Keithley 242 Service 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 repair 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 representative 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
GENERAL DESCRIPTION ........
1.
l-1.
1-2.
General ...........
Features ..........
l-3. Applications ........ 1
1-4. Specifications ....... 2
1-5.
Accessories ......... 2
l-6. Equipment Received ..... 2
2.
OPERATION.
2-1.
Front Panel Controls and
Terminals
2-2.
2-3.
2-4.
CIRCUIT DESCRIPTION ........
3.
3-l.
3-2.
Operating Procedures .... 4
Overloads
Floating Operation .....
General ...........
AC Control Circuits ..... 5
............
.........
..........
3-3; Low and High Voltage Supplies 5
3-4.
4.
SERVICING. ............
4-1.
4-2.
Regulator Circuit ......
General ...........
Servicing Schedule .....
Page
1
1
1
3
3
4
4
5
5
6
7
7
7
Section
4-3.
4-4.
4-5.
Parts Replacement . . . . . . 7
Troubleshooting . . . . . . . 7
Procedures to Guide
Page
Troubleshooting , . . . , , 8
4-6.
4-7.
Correcting for No Output , . 9
Correcting for Inaccurate
Voltage Output . . . . . . . 9
4-a.
Bias Adjustment , . . . . . .lO
4-9. Improperly Working Overload .lO
5.
CALIBRATION . . . . . . . . . . . .ll
5-1.
5-2.
5-3.
REPLACEABLE PARTS . . . . . . . . .17
6.
6-l.
6-2.
General . . . . . . . . . . .ll
Calibration Schedule . . . .ll
Output Calibration . . . . .ll
Replaceable Parts List . . .17
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
GENERAL DESCRIPTION
SECTION 1.
l-l. GENERAL.
stable outputs between 300 and 3500 volts dc.
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,
iO.OOS% from zero to full load.
l-2.
trol interpolates between steps with resolution better than 15 millivolts.
selected positive or negative, or floated up to 1000 volts off chassis ground.
zener diode reference, a photo-modulator comparator and wirewound sampling resistors.
ulator and epoxy encapsulated power transformers.
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.
applications include use in calibration tests, voltage gradient studies, insulation tests
and leakage resistance measurements.
FEATURES.
Four in-line calibrated dials set the output voltage in l-volt steps.
a.
Cumulative drift is prevented by a chopper circuit with a temperature-compensated
b.
c. The Model 242 uses heavy duty thermionic rectifiers,
The Keithley Model 242 Regulated High Voltage Supply provides accurate,
GENERAL DESCRIPTION
Accuracy is -0.1% for all loads up to 25
and load regulation is within
A TRIM Con-
Output can be
a maintenance-free photo-mod-
All high voltage capacitors are of oil-
Other
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.
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1
GENERAL DESCRIPTION
MODEL 242 VOLTAGE SUPqLY
l-4.
OUTPUT:
ACCURACY:
RESOLUTION:
ter than 15 millivolts.
RESETABILITY: 20.01%.
STABILITY:
TEMPERATURE COEFFICIENT: &5 ppm/OC maximum.
LINE REGULATION:
LOAD REGULATION:
RIPPLE AND NOISE:
SPECIFICATIONS.
Voltage:
Current:
Polarity:
Floating:
300 to 3500 volts dc in l-volt steps.
25 milliamperes dc maximum.
20.1% of dial setting.
20.01% per 6 months after a 30-minute warm-up.
Positive or negative.
1000 volts maximum off chassis ground.
A,TRIM Control permits interpolation between steps with a resolution of bet-
+0.005% for 10% change in line voltage.
;fO.OOS% from zero to full load.
Less than 2 millivolts rms above. 5 cps.
OVERLOAD PROTECTION:
approximately 27 milliamperes.
CONNECTORS:
POWER: 105-125 volts, SO-60 cps, 250 watts. 210-250 volt models available.
DIMENSIONS, WEIGHT:
pounds.
ACCESSORIES SUPPLIED: Mating connectors.
l-5. ACCESSORIES.
2421 End Frames for bench use.
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.
cap and the other with a plain protective cap.
struction Manual and two mating output plugs.
Before shipping the Model 242, always remove tubes V8 and Vl2.
separately to prevent damage during shipping.
Output:
Output is disconnected within 25 mFlliseconds if current exceeds
Two Teflon-insulated hn type. Ground:
7 inches high x 19 inches wide x 13-l/4 inches deep; net weight, 39
The Model 242 is designed for rack mounting and requires the Model
The pair of end frames, which comes with feet and mount-
Both output receptacles are capped, one with a red shorting
The shipping carton also contains the In-
NOTE
Binding post.
Pack these
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MGDEL 242 VOLTAGE SUPPLY
OPERATION
SECTION 2.
2-1. FRONT PANEL CONTROLS AND 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.
TRIM Control. The TRIM Control extrapolates between l-volt settings with a resolu-
b.
tion to 15 millivolts. The span of the control is 1.5 volts.
c. POWER Switch. A toggle switch energizes the filament and low voltage supplies to
the Model 242. When the Switch is ON, the POWER pilot 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.
also resets the High Voltage Supply after an overload.
the RIGH VOLTAGE Switch is ON. When the Switch is off, the high voltage output is disconnected, although the filament and low voltage circuits continue operating.
