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.
1065R

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

SERVICING MODEL 242 VOLTAGE SUPPLY
Trouble
Probable Cause Remedy
J output Blown fuse, Fl or F2 Replace fuse; if fuse again
blows, check further
Defective VlO, Vll, R113 See paragraph 4-6
or cl06
Defective series regulator See paragraph 4-6
v12
utput voltage not stable Defective Cl15 Check capacitor; replace if
faulty
utput voltage not accurate Power Supply out of Recalibrate per paragraph 5-3
calibration
Improper zener adjustment See paragraph 4-7
xcessive noise in output Bias out of adjustment See paragraph 4-8
verload circuits cuts off Defective R118 or Kl See paragraph 4-9
ith no overload Defective Cl15 Check capacitor; replace if
faulty
oltage Supply operates Cl16 shorted Check capacitor; replace
ven when overloaded if faulty
TABLE 2. Model 242 Troubleshooting. Read paragraph 4-5 before performing any repairs.
b. Table 2 lists problems which might occur. If the repair indicated does not work,
check through each circuit as~described in the following paragraphs. Refer to the cir-
cuit description in Section 3 to find the more crucial components and to determine their
function. The complete
circuit diagram, 14661D, is in Section 6.
4-5. PROCEDURESTO GUIDE TROUBLESHOOTING.
a. Before troubleshooting the Voltage Supply, check the external circuits. Check the
fuse, power cord and power source. The Schematic Diagram indicates the tube element volt-
ages referenced to chassis ground. Measure the dc voltages to rlO% of the indicated values.
b. Check the vacuum tubes. Normally, replacing tubes will clear up any difficulty.
All tubes except V4, VlO, Vll and V12 are readily tested on a grid-modulated tube tester.
Check the remaining four tubes by replacing them.
NOTE
Before removing the top or bottom cover, make sure the power cord is disconnected.
After removing the cover, discharne all metal-cased capacitors before proceeding
with repairs, Discharge all high voltages through a bleeder.
c. Remove the Model 242 covers. Do not put a load on the instrument. Put the red
8
0865R

MODEL 242 VOLTAGE SUPPLY SERVICING
shorting cap on the - OUTPUTReceptacle. Set the Voltage Switches for a 300-volt output;
make sure the TRIM Control is off. Put on the POWERand HIGH VOLTAGE Switches, as outlined
in paragraph 2-2.
NOTE
Be careful working inside the chassis. If the POWERSwitch is on, up to 200 volts
may appear at points in the control circuits. High voltage appears on both sides
of the chassis. Before working on a faulty Voltage Supply, disconnect the power
cord and discharge all filter capacitors, as Cl06 and C115.
4-6. CORRECTINGFOR NO OUTPUT.
Check both front panel fuses. The HIGH VOLTAGE Pilot Light indicates when it is
pozsible to have a high voltage output ; however, it can light even when the fuse is blown.
Replace defective fuses. If the fuse again blows, check further for the cause.
b. Measure the plate voltage of series regulator tube V12. Use a dc voltmeter which
measures to 4000 volts. Make sure the Voltage Switches are set at 300 volts before making
this measurement. The plate voltage should be 2000 volts f20%.
NOTE
Discharge high voltage from tube V12 through a bleeder. Discharging the cap
directly to the chassis may damage resistor R113 or diode DlOl.
c. If the plate voltage is not correct, check rectifiers VlO and Vll, the series resis-
tor R113 (Figure ll), the filter capacitor Cl06 (Figure 8) and the primary winding of
transformer T2 (Figure 8) for 117 volts.ac. If no voltage appears on the transformer pri-
mary, check the overload relay, Kl (Figure lo), and the control relay, K2 (Figure 8).
d. If the plate voltage of tube V12 is correct, check the cathode voltage across pin 1
or 7 to ground. The voltage should be 300 volts *LO%.
e. Incorrect voltage at the cathode indicates tube V12 is faulty or incorrect biasing
conditions exist. If the voltage is still not correct after tube V12 is replaced, check
tube V4 (Figure lo), the dc amplifier tube V3 (Figure lo), the ac amplifier tube V2 (Fig-
ure lo), and the diodes in the demodulating bridge, D106 to DlC9 (Figure 10).
f. If the plate voltage of V12 is correct, check the overload relay Kl. Refer to par-
agraph 4-9.
4-7. CORRECTINGFOR INACCURATE VOLTAGE OUTPUT. Poor regulation or excessive drift will
affect the accuracy of the voltage output. Several factors cause this; check the following.
a. A defective capacitor Cl15 (Figure 8) will cause the output to be unstable.
b. Although it normally does not need replacement, check the zener diode D103 (Figure
8). Since adjusting the diode circuit requires recalibrating the entire instrument, check
the zener diode only after all other possible trouble sources are eliminated.
1. Remove the jumper in series with diode D103 (the jumper is next to R133, Figure 9).

