Tektronix 1A1, Инструкция по эксплуатации, Страница: 40 / 137

Maintenance— Type 1A1
6
. Autotransformer with an output voltage variable be­
tween 103 and 127 volts (or 206 and 254 volts if oscillo­
scope used with the Type 1A1 is wired to operate within
this range). Minimum rating depends on current drawn by
the oscilloscope with its plug-in unit(s). Used to apply de­
sign-center line voltage to the oscilloscope/Type 1A1 com­
bination.
7. RMS-calibrated AC voltmeter capable of indicating
the voltage output of the autotransformer (item
6
).
8
. 30-inch long flexible cable plug-in extension. Per­
mits operating the Type 1A1 out of the plug-in compartment
so that all sides of the unit are accessible for servicing. Tek­
tronix Part No. 012-0028-00.
9. Circuit extender card complete with four signal ex­
tension leads and two ground extension leads attached to
the card. Permits operating the circuit partially out of the
unit for troubleshooting. Tektronix Part No. 012-0100-00.
10. Adapter with BNC and UHF connector fittings. Fits
BNC plug and UHF jack connectors. Used if test oscillo­
scope (item 4 and 5) external trigger connector is a UHF
jack type connector. For use in a low-frequency signal­
tracing setup to check phase relationship of calibrator sig­
nal at output of each amplifier stage in the Type 1A1. (Sig­
nal tracing setup includes items 4 through 12.) Tektronix
Part No. 103-0015-00.
11. BNC T used in a low-frequency signal-tracing setup
for connecting to the two BNC coaxial cables (item 12) and
to the calibrator output connector on the oscilloscope used
with the Type 1A1. Tektronix Part No. 103-0030-00.
12. Two coaxial cables equipped with BNC connectors on
each end. Used in low-frequency signal-tracing setup to
apply the calibrator signal to the Type 1A1 and to the test
oscilloscope external trigger input connector. Tektronix Part
No. 012-0057-00.
13. Miscellaneous: Replacement tubes, transistors, and
diodes.
Interconnecting-Plug Resistance Checks
Table 5-2 lists the approximate resistance measured be­
tween the interconnecting-plug pins and ground of the 16-
pin plug located on the rear panel of the Type 1A1. These
measurements were taken with the Type 1A1 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 since
semiconductors are used in the circuitry. In addition, the
readings can vary as much as 50% due to the type of ohm-
meter in use. Therefore, empty 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 de­
livered for full-scale deflection in each range; and (3) the
scale readings on the meter itself. If ohmmeters differed
less in these respects, the resistance measurements given
in the table would be more typical.
Troubleshooting Table
Table 5-3 is a list of typical symptoms, their possible
causes and the probable circuit at fault. Since it is impos­
sible to list every kind of symptom that might happen, those
that are included here may give you a clue to the most
likely area to check.
To locate the exact cause of a trouble when it is not
listed in the table, use the conventional method of trouble­
shooting; i.e. signal tracing, voltage and resistance checks,
and parts substitution. To reduce the parts substitution
method of troubleshooting to minimum, use the conventional
method of troubleshooting first. In addition, use the infor­
mation provided on the schematics and in other portions of
this manual as an aid to isolating the trouble.
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 ± 3 cm from the oscillo­
scope vertical-amplifier electrical center, thus positioning
the trace above or below the range of visibility.
To find the oscilloscope vertical-amplifier electrical cen­
ter, short pins 1 and 3 together momentarily and note the
position of the trace. The position of the trace is the elec­
trical 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
to the viewing area.
Fig. 5-3 shows the voltage difference limits between each
stage. If the voltage limits are exceeded in one stage, the
limits will be exceeded in the following stages (looking to­
ward the output) and the trace will deflect off the screen.
For example, if the voltage between the collectors of Q124
and Q144 in the Channel 1 Input Amplifier stage reads
+0.8 volt, the voltage between the collectors of Q404 and
Q414 in the Output Amplifier first stage will read +0.6
volt, and the voltage at pins 1 and 3 of the interconnect­
ing plug will also read +
0 .6
volt, causing the trace to be
deflected upward and off the CRT.
One quick method for isolating DC imbalance either to
one of the Channels or to the Output Amplifier second
stage is to turn one channel on at a time to see if the trace
for the channel can be normally positioned on the CRT.
If the trace for one channel cannot be positioned onto the
CRT, then the DC imbalance originates in that channel.
If none of the traces appear on the CRT, then the trouble
is probably in the Output Amplifier second stage. Also, con­
sider the possibility that the trouble might be one of the
diode switches (D421, D422, D423, or D424 in Channel 1;
D451, D452, D453 or D454 in Channel 2) or in the Switch­
ing Multivibrator stage (Q305/Q315).
5-6