Wheatstone TV-80, Техническое руководство

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A P P E N D I C E S
TV80 / Sep 98
Testing a "Live" (Powered-Up) Console – Precautions
(1) If a module must be removed, but remain connected while trouble-
shooting (using the extender ribbons that come with the console), place a
piece of cardboard or other non-conducting material across the console
where the module will be placed. This will prevent shorting, and also avoid
scratching or marring the faceplates.
(2) Be extremely careful when using meter or oscilloscope test probes,
to avoid shorting a test point to an adjacent connection. This is especially
important when probing a pin 7 op-amp output, since the adjacent pin 8 is
at 18 volts.
(3) NEVER remove or insert an integrated circuit while the console is
powered up.
Integrated Circuits
The audio circuits of the console consist almost entirely of plug-in IC
op-amps. The types called out in the schematic drawings and parts lists are
chosen for optimum performance; in an emergency situation other types
of known matching pin-out and capability can be temporarily substituted.
Some useful troubleshooting hints for these circuits follow.
(1) Resistors and capacitors, including electrolytic capacitors, have a
vanishingly small failure rate in this equipment.
(2) Do not attempt to put any significance to the fact that you can
measure very low signal levels on the inverting or "minus" input of an op-
amp stage. Due to the large open-loop gain of the typical op-amp, the
inverting input of an amplifier, configured as an inverter with its non-
inverting input grounded, acts as a “virtual ground,” and signal levels at
this point can be expected to be extremely low. However, a circuit fault
could result in a large signal level at the inverting input, so it may be worth
checking.
(3) When one of these ICs fails, it commonly swings its output to one
of the power supply rails. This should be a first check when a bad IC is
suspected. Measure the output pin of the IC directly (as opposed to
measuring after a coupling capacitor) under a no-signal condition and look
for a large DC offset at the output. Note that this test is not valid for those
op-amps used in non-audio circuits such as integrators and relay drivers.
(4) All of the console modules pick up their power supply voltage from
the main distribution busses through polyswitches. These devices are
provided to limit the current drawn by the module under fault conditions
and prevent a module level fault from becoming a console level fault.
Polyswitches will generally activate when an IC fails (see item 3, previ-
ous). Whenever a fault if suspected check the voltage on the module side
of the polyswitches. When all of the circuits in a module indicate the same
fault (all outputs have no audio and a large DC value, or all meters are
pegged under no signal conditions, etc.) it is generally due to one of the
polyswitches becoming active. Do not defeat the protection offered by
these devices by replacing them with wires. Instead, determine the fault
that caused them to activate in the first place and correct it.
A polyswitch is an electronic
fuse that switches to a very
high impedance state when
it's current theshold is
reached. The device resets
to a very low impedance
state when the fault condi-
tion is cleared and they have
cooled off to normal tem-
perature.
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