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PS163
Cirquit Diagram
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Datasheet
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Part List
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Service Manual
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User Manual
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Sencore PS163 Service Manual

PS163 DUAL TRACE TRIGGERED SWEEP OSCILLOSCOPE
SENCORE
SERVICE
MANUAL
"t h e a ll a me r ica n li ne”
CIRCUIT DESCRIPTION
SYNC or TRIGGER CIRCUITS
The HORIZONTAL SYNC pushbutton switch selects the source of the sync signal. The output of the HORIZONTAL SYNC switch is AC coupled through C200 to the input protection circuit comprised of CR201 and CR202 an d associated components. C203, CR200, and R200 act as a filter to remove high fre quency noise that would otherwise result in unstable sync in the DUAL CHOPPED mode. SW200A then connects the sync signal to either the sync se parator (TR200-TR203) for the TV Vertical and TV Horizontal sweep positions, or directly to the input of the differential comparitor (TR205-TR212).
The SYNC POLARITY switch (SW2) selects which in put of the sync separator or differential comparitor will receive the sync signal, or the reference voltage from the SYNC LEVEL control (R7).
Both the sync separator and the differential com paritor are balanced differential amplifiers, and there fore will accept a signa l to either input. The balanced amplifier, in the sync separator section, has just enough gai n to reliably operate the sync separa tor transistor (TR204). The sync separator transistor (TR204) is biased so that only the pea ks of the incom ing signal will cause it to conduct producing an output signal at its collector load resistor (R222). The output of TR204 goes to both a differentiating network (C211 and R227) and to an intergrating network (C213, C214 and R226). SW200C selects and couples the output of the integrating network for the TV Vertical sweep position or the output of the dif ferentiating network for the T.V. Horizontal sweep.
The HORIZONTAL SWEEP push button (SW3) switch is designed so when in the manual triggered mode, the sync level control is connected to the reference input of the differential comparitor. While in the AUTO TRIGGERED and the FREE RUNNING modes the reference side of the comparitor is connected to ground. The differential comparitor is a very high gain differential amplifier, so even a relatively small input signal will cause TR212 to change from satura tion to cutoff, resulting in a 25 volt signal to drive the Schmitt trigger. The Schmitt trigger (TR213 and TR
214) is designed so with no input signal, TR214 is in the on state with the voltage at the emitters of both transistors at approximately plus 10 volts. As soon as the input signa l to the bas e of TR213 exceeds the
10 volt level at its emitter TR213 starts to con
duct. When TR213 start s to conduct, two thing s happen. First, the voltage ac ross the common emitter resistor R254 starts to rise, and second, the voltage at
the collector of TR213 starts to drop. TR214 then rapidly changes to its off state as a result of the com bination of a positive signal on its emitter, and a neg ative signal on its base. Thus the output of the Sch
mitt trigger coupled through C219 to the base of TR 218 is a fast (lOnSec) pulse regardless of what the signal to the input of the trigger circuit was.
TIME BASE GENERATOR
The “heart” of the PS163 is the time base generator, and its function is to provide an extremely linear
ramp (sawtooth) for use as a horizontal sweep signal. The PS163 accomplishes this by u sing a constant current source to char ge the timing capacitor (C285 - C238). The constant current source is provided by TR227. TR228 is used as a diode for temperature compensation. The base and therefore the emitter voltage for TR227 is determined by the voltage divider R286 - R289. With the emitter voltage of TR227 constant, the output current will remain at a constant value determined by the 1% resistors R290 - R301. SW200F selects the valu e of timing capacitor, and SW200G selects the value of constant current resistor. R288 is the front panel horizontal frequency vernier and is controlled by the small center knob on the TIME BASE - FREQUENCY switch. R288 must be fully clockwise to' maintain time base calibration.
TR218 and TR219 form a Schmitt trigger with TR218 held in the off condition by the negative voltage from the stability control R267. The on
condition of TR219 is directly coupled by the emit ter follower transistor TR220 producing a negative voltage at the base of TR223 holding it in the on
condition. The on condition of TR233 shunts the timing capacitor, therefore producing no sweep signal.