SERVICING MODEL 242 VOLTAGE SUPPLY
Connect a lo-ohm, l/2% resistor (Ke-ithley Part No. R12-10) at this point. Read the volt,
age across the resistor with the Model 153. Adjust the ZENER ADJ potentiometer, R133
(Figure 9), until the microvoltmeter reads the product of the rated zener current (7.5
milliamperes) times the resistance (10 ohms) or 75 millivolts *l%.
2. The voltage across the zener diode should now be between 8.55 and 9.45 volts. If
this voltage is not correct, check the regulator tube Vl (Figure lo), resistor R132
(Figure ll), the ZBNER ADJ potentiometer R133 and the zener diode D103.
3. After adjusting the voltage, recalibrate the Model 242 (paragraph 5-3). w
4-8. BIAS ADJiTSTMENT.
a. Connect the Model 242 to the circuit OscFlloscope
shown in Figure 6. Use a resistor which
will allow a full load - 25 milliamperes Model 242
- + + -
- on the Model 242. For example, set the 0 0
Voltage Supply to 1000 volts output and use
1
a 25-kilohm resistor. Use a 0.5-microfarad,
4000-volt blocking capacitor in series with
the oscilloscope for protection.
b. Turn the Voltage Supply on; monitor
the output on the oscilloscope. The oscil-
loscope should show a signal with Z-milli-
volt rms ripple or less. Adjust the BIAS FIGURE 6. Circuit Diagram for Model 242
ADJ potentiometer, R144 (Figure 9), until the Bias Adjustment. Refer to paragraph 4-8
minimum ripple appears on the oscilloscope. for procedures and Table 1 for equipment.
c. If the potentiometer does not have suf-
ficient range to lessen the noise, check capacitor ClO8. If capacitor Cl08 (Figure 10) Fs
open, it will cause excessive noise. If it is good, observe the output wave. A high level
of corona indicates the feedback capacitor Cl20 (Figure 9) is open or the core of transfor-
mer T2 (Figure 8) is grounded.
4-9. IMPROPERLY WORKINGOVERLOAD.
a. If the OVElUOAD Lamp keeps lighting when the instrument is turned on, turn it off
and remove the load. If the instrument works when the load is not present, the overload
is in the external circuit. If the instrument continues to show an overload, check the
Model 242 for the trouble. Check the circuit voltages against those marked on the Sche-
matic Diagram.
b. If the overload relay Kl (Figure 10) will not break the circuit when the Voltage
Supply becomes overloaded, check filter capacitor Cl16 (Figure 10) for a short and check
the overload relay for proper operation.
c. If the overload relay cuts off the circuit with no'overload present, check the coil
of Kl (Figure 10) against the rated value printed on it. Also check the shunting resis-
tance R118 (Figure 9) to make sure its value has not changed.
d. If previous checks have not located the source, check capacitor Cl15 (Figure,8). A
leaky or shorted capacitor here will cause troubles similar to those caused by a faulty
overload circuit.
10 1065R

MODEL 242 VOLTAGE SUPPLY CALIBRATION
SECTION 5. CALIBRATION
5-1. GENERAL.
a. The following procedures are recommended for calibrating the Model 242. It is also
recommended that the equipment listed in Table 3 be used. If difficulty is encountered,
contact Keithley Instruments, Inc., or its representative.
b. This Section covers calibrating the accuracy of the output voltage.
c. If the instrument is not within specifications after the calibration, follow the
troubleshooting procedures or contact Keithley Instruments, Inc., or its representative.
5-2. CALIBRATION SCHEDULE. Check the accuracy of the output voltage every six months.
Also recalibrate the instrument if the series regulator tube, V12, is replaced. Replacing
or adjusting the zener diode, D103, also necessitates recalibrating the output.
5-3. OUTPUT CALIBRATION.
a. Set up the Model 242 according to Figure 7. Use the Model 660A Guarded dc Differcn-
tial Voltmeter (*0.02% limit of error) and the Model 6601A Divider (ti.Ol% limit of error).
Total limit of error of this system is f0.03%. The Model 660A Differential Voltmeter may
be used directly for output voltages from 300 to 500 volts.
b. Turn on the Model 242. Set the output to 3500 volts; make surs the TRIM Control is
off before making the calibration. The voltmeter should read 3500 volts r3.5 volts. If
necessary, adjust the HV ADJ potentiometer, R136 (Figure 9), until the output reads 3500
volts on the Model 660A Voltmeter.
CAUTION
Be careful making internal adjustments. Up to 5000 volts dc and 3600 volts ac are
present inside the instrument.
C. Set the Voltage Switches to 300 volts. The voltmeter should read 300 volts $0.3
volt. If necessary, adjust the LV ADJ potentiometer, R142 (Figure 9), until the output
Teads 300 volts on the Model 660A Voltmeter.
Instrument Use
Keithley Instruments Model 660A Guarded dc
Differential Voltmeter, 20.02% limit of Calibrate accuracy of output
error, 100 millivolts to 500 volts
Keithley Instruments Model 6601A Voltage
Divider, lOO:l, 100 volts to 5000 volts,
-To. 01% accuracy
Calibrate accuracy of output
TABLE 3. Equipment Recommended for Model 242 Calibration. Use these instruments or their
equivalents.
11

