









The NORDMENDE CHASSIS F VI/90 (F6) is a modular type and is unique for portable sets like the model here shown, for bigger models were used the F6TT which is different developed.
HOW THYRISTOR LINE DEFLECTION OUTPUT SCAN STAGES WORK:
INTRODUCTION:
The massive demand for colour television receivers in Europe/Germany
in the 70's brought about an influx of sets from the continent. Many of
these use the thin -neck (29mm) type of 110° shadowmask tube and the
Philips 20AX CRT Tube, plus the already Delta Gun CRT .
Scanning
of these tubes is accomplished by means of a toroidally wound
deflection yoke (conventional 90° and thick -neck 110° tubes operate
with
saddle -wound deflection coils). The inductance of a toroidal yoke is
very much less than that of a saddle -wound yoke, thus higher scan currents are required.
The deflection current necessary for the line scan is about 12A peak
-to -peak. This could be provided by a transistor line output stage but a
current step-up transformer, which is bulky and both difficult and
costly to manufacture, would be required.
An entirely different
approach, pioneered by RCA in America and developed by them and by ITT
(SEL) in Germany, is the thyristor line output stage. In this system the
scanning current is provided via two thyristors and two switching diodes
which due to their characteristics can supply the deflection yoke
without a step-up transformer (a small transformer is still required to
obtain the input voltage pulse for the e.h.t. tripler). The purpose of
this article is to explain the basic operation of such circuits. The
thyristor line output circuit offers high reliability since all
switching occurs at zero current level. C.R.T. flashovers, which can
produce high current surges (up to 60A), have no detrimental effects on
the switching diodes or thyristors since the forward voltage drop across
these devices is small and the duration of the current pulses short. If
a surge limiting resistor is pro- vided in the tube's final anode
circuit the peak voltages produced by flashovers seldom exceed the
normal repetitive circuit voltages by more than 50-100V. This is well
within the device ratings.
Basic Transistor Circuit:
In
Blumlein's day valves had to be used to perform the switching action.
Two were required since a valve is a unidirectional device, and as we
have seen current must flow through the switch in both directions.
Nowadays we generally use a transistor to perform the switching action,
arranging the circuit along the lines shown in Fig. 2. The line output
transformer T is used as a load for the transistor and as a simple means
of generating the e.h.t. and other supplies required by the receiver.
The scan -correction capacitor Cs also serves as a d.c. block. Capacitor
Ct tunes the coils during the flyback when the transistor is cut off.
During the forward scan Cs first charges, then discharges, via the scan
coils, thus providing deflection from the left- hand side to the
right-hand side of the screen. One advantage of a transistor is that it
can conduct in either direction. Thus unless we are operating the stage
from an 1.t. line of around 11V - as in the case of many small -screen
portables - we don't need a second switching device. With a supply of
11-12V a shunt efficiency diode - connected in parallel with the
transistor, cathode to collector and anode to emitter, is required
because the linearity is otherwise unacceptable. Another advantage of a
transistor compared to a valve is that it is a much more efficient
switch. When a transistor is saturated both its junctions are forward
biased and its collector voltage is then at little more than chassis
potential. The anode voltage of a saturated pentode however is measured
in tens of volts, and this means that there is considerable wasteful
dissipation. Thyristor Switch If what we need is an efficient switch,
why not use a thyristor???
Thyristors
are even more efficient switches than transistors. They are more
rugged, can pass heavy currents, and are insensitive to the voltage
overloads that can kill off transistors. In addition, in the sort of
circuit we are about to lo
ok
at the power supply requirements can be simplified (a line output
transistor must be operated in conjunction with a stabilised power
supply: this is not necessary in the thyristor circuit since regulation
can be built in). In the nature of things however there must be
disadvantages as well - and there are! First, a thyristor will not act
as a bidirectional switch.