When a positive trigger pulse is applied to the base of TR218, it, TR219, TR220, and TR221 instantly chan ge states. The positive voltage coupled to the base of TR223 causes TR223 to stop shunting the output of the constant current source . The output of the constant current source now starts charging the timing capacitor, producing a very linear negative going ramp. The negative ramp is coupled through the high impedance source follower TR229, and am plified by TR230. The negative ramp at the junction
of R303 and R304 is coupled back to the base of
TR218 through TR226, CR209 and R269, and also charges one of the hold off capacitors selected by SW200E. As soon as the negative ramp at the ba se of TR218 reaches the lower trip point of the Schmitt trigger, TR218 and TR219 will again change states, with TR219 returning to the on condition. The on condition of TR219 will turn on TR223, returning the voltage on the timing capacitor to zero, and caus ing the beam to retrace on the CRT. The voltage stored on the hold off capacitors, C231 - C234, keeps the base of TR218 negative long enough for all circ uits to reset in preparation for the next trigger pulse .
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TR224 and TR225 serve no active purpose in the circuit and are only used to kill all output from the time base when using the VECTOR INPUT, 60Hz or EXT sweep features. The emitter follower TR231 provides an isolated output of the sawtooth for the jack on the rear of the PS163.
The square wave at the emitters of TR220 and TR221 is used to drive the CRT unblanking circuit and the Display board in the alternate sweep mode.
TIME BASE CONT ROL
CIRCUITS
The control circuit for the time base generator con sists of TR215, TR216, and TR217.
In the MANUAL TRIGGERED mode, the base of TR217 is grounded by the HORIZONTAL SWEEP push button switch SW3A. With the base of TR217 grounded, TR217 will be off, and will have no effect on the negative voltage applied to the base of TR218 through the stability control. The stability control has been adjusted so that TR218 will only conduct when it receives a proper trigger pulse from the trigger circuit. There will be no sawtooth sweep produced until the trigger circuit is adjusted to provide a proper trigger pulse.
In the FREE RUNNING mode, the base of TR216 is grounded by SW3C. The base of TR217 is ungrounded
because of the mechanical interlock of SW3. Ground
ing the ba se of TR216 causes the transistor to turn
off, raising the collector voltage to the positive 15
volt level. This positive voltage is coupled through
R262, R263 and TR217 to the junction of R265 and
R266, which changes the voltage on the stability con trol enough so that TR218 will conduct as soon as the hold off capacitor has discha rged. The time base
generator will now free run, but it will be sensitive to trigger pu lses coupled into the base of TR218 during the time that it take s for the hold off capacitor to dis charge .
In the AUTO TRIGGERED mode, the base of TR216,
is grounded through resistor R259, and with no trigger pulses, the action is the sam e as in the FREE RUNN ING mode. If trigger pulses are present, they will be coupled to the base of TR215 through C220. The trigger pulses will be amplified by TR215 and coupled to CR205. CR205 will detect the trigger pulses, pro ducing a positive voltage at the base of TR216, caus ing it to conduct. Its collector voltage will drop,
cutting off TR217, and allowing the stability control to return to the sa me negative voltage a s in the
MANUAL TRIGGERED MODE. C222 is used to
hold the base of TR217 negative between trigger
pulses. In this mode the time base will free run in the
absence of trigger pulses. The presence of trigger
pulses will ca use the time base to automatically
switch to a triggered mode.
TR233 and TR234 amplify the horizontal sweep signal, and together with constant current source TR232, establish the push-pul l signal necessary to drive the output amplifier. R l l is the front panel horizontal position control. R317 adjusts the gain of TR233 and TR234 to produce a horizontal trace 11cm long. SW5 is a push-pull switch mounted on the rear of the
HORIZONTAL POSITION control. When the shaft is pulled to its out position, the switch connects R319 in parallel with R317. R319 is adjusted for 5X in cre ase in the gain of TR233 and TR234. R321 and
R322 are ganged together, and are used to match the
horizontal position of the expanded sweep to that of
the normal sweep.
TR235 a nd TR236 are used in the EXT horizontal and
60Hz sweep positions to amplify the incoming signal.
The output of TR235 is coupled through SW200H
to the base of TR233. R331 is the front panel hor
izontal gain control (small center knob on TIME
BASE - FREQUENCY), and R8 (front panel LINE
SWEEP PHASE) is used to adjust the pha se of the
60Hz sweep.
PREAMPLIFIERS
Both of these amplifiers are identical, so we will look
only at the operation of the CHANNEL A pre amplifier. The signal at the front panel input is coupled to the input of the preamp through the IN PUT COUPLING switch. When this switch is in the DC coupled position, the input signal is direct coupled to the input of the pre amp through R4, the 47 ohm resistor. In the AC position, the signal is AC coupled
to the input of the preamp through C2 and R4. In the
ground position, the input sig nal is grounded through
R3 (10K) and C2, while the input of the preamp is
tied to ground through R4.
