Keithley 242 User manual

INSTRUCTION MANUAL
MODEL 242
REGULATED HIGH VOLTAGE SUPPLY

WARRANTY
We warrant each of our products to be free
from defects in material and workmanship. Our
obligation under this warranty is to repair ‘or
replace any instrument or part thereof (except
tubes and batteries) which, within a year after
shipment, proves defective upon examination.
We will pay domestic surface freight costs.
To exercise this warranty, call your local
field representative or the factory, DDD 216.
795-2666. You will be given assistance and
shipping instructions.
REPAIRS AND RECALIBRATION
Keithley Instruments maintains a complete re-
pair service and standards laboratory in Cleve-
land, and has an authorized field repair
facility in Los Angeles.
To insure prompt repair or recalibration serv-
ice, please contact your local field representa-
tive or the plant directly before returning the
instrument.
Estimates for repairs, normal recalibrations,
and calibrations traceable to the National Bureau
of Standards are available upon request.

MODEL 242 VOLTAGE SUPPLY CONTENTS
TABLE OF CONTENTS
section
1.
2.
3.
4.
Page
GENERALDESCRIPTION ........ 1
l-1. General ........... 1
1-2.
Features .......... 1
l-3. Applications ........ 1
1-4. Specifications ....... 2
1-5.
Accessories ......... 2
l-6. Equipment Received ..... 2
OPERATION. ............ 3
2-1.
Front Panel Controls and
Terminals ......... 3
2-2.
Operating Procedures .... 4
2-3.
Overloads .......... 4
2-4. Floating Operation ..... 4
CIRCUIT DESCRIPTION ........ 5
3-l.
General ........... 5
3-2. AC Control Circuits ..... 5
3-3; Low and High Voltage Supplies 5
3-4. Regulator Circuit ...... 6
SERVICING. ............ 7
4-1.
General ........... 7
4-2. Servicing Schedule ..... 7
Section Page
5.
6.
4-3. Parts Replacement . . . . . . 7
4-4.
Troubleshooting . . . . . . . 7
4-5. Procedures to Guide
Troubleshooting , . . . , , 8
4-6.
Correcting for No Output , . 9
4-7. Correcting for Inaccurate
Voltage Output . . . . . . . 9
4-a.
Bias Adjustment , . . . . . .lO
4-9. Improperly Working Overload .lO
CALIBRATION . . . . . . . . . . . .ll
5-1.
General . . . . . . . . . . .ll
5-2.
Calibration Schedule . . . .ll
5-3. Output Calibration . . . . .ll
REPLACEABLEPARTS . . . . . . . . .17
6-l. Replaceable Parts List . . .17
6-2.
How to Order Parts . . . . .17
Model 242 Replaceable Parts
List . . . . . . . . . . .~ .18
Model 242 Schematic Diagram
14661D . . . . , . , . . . .25
'*Change Notice
QkYellow Change Notice sheet is included only
for instrument modifications affecting the
Instruction Manual.
0865R i

KODEL 242 VOLTAGE SUPPLY
SECTION 1. GENERAL DESCRIPTION
GENERALDESCRIPTION
l-l. GENERAL. The Keithley Model 242 Regulated High Voltage Supply provides accurate,
stable outputs between 300 and 3500 volts dc. Accuracy is -0.1% for all loads up to 25
milliamperes. Stability is 20.01% for six months after a 30-minute warm-up. Line regula-
tion is within ?O.OOS% for a 10% change in line voltage, and load regulation is within
iO.OOS% from zero to full load.
l-2. FEATURES.
a. Four in-line calibrated dials set the output voltage in l-volt steps. A TRIM Con-
trol interpolates between steps with resolution better than 15 millivolts. Output can be
selected positive or negative, or floated up to 1000 volts off chassis ground.
b. Cumulative drift is prevented by a chopper circuit with a temperature-compensated
zener diode reference, a photo-modulator comparator and wirewound sampling resistors.
c. The Model 242 uses heavy duty thermionic rectifiers, a maintenance-free photo-mod-
ulator and epoxy encapsulated power transformers. All high voltage capacitors are of oil-
filled, metal-cased construction with mylar dielectric.
l-3. APPLICATIONS. The Model 242 is an excellent source for phototubes, photo-multiplier
tubes and other detectors requiring high voltage biasing with superior stability. Its
current capability permits parallel operation of photo-multiplier divider strings. Other
applications include use in calibration tests, voltage gradient studies, insulation tests
and leakage resistance measurements.
FI:GURE 1. Keithley Instruments Model 242 Regulated High Voltage Supply. The Voltage
Supply is shown with the Model 2421 End Frames, for bench use.
0865R 1