CALIBRATION MODEL 242 VOLTAGE SUPPLY
d. Repeat the 3500 and 300-volt adjustments until there is no discrepency between the
Model 242 output setting and the Model 660A Voltmeter reading.
FI LE 7. Block Diagram for Calibrating Model 242 Output Accuracy. See Table 3 for
equipment. Use coaxial cable to connect the Models 242 and 6601A.
Control
Zener Adjustment
Circuit
Desig.
R133
Fig.
Ref.
9
Refer to
Paragraph
4-7
High Voltage Adjustment
Low Voltage Adiustment
R136 9 5-3
R142 9 5-3
I
Bias Adjustment
I
Rl44
TABLE 4. Model 242 Internal Controls. The Table lists all internal controls, the figure
picturing the'location and the paragraph describing the adjustment.
12 1065R

MODEL 242 VOLTAGE SUPPLY CALIBRATION
(W
R21!
(R20
Rm
SH Figs. 10, 11 .“”,,
FIGURE 8. Top View of Model 242 Chassis. The illustration shows the location of com-
ponents and printed circuit. Refer to Figure 9 for a bottom view.
0865R 13

CALIBRATION MODEL 242 VOLTAGE SUPPLY
, ::z
FIGURE 9. Bottom View of Model 242 Chassis. The illustration shows the location of corn
ponents and internal controls. Refer to Figure 8 for a top view.
14 086'5R

MODEL 242 VOLTAGE SUPPLY CALIBRATION
FIGURE 10. Tube, Diode, Capacitor and Modulator Locations on Printed Circuit Board PC-100.
Resistors are shown in Figure 11.
0865R 15

CALIBRATION MODEL 242 VOLTAGE SUPPLY
FIGURE 11. Resistor Locations on Printed Circuit Board PC-100. Other components are
shown in Figure 10.
16 0865R

MODEL 242 VOLTAGE SUPPLY REPLACEABLEPARTS
SECTION 6. REPLACEABLE PARTS
6-l. REPLACEABLEPARTS LIST. The Replaceable Parts List describes the components of the
Model 242 Regulated High Voltage Supply. The List gives the circuit designation, the
part description, a suggested manufacturer, the manufacturer's part number and the Keithley
Part Number. The last column indicates the figure picturing the part. The name and ad-
dress of the manufacturers listed in the "Mfg. Code" column are contained in Table 6.
6-2. HOWTO ORDERPARTS.
a. For parts orders, include the instrument's model and serial number, the Keithley
Part Number, the circuit designation and a description of the part. All structural parts
and those parts coded for Keithley manufacture (80164) must be ordered from Keithley
Instruments, Inc. , or its representative. In ordering a part not listed in the Replaceable
Parts List, completely des&ibe the part, its function and its location.
b. Order parts through your nearest Keithley representative or the Sales Service Depart-
ment, Keithley Instruments, Inc.
=mP
CbVar
CerD
camp
CompV
DCb
ETB
ETT
f
k
ampere
Carbon Variable
Ceramic, Disc
composition
Composition Variable
Deposited Carbon
Electrolytic, tubular
Electrolytic, tantalum
farad
kilo (103)
M or meg
kg.
Mil. No.
MY
n
P
i-I ’
v
Var
zw
WWVar
mega (106) or megohms
milli (10-3)
Manufacturer
Military Type Number
Mylar
pica (lo-121
micro (10-6)
volt
Variable
watt
Wirewound
Wirewound Variable
TABLE 5. Abbreviations and Symbols.
17