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Model 35601A
Service
6-70. FET Switch Drivers
All control signals from the HP-IB Interface board (Al) for both A4 and A5 boards are
sent, via Al J2 and a 50 pin cable, to J2 on the A5 board. The control signals used on the A4
board are simply passed from Jl of the A5 board to J14 of the A4 board. The control signals
that operate the CMOS devices of A5 (U2, U3, U4) are tied to + 5 volts with resistor pack
R89. The + 5 volts is necessary to guarantee that the CMOS will be switched properly. All
the control signals that operate the FET switches of A5 (U5, U6, U7, U17) are inverted with
U8, U9, and U10 so that any FET switch will be opened when its control line is low.
6-71. Overload Flip-Flop
The Phase-Locked Loop Control Board (A5) contains Overload Flip-Flop U11B. If an
overload occurs, Jl pin 7 is pulled low which pre-sets Ul IB; the Q output (pin 9) goes high.
This output is inverted with U8E and sent to the A4 board where it disables some of the relay
drivers. The inverted output of U11B (pin 8) is sent to Ql to drive the front panel overload
indicator.
6 72. Out of Lock Flip-Flop
The Out-of-Lock Flip-Flop is U11A; it operates the "Out-of-Lock" indicator on the front
panel. In addition, U11A closes U17D to short the capacitor of the Integrator. The output
of U11A is then inverted with U8D; this line controls U17D and can be clamped to ground by
Q3. This is done by the same line that controls U7B (which closes phase lock loop). If the
phase-locked loop is open, the control line feeding U7B is high. This causes Q3 to saturate
and guarantees that the integrating capacitor is always shorted by U17D. Then, if the soft
ware toggles the flip-flop U11A, the "Out-of-Lock" light will be out yet the integrating
capacitor will be shorted. This permits the meter to read correctly when there is no phase-
locked loop. The meter then indicates the quadrature on the input mixers by a reading of
zero.
6-73. Digital to Analog Converter
The D-to-A converter Ul takes its inputs directly from the HP-IB Control board (Al). The
output is converted from current to voltage by U12A; U12B inverts this signal. U17A and
U17B select a positive or negative voltage to be connected to the summing node of U19A.
REMOVING THE A5 BOARD FOR SERVICING
When servicing is required, the A5 board may be accessed by following these procedures:
1. Remove the trim strips from the top and bottom of the front panel frame.
2. Remove the retaining screws for the front panel; these are normally hidden by the trim
strips. The front panel assembly should now be easily removable; the various cables will still
be attached, of course.
3. The A5 board should be removed by sliding it off the plastic retaining pins and the con
nector that joins it to the A4 board.
4. The A5 board may be plugged into the A4 board using the right angle connector (-hp-
part number 035601-66508) provided in the accessory kit (-hp- part number 035601-84401).
6-67/6-68
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