Display Board Circuit Theory; Power Supply Circuit Theory - Keithley 2002 Repair Manual

2.9.2 Display board circuit theory

The following information provides some basic circuit theo-
ry that can be used as an aid to troubleshoot the display and
keyboard.
Display microcontroller
U902 is the display microcontroller that controls the VFD
(vacuum fluorescent display) and interprets key data. The
microcontroller has four peripheral I/O ports that are used
for the various control and read functions.
Display data is serially transmitted to the microcontroller
from the digital board via the TXB line to the microcontrol-
ler PD0 terminal. In a similar manner, key data is serially
sent back to the digital board through the RXB line via PD1.
The 4MHz clock for the microcontroller is generated on the
digital board.
Vacuum fluorescent display
DS901 is the VFD (vacuum fluorescent display) module,
which can display up to 49 characters. Each character is or-
ganized as a 5 × 7 matrix of dots or pixels and includes a long
under-bar segment to act as a cursor.
The display uses a common multiplexing scheme with each
character refreshed in sequence. U903 and U904 are the grid
drivers, while U901 and U905 are the dot drivers. Note that
dot driver and grid driver data is serially transmitted from the
microcontroller (PD3 and PC1).
The VFD requires both +60VDC and 5VAC for the fila-
ments. These VFD voltages are supplied by U625, which is
located on the digital board.
Key matrix
The front panel keys (S901-S931) are organized into a row-
column matrix to minimize the number of microcontroller
peripheral lines required to read the keyboard. A key is read
by strobing the columns and reading all rows for each
strobed column. Key down data is interpreted by the display
microcontroller and sent back to the main microprocessor
using proprietary encoding schemes.

2.9.3 Power supply circuit theory

The following information provides some basic circuit theo-
ry that can be used as an aide to troubleshoot the power sup-
ply.
Pre-regulator circuit
The pre-regulator circuit regulates power to the transformer.
When power is applied to the instrument, a power transform-
er secondary voltage (pins 12 and 13) is rectified (CR622),
doubled (C624, C630, CR624 and CR625) and applied to
U619, which is a +5V regulator. This +5V (+5VC) is used for
the pre-regulator circuit.
The pre-regulator circuit monitors the voltage level on C611
using an integrator (U627). The voltage on C611 (typically
around 7.5V) is divided by three through R712 and R713 and
applied to the inverting input (pin 2) of the integrator. The
+5V (+5VC) is divided by two through R706 and R708. This
2.5V reference is applied to the non-inverting input (pin 3) of
the integrator.
When the voltage on the inverting input of the integrator is
less than the 2.5V reference on the non-inverting input, the
integrator output ramps in the positive direction. This posi-
tive ramp turns on Q608, which pulls the CONT line low to
digital common. With CONT connected to common, current
flows through the photodiode of U100 and generates a posi-
tive voltage at the gate of FET Q528. As Q528 turns on, the
470Ω resistor (R100) becomes shunted and results in less ef-
fective resistance to the transformer. The resultant increase
in current (power) will increase the voltage on C611.
Conversely, when the voltage on the inverting input of the in-
tegrator is more than the 2.5V reference, the integrator out-
put ramps in the negative direction and begins to turn Q608
off. This will decrease current through U100, decrease the
positive voltage on Q528 and thus, increase the effective re-
sistance to the transformer. The resultant decrease in current
(power) will decrease the voltage of C611.
This constant regulation of effective resistance in series with
the transformer regulates the power delivered to the instru-
ment.
Line voltage (110V/220V) selection circuit
This circuit automatically selects the proper power line volt-
age setting for the instrument. The line selection circuit de-
rives its power from the AC1 and AC2 lines on the primary
side of the transformer. Rectifier CR104 applies approxi-
mately +18V to regulator U105. The output of U105 pro-
vides the +8V for the line voltage selection circuit and the
HI/LO voltage control circuit.
U102 is a comparator that has a +4V reference (via voltage
divider R111 and R112) applied to its non-inverting input.
The inverting input monitors the voltage on C103. When the
voltage at the inverting input is greater than 4V, the output of
U102 goes low and turns on FET Q106. With Q106 on, +8V
Troubleshooting
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