Display Board Circuit Theory; Power Supply Circuit Theory - Keithley 2001 Repair Manual

2.10.1 Display board circuit theory

The following information provides some basic circuit
theory that can be used as an aide to troubleshoot the
display and keyboard.
Display microcontroller
U902 is the display microcontroller that controls the
VFD (vacuum ßuorescent 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 microcon-
troller from the digital board via the TXB line to the mi-
crocontroller 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 micro-
controller is generated on the digital board.
Vacuum fluorescent display
DS901 is the VFD (vacuum ßuorescent display) mod-
ule, which can display up to 49 characters. Each char-
acter is organized 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 se-
rially transmitted from the microcontroller (PD3 and
PC1).
The VFD requires both +60VDC and 5VAC for the Þla-
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 micro-
controller peripheral lines required to read the key-
board. 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.

2.10.2 Power supply circuit theory

The following information provides some basic circuit
theory that can be used as an aide to troubleshoot the
power supply.
Pre-regulator circuit
The pre-regulator circuit regulates power to the trans-
former. When power is applied to the instrument, a
power transformer secondary voltage (pins 12 and 13)
is rectiÞed (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 integra-
tor is less than the 2.5V reference on the non-inverting
input, the integrator output ramps in the positive di-
rection. This positive ramp turns on Q608 which pulls
the CONT line low to digital common. With CONT
connected to common, current ßows through the pho-
todiode of U100 and generates a positive voltage at the
gate of FET Q528. As Q528 turns on, the 470Ω resistor
(R100) becomes shunted and results in less effective re-
sistance 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 integrator is more than the 2.5V reference, the inte-
grator output ramps in the negative direction and be-
gins to turn Q608 off. This will decrease current
through U100, decrease the positive voltage on Q528
and thus, increase the effective resistance to the trans-
former. 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 instrument.
Troubleshooting
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