THE E.H.T. SUPPLY UNIT

The E.H.T. supply for the Projection Television System is generated by what is known as the 'pulsed oscillator' method. There are three principal versions of the circuit:

FRAME No. 25 - E.H.T. SUPPLY SYSTEM (ORIGINAL CIRCUIT)

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In the diagram which shows the original circuit for this form of supply unit, the triode section of VI is connected as a conventional blocking oscillator, and the resulting saw-tooth waveform, which has a repeti- tion frequency of about 1000 cycles per second, is applied to the grid of the driver valve, V2. This valve is biased well below cut-off so that anode current flows during only a part of the saw-tooth cycle. When anode current ceases, the energy stored in the choke which forms the anode load is dissipated as a train of damped oscillations, the frequency of which depends upon the inductance of the choke and the stray capacitance of the circuit. This frequency is of the order of 25kc/s, and the peak voltage is of the order of 8 kilovolts. These peaks are applied to a three-stage voltage tripler rectifying circuit employing three EY51 half-wave rectifiers, and giving a rectified output of 25kV. Good voltage regulation is obtained by applying a form of A.G.C. to the driver valve, using the diode sections of VI on the 'ringing' transformer.

FRAME No. 26 - E.H.T. SUPPLY SYSTEM (SECOND VERSION)

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The second version of the E.H.T. generator, shown here, is basically the same, but the ratio of the A.G.C. obtained from the diode section of VI is increased in order to improve the voltage regulation of the unit and thus prevents a certain amount of de- focusing which occurred with the earlier unit. The damping circuit C8, RIO serves to eliminate the risk of instability due to the increased A.G.C.

FRAME No. 27 - E.H.T. SUPPLY SYSTEM (THIRD UNIT USING TRIODE-PENTODE FOR V1)

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In the third version illustrated, a triode pentode Type ECL80 is employed instead of the double diode triode EBC33, and the output pentode Type PL820 is substituted for Type EL83. Otherwise the circuit is basically the same, the triode section of the ECL80 being operated as a diode to provide the A.G.C. voltage.

The ringing transformer and rectifier valves with their associated E.H.T. capacitors are sealed under oil in the metal can shown. Terminals 1, 2, 3 and 4 correspond to the reference in Frames 25, 26 and 27.
	FRAME NO. 28—SEALED CAN CONTAINING E.H.T. TRANSFORMER AND RECTIFIERS.

SERVICING THE E.H.T. UNIT

The following symptoms indicate possible faults in the E.H.T. unit:

1. No picture. This may be due to no output from the E.H.T. unit. Faults in the E.H.T. unit may be located by systematic testing in the following way. If no E.H.T. voltage is available, i.e. it is im- possible to draw sparks from the E.H.T. terminal, the presence of E.H.T. at the anode of V2 should be checked by endeavouring to draw sparks there-from. If sparks can be obtained it is clear that the E.H.T. rectifier can is faulty, and must be replaced. If, however, no spark can be drawn from the anode of V2, either the E.H.T. can is faulty or the trouble lies in the circuit of VI or V2. The control- grid voltage of VI should be checked. If the reading is zero, the blocking oscillator circuit should be checked to locate the fault. If, however, the control grid voltage of VI is normal, i.e. 50 volts (a.c.) or -70 volts (d.c.), the trouble is in the circuit of V2. The screen voltage should be 345V (Frames 25 and 26) or 210V (Frame 27) and the cathode voltage 3V (Frames 25 and 26) or IV (Frame 27). The control grid voltage of V2 should therefore be checked. If zero, the coupling between the oscillator circuit (VI) and the drive circuit (V2) is probably faulty. If the control grid voltage of V2 is normal (50 volts a.c. and a negative d.c. voltage) a faulty E.H.T. can is confirmed. The H.T. feed voltage to the E.H.T. unit should be 250V, and the current consumption from 30 to 75mA according to the E.H.T. load.
2. Picture too small and size changes with brightness.
   (i) Fault in the A.G.C. line or faulty diode section of V1.
3. Picture normal size but increases rapidly as bright- ness increases.
   (i) One of the E.H.T. rectifiers faulty.
   (ii) V2 low emission.
   (iii) Blocking oscillator frequency incorrect.
   (iv) Open circuit in cathode by-pass capacitor of V2.
4. Fluctuating picture size
   (i) Fault in A.G.C. circuit causing too much feedback and hence instability.
   (ii) Faulty smoothing resistor in E.H.T. cable.