Once inside the preamp, the input signal must be att
enuated to a level that will not over drive the FET in
puts. The attenuation ratios are: 1:1, 10:1, 100:1
and 1,000:1, and are selected by SW100A and SW
100B . In order to maintain exact attenuation at
higher frequencies, the 1,000:1 attenuator section is
actually a 10:1 and a 100:1 connected in series. The
capacitors in the attenuator are used for frequency
compensation. The input FET TR100 is connected as
a source follower to match the 1 meg input imped
ance to the low impedance transistor circuitry, and is
protected by CR100 and CR101 which clamp the in
put to plus and minu s 2 volts. TR101 along with the
DC BALANCE control R120 provide a DC reference
for the balanced differential amplifiers that follow.
TR103A and TR103B along with constant current
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source TR104 produce the necessa ry push-pull si gnal for deflection.
TR103A in cascode with TR105 is actually the first stage of voltage amplification. SW100C selects dif ferent values of emitter resistors for TR103 to change the gai n of this stage in the 1,2,5 sequen ce necessa ry to produce the full ran ge of input sensitivities of the preamp. TR108 operates as a current amplifier to drive the low impedance input of the output amplifier and in addition it changes the DC level of the signal. R156 is the front panel vertical gain vernier, and is
controlled by the small center knob of the PPV PER DIV. switch. R156 must be fully clockwise to main tain calibration of the preamplifier. R5 and R6 are a dua l control used as the front panel TRACE POSITION control.
VERTICAL INPUT
(Switch a nd Di spl ay Boa rd)
The actual switching of either CHANNEL A or CHANNEL B to the vertical output amplifier is ac complished by diodes CR403 - CR410. If a positive
voltage is applied to the junction of CR404 and CR 405, they will be reverse bias ed, and therefore have no effect on the signal from CHANNEL A. CR403 and CR406 will then be forward biased by the difference in DC voltage level between the preamp and the out put amplifier, and the signal from CHANNEL A will be coupled to the vertical output amp. If a negative voltage is applied to the junction of CR404 and CR
405, they will be forward biased, and the signal from CHANNEL A will be shunted to ground through TR403. CR403 and CR406 would then be reverse biase d by the negative voltage on their anodes, block ing the CHANNEL A sign al from reaching the output
amplifier. Multivibrator TR404 and TR405, along with emitter followers TR403 and TR406 are con structed so that if a negative voltage is applied to the
junction of CR404 and CR405 from the emitter of
TR403, then a positive voltage will be applied to the
junction of CR408 and CR409 from the emitter of
TR406. With a positive voltage at the junction of CR 408 and CR409, they will be reversed biased, while
CR407 and CR410 will be forward biased, connect ing the output of the CHANNEL B preamp to the vertical output amp.
Pushing the CHANNEL A VERTICAL INPUT button shorts the base and emitter of TR404 resulting in a positive voltage at its collector. This positive voltage is coupled through emitter follower TR403 to the junction of CR404 and CR405, connecting the
CHANNEL A preamp to the vertical output amp.
is coupled through emitter follower TR406 to the junction of CR408 and CR409, connecting the output of the CHANNEL B preamp to the vertical output amplifier.
Pushing the DUAL ALTERNATE VERTICAL INPUT button connects the blanking waveform from the junction of the emitters of TR220 and TR221 on the time base board to the base of TR407. During the re trace time of the sweep, a negative pulse will be coupled to the ba se of TR407 turning it on. When TR407 turns on, it will sh unt the minus 25 volt supply for the multivibrator to ground. With the emitters of TR405 and TR404 shorted together, by the normally closed contacts of the DUAL CHOPPED VERTICAL INPUT switch, the multivibrator is wired a s a bistable that will switch states every time the minus 25 volts is removed. Therefore every time the sawtooth from the time ba se retraces, the multivibrator will change stat es producing an alternate connection of the CHANNEL A and CHANNEL B preamps to the vertical output amplifiers.
Pushing the DUAL CHOPPED VERTICAL INPUT button removes the short between the emitters of TR 403 and TR404, an d applies power to the multi vibrator at the junction of R419 and R420. The mult ivibrator will now free run at approximately lOOKHz, with the result that CHANNEL A and CHANNEL B preamps will be switched to the vertical output amp lifier during alternate half cycles of the lOOKHz chopping frequency. Plus 15 volts is connected by the remaining pair of contacts to R200 on the time base board, activating the low pass filter at the input of the trigger circuit.
