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Theory of Operation
Calibration
During calibration the #1 relay selects Cal (disconnects the input signal), the #2 relay selects DC, and the two
lOX/lX relays turn on and off appropriately until calibration is complete.
Input Overload Sensing
The Input Overload Sensing circuit gives the 11A52 the ability to detect input voltages in excess of 5 Vrms.
The OVERLOAD SENSE signals for channels 1 and 2 are TTL level signals from U945 and U940. ICs U945 and
U940 are RMS signal detectors; they produce a high on the CH 1 OVERLOAD $ENSE or CH 2 OVERLOAD
SENSE line whenever the respective input exceeds 5 Vrms. In the event of an overload, the processor reads the
overload signal through U910 (diagram 2) and disconnects the input signal. Capacitors C940 and C830 average
a current that is proportional to therms level of the input signals. Capacitors C949 and C941 bypass U945 and
U940's internal references. The detect outputs of U945 and U940 are open collectors with R948 and R939 serving
as pull-up resistors. Resistors R840, R841, R842 and R940, along with U840 are used to self-test U940 and U945.
With the signal RMSTEST high and RMSINV low, each RMS detect IC receives a simulated + 10 V overload
(as if each input channel were overdriven by a + 10 V power supply). With RMSTEST high and RMSINV high,
the simulated overload would appear to be-10 V.
Fuse Check Circuit
The Fuse Check Circuit produces +l.18 V for Analog-to-Digital converter (A/D) U750 when fuses FlOOl, F1002,
F1003, and F1004 are intact. In normal operation, the +l.18 Vindicates that the probe power supplies are all
functioning properly.
If
any fuse fails, the FUSE CHECK output will change by at least ±1 V and the DAC, as
directed by the processor, will acquire the new signal level.
If
the FUSE CHECK signal
is
not close to +1.18 V,
the processor will send an error message to the oscilloscope display after power-up. Diodes CR1045 and VR975
limit the FUSE CHECK voltage to-0.3 V minimum and +5.1 V maximum.
Relay Fast Power On (RFPO)
The RFPO circuit controls operating power to U850, the IC that drives the attenuator relays. At power-up,
U850 receives operating power when the+ lSA supply reaches+ 13.6 V. At power-down, power to U850 is turned
off when the output of the +lSA supply falls below +12.9 V.
IC U350 detects that both the + lSA and +SD power supplies have attained a large enough voltage to guarantee
proper attenuator relay state changes. Also, at power-down U350 rapidly shuts off the relay supply current
when the + lSA supply begins to drop. VR452 sets the + lSA "supply up" trip point. Resistors R344 and R345
divide the + lSA voltage so it ca1,1. be compared to the VR452 voltage. The relay power is turned on when + lSA
=
+ 13.6 V (double the voltage across VR452). The relay power is turned off when + lSA
=
+ 12.9 V [2(VR452)-
0.7V]. The difference in the "relay power on" and "relay power off' voltage trip points (0.7 V) ensures that
U350 does not oscillate during power-up or power-down.
Positive feedback through C350 to U350 causes U350 to make fast on/ off transitions. Capacitor C450 filters
digital noise. Diode VR451 lengthens the RESET(L) pulse at power-down. IC U350 drives Q449 through R448.
2-6
11A52 Extended Service Manual
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Table of Contents
Troubleshooting