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.
post (GND) is between the receptacles.
receptacle during normal operation.
242 is not operating.
A toggle switch controls the high voltage output circuits and
The red cap is a shorting cap to ground the unused
The other cap protects the receptacle when the Model
OPERATION
The amber pilot light indicates
The Fuse for
The ground
f. OVERLOAD Pilot Light.
the overload relay opens the high voltage output circuit and lights the red
Light. The Pilot Light also lights for settings below 300 volts.
FIGURE 2.
Replaceable Parts List and the schematic diagram.
Model 242 Front Panel Controls and Terminals, Circuit designations refer to
When the output current exceeds approximately 27 milliamperes,
Xlmm "olfaga switch CSS)
Xl00 "okage Stitch (56)
s"itd,
OVER,l,OAD
Pilot
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OPERATION
MODEL 242 VOLTAGE SUPPLY
2-2.
OPERGTING PROCEDURES.
a. Keep the POWER Switch, HIGH VOLTAGE Switch and TRIM Control off. Connect the power
cord at the back of the cabinet to an outlet.
ment ,
to the + OUTPUT Receptacle;
ground the negative line.
the Model 242 connects into 105-125 volt, 50-60 cps sources.
Connect the load to the Model 242. For positive'polarity output, connect the cable
b.
connect the red shorting cap to the - OUTPUT Receptacle to
The + OUTPUT Receptacle has its center terminal at positive
polarity and its shell at chassis ground.
Unless otherwise specified on the instru-
Reverse procedures for negative polarity
output.
Set the POWER Switch to ON.
C.
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.
Warm up the Model 242 for 30 seconds.
d.
Put the HIGH VOLTAGE Switch in ON position.
Warm up the Model 242 for approximately 30 minutes for maximum output stability.
NOTE
The OVERLOAD Pilot Light will light if the HIGH VOLTAGE Switch is ON when
the POWER Switch is turned on. OVERLOAD will also light if the circuits are not
warmed up.
When the OVERLOAD bulb 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.
When resetting output voltage or changing terminals, put the HIGH VOLTAGE Switch to
f.
OFF position.
for the output change.
2-3.
OVERLOADS.
The POWER Switch can remain on as long as the HIGH VOLTAGE Switch is off
Only the green light should be on for this step.
If the red OVERLOAD Pilot 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 OVERLOAD Pilot 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.
of the high voltages present,
make sure all instructions are understood and fol-
Because
lowed before attempting floating operations.
4
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MODEL 242 VOLTAGE SUPPLY
CIRCUIT DESCRIPTION
SECTION 3. CIRCUIT DESCRIPTION
3-l.
stable outputs from 300 to 3500 volts dc.
tionship between the operating circuits.
selects the output voltage.
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.
maintains the output voltage constant at a
level determined by the divider ratio.selected with the front panel Voltage Switches.
3-2. AC CONTROL CIRCUITS.
ure 4.)
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
control relay, K2, will close.
to transformer T2, which supplies high voltages to the series regulator, can now operate when the HIGH VOLTAGE Switch is turned
On.
GENERAL.
Refer to Schematic Diagram 14661D
for circuit designations.
Closing the POWER Switch, Sl, com-
a.
The Keithley Model 242 Regulated High Voltage Supply furnishes accurate,
Setting the sampling resistors in the divider
The maintenance-
This
NOTE
(Refer to Fig-
The circuit
The block diagram, Figure 3, shows the rela-
LkID"r
I -
I
,&Qgig+Il
FIGURE 3.
Ll".
mwc
Model 242 Block Diagram.
If the HIGH VOLTAGE Switch is already
b.
ON when the POWER Switch is put ON, the circuit activating the control relay is open.
Therefore, transformer T2 can not function,
even though the other two transformers are
operating. FIGURB 4.
Circuits.
c. The Model 242 has several safety de-
vices in its ac control circuits.
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. LOW AND HIGH VOLTAGE SUPPLIES.
Transformer Tl supplies low voltages to bias the circuits and to furnish the voltage
a.
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.
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The over-
Circuit designations refer to schematic digram.
Simplified Diagram of AC Contra
All circuits are shown open.
The transformer also provides
5
CIRCUIT DESCRIPTION
MODEL 242 VOLTAGE SUPPLY
the gating voltage for the diode bridge demodulator, D106 to D109.
Transformer T2 supplies the high voltages for the series regulator.
b.
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.
REGULATOR CIRCUIT. (See Figure 5.)
The regulator circuit uses a photo-
a,
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.
For low-frequency fluctuations at the
b.
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
is then amplified by tube V2 and demodulated
FIGURE 5.
Circuit.
Simplified Diagram of Regulator
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.
t
For high-frequency fluctuations and transients in the output, the feedback loop is
c.
through capacitor Cl20 to the grid of tube V4, which applies the signal to the bias of
the series regulator tube, V12.
The voltage switching is done by current sensing, which means no switch contact is
d.
required to
carry
more than 5 milliamperes or switch more than 9 volts.
6
0965R
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