage on each side of
the diode during its quiescent state as given on the sche-
matics, then determine whether the difference between volt-
ages is normal or not.
If you are in doubt whether a diode is defective, unsolder
one end and check the forward-to-back resistance ratio.
If the ohmmeter check proves unsatisfactory, replace the
diode.
NOTE
As a general rule, do not use the R X 1 and R
X 10 ohmmeter ranges. Use the higher ranges
where the current is limited to less than 2 mA.
You can quickly check the current by inserting a
milliammeter between the ohmmeter leads, and
then noting the current for each range you intend
to use. Internal voltage of the ohmmeter should
not exceed 22 V.
Interconnecting-Plug Resistance Checks
Table 5-1 lists the approximate resistances measured
between the interconnecting-plug pins and ground of the 16-
pin plug located on the rear panel of the Type 1A2. These
measurements were taken with the Type 1A2 disconnected
from the associated oscilloscope. The measurements are
particularly useful for locating a possible short circuit or low-
resistance path in the unit, if such trouble should occur.
The resistance measurements vary considerably due to the
semiconductors in the circuitry. In addition, the readings
can vary as much as 50% due to the type of ohmmeter in
use, even when using the same ranges. Therefore, blank
columns are provided in the table for logging your own
measurements, and the type of meter used, for future
reference.
Significant differences between ohmmeter types are: (1)
the amount of internal voltage used, (2) the currents delivered
for full-scale deflection in each range, and (3) the scale read-
ings on the meter itself.
Isolating DC Imbalance
For free-running traces to appear within the usable view-
ing area of the CRT screen, the DC voltage as measured
between pins 1 and 3 of the interconnecting plug to the
oscilloscope must be less than 0.3 volt. A voltage differ-
ence which exceeds =£0.3 volt between these two points may
position the trace more than --3cm from the oscilloscope
vertical-amplifier electrical center, thus positioning the trace
above or below the range of visibility.
To find the oscilloscope vertical-amplifier electrical center,
short pins 1 and 3 together momentarily and note the posi-
tion of the trace. The position of the trace is the electrical
center. When shorting the pins, use care to avoid shorting
to other pins or to ground.
The DC voltages at pins 1 and 3 of the interconnecting
plug depend on the DC balance of all amplifier stages in
both channels. Since all the amplifier stages are DC coupled,
any excessive imbalance between input and output can
unbalance the output and cause the trace to deflect out of
the viewing area.