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T E L E Q U I P M E N T
OSCILLOSCOPE
TYPE S51B A N D S51E
All TelequipmenT instruments are the subject of
continuous development and improvement, and,
in consequence, may incorporate minor detail
changes from the information contained herein.
TelequipmenT is a registered trade mark of
TELEQUIPMENT LIMITED*
313 Chase Road
Southgate
London, N. 14.
England.
Telephone: 01-882 1166
Cables: TELEQUIPT LONDON N 14
CIRCUIT DESCRIPTION
VERTICAL AMPLIFIER
The vertical amplifier consists of three double triodes. Vla and b form a
cathode coupled amplifier stage with RV6 acting as the gain control.
The signal is taken from Vlb anode and drives the output stage V3 via
cathode follower V2a.
V3 consists of a long-tailed pair with the signal taken to V3a and the vertical
shift applied to V3b.
V2b acts as a low impedance HT supply for V1, also providing approximately
15OV for Y shift and timebase circuits.
The signal at the anode of V3b is taken to the INT-EXT trigger switch.
The HT supply for the trigger circuit, V4 and V5b is taken from a common
resistor R15 in the cathode circuit of V3.
TRIGGER CIRCUIT
The trigger selection is performed by three switches in the S51B and one in the
S51E; S2 selects the INT-EXT triggering signal and connects it to the grid of phase
splitter V4b. S3 selects the positive or negative output of the phase splitter and
connects it to S4 which either passes the signal directly or via an integrating circuit
to V4a. The latter position, TV FIELD, integrates the field pulses of a television
waveform and attenuates the line pulses allowing the time base to be triggered by
the field sync pulses.
From S4 the signal is fed to V4a and V5b which form a cathode coupled
bistable switch, the switching level being adjusted by the grid potential of V4a.
On the AUTO position the switch S5 is opened and the grid of V4a is returned to
the grid of V5b. In the absence of a signal V4a and V5b oscillate at a frequency
determined by the time constant R42 and C19, but as soon as any input signal
between about 50Hz and 1MHz is applied to the grid of V4a, the self-oscillation
ceases and the multivibrator synchronises to the signal frequency. The square wave
appearing at the anode of V5b is differentiated, the positive pulse removed by MR7
and the negative pulse used to trigger the time base.
TIMEBASE & H O R I Z O N T A L AMPLIFIER
V6a is the Mi l ler sweep generator, the speed of run down being controlled by
S8 and the VARIABLE speed control.
The Miller valve is keyed by V5a and V6b which together form a d .c. c o u p l e d
multivibrator.
In the rest position of the timebase, V5a is off and its anode potential is
clamped by MR8 and 9. V6b is conducting. A negative trigger pu Ise from V5b
lowers the grid potential of V6b and hence that of the anode of V6a. This is
coupled to the grid of V6a, so reducing the valve current. The resulting rise in
screen potential developed across R65 is coupled via R54 and S6a to the grid of
V5a. This causes V5a anode voltage to fall; which leads to regenerative action
whereby V6b becomes quickly cut off and V5a hard on.
When the run down has reached the point where the cathode voltage of V6b
has fallen to near that of the grid, V6b starts to conduct and a negative pulse is
applied to the grid of V5a. Regeneration again takes place driving V5a to cut off
and V6b to conduction.
The positive going voltage at V6b anode, developed during the sweep, is
coupled via S6b to the modulator plate of the CRT, to provide trace unblanking.
The STABILITY control RV49 serves to set the grid potential of V5a to the
point just short of the free running condition of V5a and V6b.
The linear sweep voltage at the anode of V6a is taken via R64, C41 and R73
to one grid of the horizontal amplifier V7, horizontal shift voltage is applied to the
other grid and the output at the anodes drives the CRT X plates in push-pull.
Transistor TR1 in the common cathode circuit, acts as a constant current
source and provides a balanced output at the two anodes.
An external signal may be applied through a connector at the rear of the
instrument. In this condition, the timebase should be switched off with the
VARIABLE speed control. V5a is then switched for use as a preamplifier, thesignal
being applied to its grid. Cathode bias is provided by RV56, decoupled by C25.
The signal at V5a anode is coupled via V6b, acting as a cathode follower, and S6d
to the horizontal output amplifier V7. In order to keep the d.c. conditions correct,
V5a and V6b anodes are switched by S6b to the 150V HT supply and in addition V6b
anode is decoupled by C26.
10
POWER SUPPLY
Silicon rectifiers MR1 and 2 in a voltage doubling circuit supply the various
HT voltages via smoothing resistor R24.