Pus hing the VECTOR VERTICAL INPUT button does the following: switches the output of the time base board from the horizontal output amplifier to ground , connects the CHANNEL B preamp to the horizontal output amplifier, applies minus 25 volts to R284 on the timebase board stopping the operation of the timebase generator, and shorts base to emitter of TR 404 connecting the CHANNEL A preamp to the vert ical amplifier.
The display board also contains the circuit for the 2 volt square wave calibrator. TR400 and TR401 form a free r unning multivibrator operating at approximately lKHz. TR402 is a squaring amplifier to improve the rise time of the square wave. R409 is an internal con trol to set the level of the 2 volt output.
OUTPUT AMPLIFIERS
(Hori zontal a nd Vertical)
Pushing the CHANNEL B VERTICAL INPUT button sho rts the ba se and emitter of TR405 resulting in a positive voltage at its collector. This positive voltage
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The horizontal and vertical output amplifiers are id entical in the way that they process the incoming
signal. The only difference is that the horizontal amp
has 25% more gain to compensate for the lower sen sitivity of the horizontal deflection plates in the CRT.
TR500 and TR501 provide a low impedance termina tion for the input of the output amplifier, and are actually in cascode with the last sta ge of the preamp or time base. R504 and R505 ar e the collector load resistors for this cascode stag e. R500 is found only on the vertical output amplifier, and is used to reduce the gain of the cascode stage, to partially com pensate for the higher sensitivity of the vertical de flection plates. TR502 and TR503 are emitter fol lowers to provide a low impedance drive for the out put sta ge. TR504 in cascode with TR506 along with TR505 in cascode with TR507 form the output volt age amplifier sta ge. TR508 and TR509 form a con
stant current source to prevent the output stage from saturating, even if overdriven. R 501 is found only in the horizontal output amplifier, and is used to in crease the gain to partially compensate for the lower sensitivity of the horizontal deflection plat es. Access to the vertical deflection plates is available through VERT. DEF. PLATES jacks with R ll and R12 used to isolate the vertical output amplifier from the ex ternal signal.
SW5 is the VERT DEF PLATES switch and connects the external signal to the deflection plates in the EXT position, and the vertical output amplifier to the
pla tes in the NORMAL position. This switch should
be in the NORMAL position for normal operation.
HV POW ER SUPPLY
an d DC Co up led
CRT U nb la n kin g Circuit
The PS163 incorporates a 2000 volt high voltage power supply with advanced design, normally found only in much higher priced equipment. Instead of a 60 Hz supply with all the inherent problems of high volt age breakdown and large physical size, the PS163 uses a
Hartley oscillator supply operating at 30KHz. The small physical size of the 30KHz transformer and capac itors results in a considerable savings in weight, along with an increase in reliability. TR602 and the primary of the high voltage transformer T600 are the active components of the oscillator. C604 is used to reson ate the primary of the transformer to 30KHz. Feedback to sus tain oscillation is provided by the small section of the primary winding below the center tap. R609, the 3K resistor, is only used to start the oscillation.
The high voltage transformer is equipped with two
secondary windings, one to supply voltage to the cath
ode of the CRT, and one to provide voltage to the control grid of the CRT. The two windings are isolated from each other and from ground, so that the direct
coupled CRT unblanking signal may be applied to the low side of the control grid secondary. The unblank ing signal is coupled from the junction of the emitters of TR220 and TR221 on the time base board to the emitter of TR600. This sign al is positive when the time base is producing a sweep, and negative when the time base is in the hold off condition.
As soon as the time base starts to sweep, and a pos itive voltage is coupled to the emitter of TR600, TR 600 will go to the off state. With TR600 off, its collector voltage will go to plus 75 volts, turning on TR601, and placing plus 75 volts on the emitter of TR601. The positive 75 volts change at the emitter
of the TR601 will be coupled by R604 - R608 and
R608 and C602 to the control grid of the CRT, caus ing the CRT to conduct producing a trace on the screen. R604 is adjusted to provide just enough neg ative voltage to the grid of the CRT so the CRT will be turned off in the absenc e of the unblanking voltage at the emitter of TR600.
With a negative signal applied to the emitter of TR 600, it will be in the on condition, resulting in zero voltage at the collector. This voltage will also be pre sent at the b ase of TR601, cau sing TR601 to be in the
off condition. With TR601 in the off condition, its
emitter voltage will be near zero determined by the
forward bias condition of CR603. This voltage is
coupled to the CRT grid holding it in cutoff.