GENERALDESCRIPTION MODEL 242 VOLTAGE SUPqLY
l-4. SPECIFICATIONS.
OUTPUT:
Voltage: 300 to 3500 volts dc in l-volt steps.
Current: 25 milliamperes dc maximum.
Polarity: Positive or negative.
Floating: 1000 volts maximum off chassis ground.
ACCURACY: 20.1% of dial setting.
RESOLUTION: A,TRIM Control permits interpolation between steps with a resolution of bet-
ter than 15 millivolts.
RESETABILITY: 20.01%.
STABILITY: 20.01% per 6 months after a 30-minute warm-up.
TEMPERATURECOEFFICIENT: &5 ppm/OC maximum.
LINE REGULATION: +0.005% for 10% change in line voltage.
LOAD REGULATION: ;fO.OOS% from zero to full load.
RIPPLE AND NOISE: Less than 2 millivolts rms above. 5 cps.
OVERLOADPROTECTION: Output is disconnected within 25 mFlliseconds if current exceeds
approximately 27 milliamperes.
CONNECTORS: Output: Two Teflon-insulated hn type. Ground: Binding post.
POWER: 105-125 volts, SO-60 cps, 250 watts. 210-250 volt models available.
DIMENSIONS, WEIGHT: 7 inches high x 19 inches wide x 13-l/4 inches deep; net weight, 39
pounds.
ACCESSORIES SUPPLIED: Mating connectors.
l-5. ACCESSORIES. The Model 242 is designed for rack mounting and requires the Model
2421 End Frames for bench use. The pair of end frames, which comes with feet and mount-
ing hardware, fastens to the Voltage Supply sides and front panel.
l-6. EQUIPMENT RECEIVED. The Model 242 Voltage Supply is factory calibrated. Two vacuum
tubes, the 7235 (V4, Figure 10) and the 4-658 (V12, Figure 8) are packed separately to
prevent damage during shipping. Both output receptacles are capped, one with a red shorting
cap and the other with a plain protective cap. The shipping carton also contains the In-
struction Manual and two mating output plugs.
NOTE
Before shipping the Model 242, always remove tubes V8 and Vl2. Pack these
separately to prevent damage during shipping.
1065R