The negative EHT supply for the CRT and STABILITY control via R29 is
provided by MR3, 4 and 5, C8, 9 and 11. It is applied via voltage reference
n e o n s N and 2, in parallel with the BRIGHTNESS control, to the cathode of the
1
CRT.
The positive EHT supply is derived from MR6 and C12. The negative end of
C 12 is returned to the HT line.
CRT
Intensity modulation signals may be applied via Cl6 to the grid of the CRT.
The interplate shield potential is adjusted by RV88 to the mean potential of
the deflector plates for optimum geometry or freedom from bowing of the display.
RV89 provides preset astigmatism adjustment and RV92, blanking, allows the beam
intensity to be adjusted to a maximum by varying the first anode potential.
MAINTENANCE ADJUSTMENTS
The simplicity of the circuitry of the S51 makes it an extremely reliable
instrument, and for the most part servicing will be limited to the replacement of
defective va Ives. Valve replacement will have little effect on performance and
should not necessarily entail readjustment of preset controls. If for any reason the
internal presets do require adjustment, the following detailed instructions will
al low this to be done quickly and accurately.
The case may be removed after unscrewing the two screws at the rear of the
instrument.
INPUT ATTENUATOR & PROBE
In order to adjust the input attenuator compensation a squarewave generator is
required with a frequency of approximately 2kHz and whose output can be varied
between 0.2 and 1OOV. The rise time of the squarewave need not be particularly
fast, but it must have a flat top and bottom. Connect the squarewave generator to
the input socket and adjust the output to approximately 0.2V. Set the input
attenuator to . and adjust the sweep controls so that 3 cycles of the squarewave
1
are displayed. Now carry out the following procedure step by step, adjusting
each trimmer to give a square corner to the squarewave. On each setting of the
input attenuator the output of the squarewave generator should be adjusted to give
a display of approximately 2-3 cm amplitude.
Set VOLTS/CM to: Adjust
.2 C112
.5 C111
1 Cl04
2 Cl08
5 Cl09
10 C105
If these adjustments have been carried out correctly the 20 and 50V/cm ranges
are automatically compensated.
In order to adjust Cl02 and 103 it is necessary to use a high impedance probe,
as these two capacitors only affect compensation when a probe is in use. Remove
the squarewave generator from the input socket and plug in the high impedance
probe, connect the output of the squarewave generator to the probe tip and set the
input attenuator to .l, set the output of the squarewave generator to give approxi-
,
mately 2cm amplitude and adjust the probe trimmer to give a flat top to the square-
wave. Now switch the input attenuator to 1, readjust the output of the squarewave
generator and adjust C102. Set the input attenuator to 10, readjust the generator
and adjust C 103. All other ranges will automatically be correct.
VERTICAL AMPLIFIER
Adjustment of the high frequency compensation of the vertical amplifiershould
only be carried out if a squarewave generator is available which is capable of
producing an accurate squarewave at a frequency of about 250kHz with a rise time
of less than 1OOns and which is known to be absolutely free from ring or overshoot.
The compensation circuits in the vertical amplifier are extremely stable and, unless
such a generator is employed readiustment is undesirable.
Set the input attenuator switch to 0.1 and adjust the output of the squarewave
generator to give a trace of approximately 2-3cm amplitude (the output frequency
of the generator should be between 200 and 300kHz). L is adjusted to give a flat
1
topped squarewave with a fast -rise time, square corners and no overshoot.
,fWB *
The only other variable on the vertical amplifier is the SET Y G A I N c o n t r o l ,
which should be adjusted with a signal of known amplitude with appropriate setting
of the VOLTS/CM switch.
TRIGGER CIRCUIT
The only adjustment necessary in the trigger circuit is an occasional resetting
of the trigger sensitivity control RV48; this should be set so that the trigger circuit
will operate when the trace amplitude on the screen exceeds 2mm. If any attempt
is made to increase the sensitivity beyond this point erratic operation will almost
inevitably result. Connect a signal, say a 2kHz squarewave, to INPUT. Now
set the input attenuator and squarewave generator to give a trace 2mm high and
adjust the TRIG SENS control SO t h a t , at a critical setting of the TRIG LEVEL
control, the sweep will just trigger. Reduce the trace amplitude to lmm and make
sure that the sweep will not trigger on this signal.
SWEEP GENERATOR & HORIZONTAL AMPLIFIER
To make a complete readjustment of the sweep generator and horizontal
amplifier, carry out the following procedure:-
Remove the lead to the modulator anode, pin 7 on the CRT. Set the TIME/
CM switch to 1 O O