The cathode voltage on the CRT is adjusted by the in
tensity control R13 to provide manual control of the
trace brightne ss. The focus control, R15, is in the
same voltage divider as the intensity control so as
the intensity control is varied, the focus voltage will
follow it. The result is the CRT beam remains focused
at any intensity setting.
Provision for Z axis modulation is provided by C6
coupling into the cathode of the CRT. With this input,
a positive signal will decrease the brightness of the
trace, and a negative signal will increase the bright
ness of the trace. SW8, the Z AXIS input switch, is
used to connect the Z AXIS jack to the cathode when
this input is desired. This switch should be in the off
position for normal scope operation.
LO W VO LTA GE POW ER
SUPPLY
The PS163 low voltage power supply incorporates a ferroresonant power transformer (T l), and full wave rectification, to provide regulated voltage outputs of pl us 200 volts, plus 75 volts, plus 25 volts, plus 15 volts and minus 25 volts. The blue/white, blue/white
winding of T l is resonated by capacitor C7 to the 60Hz power line frequency. Because of the energy stored in this parallel resonant circuit, the output voltage of T l
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will remain constant over a wide range of input volt age variations, and load current variations.
TR700 - TR702 are electronic ripple filters with a filtering action equal to the beta of the transistor times the capacitor from base to ground. These tran sis tors are not protected against a shorted output, so care should be taken when working inside the PS163 not to short the plus 25, plus 15 or minus 25 volt
DISASSEMBLY INSTRUCTIONS
CASE
supplies to ground. If thes e transistors ar e defective they may not affect the output voltage from the supplies, but will cause an increase in the ripple on the supply lines.
R19, the ASTIGMATISM control on the back panel, is used in conjunction with the FOCUS control to ad just the sharp ness of the trace.
The case of the PS163 is in two parts. The first sec tion is the top and side wrap. Remove the 4 philips head screws from the top of the cas e, and the 5 philips head screws from each side of the cas e. Slide the wrap about lA inch forward, and lift it up and off. The next section is the bottom and rear cover. Remove the three philips hea d screws holding the cord wrapper on the back of the PS163. Place the PS163 face down on a soft cloth and remove the two philips screws from the aluminum trim bar along the front edge of the bottom. Remove the remaining philips head scre ws from the bottom of the case, and lift the case straight up being careful not to damage the cir
cuit breaker or astigmatism control. Reverse this pro cedure for reassembly.
CRT
Place the PS163 face down on a soft cloth and re move the philips screw that secures the rear support bracket for the CRT from the under side of the
CALIBRATION
chassis. Return the PS163 to its normal up-right position. Remove the four philips screws that secure the bottom edge of the output amplifiers from the rear of the chassis. Remove the two screws that hold the output amplifier support bracket from the top of the ch assis. Remove the socket from the rear of the CRT. Remove the 4 bezel nuts, the bezel, the green filter, and the grid graticule from the front of the PS163. Remove the 4 black plastic light shields, and note that there ar e two different types of shields. The sh ields in the upper right and lower left have the open ing in front of the collar, and are used to illuminate the grid graticule. Remove the vector graticule , and note the 4 philips screws that are recesse d behind it.
Carefully remove these 4 screws while holding the
CRT. After the 4 screws are removed, push the CRT
to the rear, and swing it out toward the open side of
the chassis. Be sure that the front support bracke ts clear the wiring for the grid lights.
Reverse thi s procedure to reinstall the CRT. Refer
to adjustment procedure for CRT tilt.
PREAMPLIFIERS CH ANN EL
A a nd B
DC B A L A N CE
NOTE: It is not necessary to remove the PS163 from its case to adjust DC BALANCE. A misadjustment of the DC BALANCE will result in a position shift of the trace as the preamplifier PPV PER DIV. switch is switched in the 1, 2, 5 sequence.
1. Depress the AUTO TRIGGERED HORIZONTAL SWEEP push button, set the TIME BASE - FRE QUENCY switch to the lmSec./DIV position, and
set the preamp INPUT COUPLING switch to the
center ground position.
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2. Set the preamp PPV PER DIV. switch to the .5 position, and adjus t the TRACE POSITION control to center the trace on the major horizontal grid line.
3 . Turn the preamp PPV PER DIV. switch to the .05 position, and adjust the DC BALANCE control to re- center the trace.
4. Repeat steps 2 and 3 until there is no shift in the
trace position.
C AL I B RA T IO N
The preamplifier calibration adju stments (R131, R 134, R137, and R140) are horizontally mounted P.C. pots located on the top side of the preamplifier P. C, boards. Access to these controls for adjustment is
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