MGDEL 242 VOLTAGE SUPPLY
SECTION 2. OPERATION
OPERATION
2-1. FRONT PANEL CONTROLSAND TERMINALS (See Figure 2).
a. Voltage Switches. Four skirted dials, calibrated in 1000, 100, 10 and l-volt steps,
set the output voltage from 300 to 3500 volts dc.
b. TRIM Control. The TRIM Control extrapolates between l-volt settings with a resolu-
tion to 15 millivolts. The span of the control is 1.5 volts.
c. POWERSwitch. A toggle switch energizes the filament and low voltage supplies to
the Model 242. When the Switch is ON, the POWERpilot light will glow green to indicate
the instrument is on. The Fuse for the control circuits is next to the Switch.
d. HIGH VOLTAGE Switch. A toggle switch controls the high voltage output circuits and
also resets the High Voltage Supply after an overload. The amber pilot light indicates
the RIGH VOLTAGE Switch is ON. When the Switch is off, the high voltage output is discon-
nected, although the filament and low voltage circuits continue operating. The Fuse for
the high voltage circuit is next to the Switch.
e. OUTPUT Receptacles. Rro hn-type receptacles (UG 496/U) are on the front panel,
marked for negative (-) and positive (+) outputs with respect to the chassis. The ground
post (GND) is between the receptacles. The red cap is a shorting cap to ground the unused
receptacle during normal operation. The other cap protects the receptacle when the Model
242 is not operating.
f. OVERLOADPilot Light. When the output current exceeds approximately 27 milliamperes,
the overload relay opens the high voltage output circuit and lights the red OVER,l,OAD Pilot
Light. The Pilot Light also lights for settings below 300 volts.
Xlmm"olfagaswitchCSS)
Xl00"okageStitch(56)
s"itd,
FIGURE 2. Model 242 Front Panel Controls and Terminals, Circuit designations refer to
Replaceable Parts List and the schematic diagram.
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OPERATION MODEL 242 VOLTAGE SUPPLY
2-2. OPERGTING PROCEDURES.
a. Keep the POWERSwitch, HIGH VOLTAGE Switch and TRIM Control off. Connect the power
cord at the back of the cabinet to an outlet. Unless otherwise specified on the instru-
ment , the Model 242 connects into 105-125 volt, 50-60 cps sources.
b. Connect the load to the Model 242. For positive'polarity output, connect the cable
to the + OUTPUTReceptacle; connect the red shorting cap to the - OUTPUT Receptacle to
ground the negative line. The + OUTPUT Receptacle has its center terminal at positive
polarity and its shell at chassis ground. Reverse procedures for negative polarity
output.
C.
Set the POWERSwitch to ON. Select the output voltage by setting the four Voltage
Switches between 300 and 3,500 volts.
NOTE
Make sure the TRIM Control is set to OFF. If it is on, up to 1.5 volts will be
added to the displayed output.
d. Warm up the Model 242 for 30 seconds. Put the HIGH VOLTAGE Switch in ON position.
Warm up the Model 242 for approximately 30 minutes for maximum output stability.
NOTE
The OVERLOADPilot Light will light if the HIGH VOLTAGE Switch is ON when
the POWERSwitch is turned on. OVERLOADwill also light if the circuits are not
warmed up. When the OVERLOADbulb lights, put the HIGH VOLTAGE Switch off and
wait 30 seconds. Then put on HIGH VOLTAGE Switch again.
e. If desired, use the TRIM Control for settings between the l-volt steps. Resolution
is better than 15 millivolts.
f. When resetting output voltage or changing terminals, put the HIGH VOLTAGE Switch to
OFF position. The POWERSwitch can remain on as long as the HIGH VOLTAGE Switch is off
for the output change. Only the green light should be on for this step.
2-3. OVERLOADS. If the red OVERLOADPilot Light lights, the circuit is overloaded and
the output is disconnected. To reset the Model 242, snap the HIGH VOLTAGE Switch off,
wait 30 seconds, then put it on again. Refer to Section 4 if the OVERLOADPilot Light
continues to light.
2-4. FLOATING OPERATION. Remove the shorting cap from the receptacle. Connect two
separate shielded coaxial cables, type RG-149/U or equivalent, to the OUTPUT Receptacles,
one for positive and one for negative polarity, Use only the two furnished connectors
and the specified shielded cables because of the high potentials present.
NOTE
Do not float the Voltage Supply more than 1000 volts off chassis ground. Because
of the high voltages present, make sure all instructions are understood and fol-
lowed before attempting floating operations.
4 1065R

MODEL 242 VOLTAGE SUPPLY
SECTION 3. CIRCUIT DESCRIPTION
CIRCUIT DESCRIPTION
3-l. GENERAL. The Keithley Model 242 Regulated High Voltage Supply furnishes accurate,
stable outputs from 300 to 3500 volts dc. The block diagram, Figure 3, shows the rela-
tionship between the operating circuits. Setting the sampling resistors in the divider
selects the output voltage. The maintenance-
free photo-modulator compares a portion of
the output to the sener diode reference.
The difference is amplified and used to
control the series regulator tube. This LkID"r
maintains the output voltage constant at a
level determined by the divider ratio.se- I -
lected with the front panel Voltage Switches. I
3-2. AC CONTROLCIRCUITS. (Refer to Fig- ,&Qgig+Il
ure 4.)
NOTE
Refer to Schematic Diagram 14661D
for circuit designations.
a. Closing the POWERSwitch, Sl, com- FIGURE 3. Model 242 Block Diagram.
pletes the circuit from the power line to
transformer Tl, which furnishes the bias
voltages, and to filament transformer T3..
If the HIGH VOLTAGE Switch, 54, is off, the Ll".
control relay, K2, will close. The circuit mwc
to transformer T2, which supplies high vol-
tages to the series regulator, can now op-
erate when the HIGH VOLTAGE Switch is turned
On.
b. If the HIGH VOLTAGE Switch is already
ON when the POWERSwitch is put ON, the cir-
cuit activating the control relay is open.
Therefore, transformer T2 can not function,
even though the other two transformers are
operating. FIGURB 4. Simplified Diagram of AC Contra
Circuits. All circuits are shown open.
c. The Model 242 has several safety de- Circuit designations refer to schematic di-
vices in its ac control circuits. The over- gram.
load relay, Kl, breaks the circuit to the
control relay and to transformer T2 if the
output current exceeds approximately 27 milliamperes. The decade interlocks, 55 and 56,
will not close for outputs less than 300 volts.
3-3. LOWAND HIGH VOLTAGE SUPPLIES.
a. Transformer Tl supplies low voltages to bias the circuits and to furnish the voltage
for the sener diode reference. A separate winding, along with tube V9, provides regulated
105 volts to the screen of the series regulator tube, V12. The transformer also provides
1065R 5