CHECKING THE BLOCKING OSCILLATOR

The repetition frequency of the blocking oscillator not only governs the E.H.T. voltage but also governs the power delivered to the filaments of the three EY51 rectifiers. In order to ensure the correct E.H.T. voltage and also to avoid over-running or under-running the rectifier filaments, the blocking oscillator frequency must be kept within the limits of 930c/s to 11OOc/s. Adjustment of this frequency should normally be necessary only when a frequency-determining element is replaced. Small variations from the specified frequency can be corrected by replacing Cl and R2 (Frame 27) with components of slightly different value. If, however, the frequency departs considerably from the correct value, the cause will probably be a defective blocking transformer. When replacing this transformer it should be remembered that the winding with the lower resistance is connected in the grid circuit of VI. The frequency of the blocking oscillator can be measured with a cathode ray oscilloscope in conjunction with an accurately calibrated A.F. generator. The 'Y' deflection terminal of the oscilloscope should be loosely coupled to the E.H.T. unit. Usually a wire connected to the 'Y' terminal and brought close to the E.H.T. unit will pick up sufficient energy for a reasonable vertical deflection.

FRAME NO. 29 — OSCILLOGRAM OF E.H.T. GENERATOR VOLTAGE.

The horizontal deflection can be obtained from the A.G. generator. The frequency of the A.G. generator should be adjusted until a steady figure similar to that shown is obtained. A steady trace of this form should occur at a frequency between 930c/s and 11OOc/s. If the A.F. oscillator frequency is outside these limits, Cl or R2 must be replaced by components of suitable value.

PRECISION COMPONENTS

Because it is essential that the E.H.T. unit has very good voltage regulation, it has been necessary to use in the circuit a number of components of close toler- ance rating. It is most important, therefore, when replacing these components, to be certain that the recommended tolerances are observed.

D—CARE OF LENSES, MIRRORS AND SCREENS

As previously stated, the optical unit is sealed, and no cleaning of the items contained therein is necessary. Dust which may collect on the large cabinet mirror may be blown off. Since the mirror is surface silvered, NORMAL MIRROR OR WINDOW CLEANING METHODS MUST NOT BE APPLIED. If it is necessary to handle the cabinet mirror, only the edges should be touched. If, through faulty handling, finger marks are produced, they should be removed immediately, using one of the following methods:

1. Moisten a piece of very soft cotton wool (Egyptian cotton wool) with industrial alcohol, and apply it to the mark with a very light stroking action, in one direction. Each piece of cotton wool should be used once only. The solvent, together with the foreign matter, should then be removed with clean dry cotton wool, using the same light stroking action. This cotton wool should also be used once only. The process may be repeated if the mark proves obstinate, but not more than twice, as the damage to the mirror through scratching can be more serious than slight discoloration.
2. Dust the mirror very carefully with a clean dry chamois leather. Wash with warm, soft (preferably distilled) water containing a suitable proportion of a wetting agent (obtainable from photographic supply stores). Dry with chamois leather.

It should be noted that any cleaning method will probably reduce to some extent the reflecting properties of the mirror, and it is therefore better to handle the mirror carefully and thus avoid the necessity of cleaning. The plastic screen on back projection receivers may be cleaned if necessary by using industrial alcohol and clean chamois leather, or warm water to which a a little soapless detergent has been added.