CIRCUIT DESCRIPTION MODEL 242 VOLTAGE SUPPLY
the gating voltage for the diode bridge demodulator, D106 to D109.
b. Transformer T2 supplies the high voltages for the series regulator. The transformer
primary is tapped to furnish a different voltage for each lOOO-volt range. This prevents
exceeding the voltage and power rating of the series regulator, Vl2, under any of the
specified line and load conditions, Two rectifiers, tubes VlO and Vll, convert the trans-
former output to full-wave rectified dc for the series regulator, V12.
3-4. REGULATORCIRCUIT. (See Figure 5.)
t
a, The regulator circuit uses a photo-
modulator to compare the output voltage to
the selected voltage, Any difference is
amplified and returned to the series regu-
lator to correct the output.
b. For low-frequency fluctuations at the
output, the voltage divider and the sampling
resistors on the decade switches divide the _
output voltage. The photo-modulator, ElOl,
compares the reference voltage across aener
diode D103 and the divided output voltage.
The photo-modulator develops an ac signal
for any dc voltage difference. The signal FIGURE 5. Simplified Diagram of Regulator
is then amplified by tube V2 and demodulated Circuit.
by the diode bridge, D106 to D109. The dc
signal is further amplified by V3 and V4
and applied to the bias of the series regulator tube, ~12.
c. For high-frequency fluctuations and transients in the output, the feedback loop is
through capacitor Cl20 to the grid of tube V4, which applies the signal to the bias of
the series regulator tube, V12.
d. The voltage switching is done by current sensing, which means no switch contact is
required to carry more than 5 milliamperes or switch more than 9 volts.
6 0965R

MODEL 242 VOLTAGE SUPPLY SERVICING
SECTION 4. SERVICINO
4-1. GENERAL. Section 4 contains the maintenance and troubleshooting procedures for the
Mode1~242 Voltage Supply. Follow these as closely as possible to maintain the specifica-
tions of the instrument.
4-2. SERVICING SCHEDULE. The Model 242 requires no periodic maintenance beyond the nor-
mal care required of high-quality electronic equipment, Occasional checks of the output,
described in Section 5, will show the need for any adjustments. No part should need fre-
quent replacement under ordinary use.
4-3. PARTS REPLACEMENT.
a. The Replaceable Parts List in Section 6 describes the electrical components of the
Voltage Supply. Replace components only a8 necessary. Use only reliable replacements
which meet the specifications.
b. The aener diode, D103, is selected, Diodes D104 and D105 are matched; replace only
as a pair. Order these replacements only from Keithley Instruments, Inc., or its repre-
sentative, as well as other parts marked for Keithley manufacture in the Replaceable Parts
List.
4-4. TROUBLESHOOTING.
a. The procedures which follow give instructions for repairing troubles which might
occur in the Model 242, Use the procedures outlined and uee only specified replacement
parts. Table 1 lists equipment recommended for troubleshooting. If the trouble cannot be
located or repaired, contact Keithley Instruments, Inc., or its representative.
Instrument
Keithley Instruments Model 153 Microvolt-
Anrmeter, 1% accuracy at 100 millLvolts
Use
Zener circuit adjustment
Tektronix Type 503 Oscilloscope, 1 mv/cm sen- Adjust bias
sitivity, dc ‘to 450 kc
dc voltmeter, 10% accuracy, to 4000 volts, Measure dc voltages
lo-megohm minimum input resistance
Grid-modulated tube tester
lo-ohm, l/2 watt, l/2% deposited carbon
resistor (Keithley Part R12-10)
Test vacuum tubes
Zener circuit adjustment
NE-1 neon tube; l/2 watt, l/Z% deposited Adjust bias
carbon resistor; 0.5-microfarad, 400C-volt
capacitor
TABLE 1. Equipment Recommended for Model 242 Troubleshooting. Use these instruments or
their equivalents.
1065R 7