There
is no great problem here however: all we need do is to shunt it with a
parallel efficiency diode. More awkward is the fact that once a
thyristor has been triggered on at its gate it cannot be switched off
again by any further action taken in its gate circuit. In fact it's this
problem of operating the thyristor switch that is responsible for the
complexity of thyristor line output circuits.
A
thyristor can be switched off only by reducing the current through it
below the "hold on" value, either by momentarily removing the voltage
across the device or by passing an opposing current through it in the
opposite direction - this latter technique is used in practical
thyristor line output circuits. Once the reverse current through the
thyristor is about equal to the forward current flowing through it the
net current falls below the "hold on" value and the thyristor switches
off.
Basic Thyristor Circuit:
There
is more than one way of arranging a thyristor line output stage. Only
one basic circuit has been used so far however, though as you'd expect
there are differences in detail in the circuits used by different
setmakers. The basic circuit was first devised and put into production
by RCA in the USA in the late 1960s. It was subsequently popularised in
Europe by ITT, and many continental setmakers have used it, mainly in
colour receiver chassis fitted with 110° delta gun c.r.t.s. They include
Finlux, Grundig, Saba, Siemens and ASA. Korting use it in their 55636
chassis which is fitted with a 90° PIL tube, while Grundig continue to
use it in their latest sets which use the Mullard/Philips 20AX tube.
Amongst Japanese setmakers, Sharp use it in their Model C1831H which is fitted with a Toshiba RIS tube.
Reduced
to its barest essentials, the circuit takes the form shown in Fig. 3.
To start with this looks strange indeed! The right-hand side however is
simply the equivalent of the scanning section of the transistor circuit
shown in Fig. 2, with TH2 and D2 replacing the transistor as the
bidirectional switch.
The
tuning capacitor however is returned to chassis via the left-hand side
of the circuit - in consequence there is no d.c. path between the
right-hand and left-hand sides of the circuit. L1 provides a load. The
efficiency diode D2 conducts during the first part of the forward scan,
after which TH2 is switched on to drive the beam towards the right-hand
side of the screen. The purpose of the left-hand side of the circuit,
the bidirectional switch TH1/D1 and L2, together with the tuning
capacitor Ct, is to switch TH2 off and to provide the flyback action.
The
output from the line oscillator consists of a brief pulse to initiate
the flyback. It occurs just before the flyback time (roughly 3µS before)
and is applied to the gate of TH1, switching it on. When this happens
L2 is connected to chassis and current flows into it, discharging Ct
(previously charged from the h.t. line). L2 is called the commutating
coil, and forms a resonant circuit with Ct. Thus when TH1 is switched on
a sudden pulse builds up and this is used to switch off TH2. In
addition to tuning L2, Ct tunes the scan coils to provide the usual
flyback action.
Roughly
speaking therefore D2 and TH2 conduct alternately during the forward
scan and are cut off during the flyback, while TH1 is triggered on just
before the flyback, TH1 and D I subsequently conducting alternately
during the flyback and then cutting off when the efficiency diode takes
over.
Thyristor Line Scan Practical Circuit:

Scanning Sequence:
It's time to look at the basic scanning sequence in more detail, basing
the description on Figs. 3 and 4. We'll start at the beginning of the
flyback. TH2 and D2 have just been switched off - we'll come to how this
is done later - while TH1 which was triggered on by a pulse from the
line oscillator is still conducting. Energy is stored in the scan coils
in the form of magnetic fields. As these collapse, a decaying current
flows via the coils, Cs, Ct, L2 and TH 1. When this current falls to
zero the charge on Ct will have reversed and TH 1 will switch off. This
completes the first half of the flyback. The left-hand plate of Ct is
charged negatively, while its right-hand plate carries a positive
charge. D1 is now biased on and Ct discharges back into the scan coils
to give the second half of the flyback. Current is flowing via D1, L2,
Ct, Cs and the scan coils. At the end of this period the circuit energy
will have been transferred once again to the scan coils - in the form of
magnetic fields. One complete half cycle of oscillation will have
occurred, returning the beam to the left-hand side of the screen. With
Ct discharged, D 1 switches off. The oscillation tries to continue in
the negative direction, but we then get the normal efficiency diode
action, i.e. D2 conducts shorting out the tuned circuit. As the fields
around the coils collapse a linearly decaying current flows via the
coils, Cs and D2. This gives us the first part of the forward scan.
Towards the centre of the screen TH2 is switched on by the pulse
obtained from L3 and the current in the scan coils reverses to complete
the scan.
Switching the Scan Thyristor Off: The
tricky part is when it comes to switching TH2 off. As we have seen, TH1
is triggered on about 3fitS before the end of the forward scan. Prior
to this Ct will have been charged to the h.t. potential via L 1 and L2.
When TH1 conducts, current flows via TH1, L2, Ct and TH2 (which is on
remember). Because of the tuned circuit formed by L2 and Ct, the current
builds up rapidly in the form of a pulse - the commutating pulse shown
in Fig. 5. When this current, which flows through TH2 in the opposite
direction to the scan current, exceeds the scan current TH2 switches
off. Once TH2 cuts off D2 is able to conduct - it is no longer reverse
biased - which it does for a short period to provide an earth return
path for the remaining duration of the commutating pulse and also to
enable the scan to be completed (Cs discharging via the scan coils).
When the reverse, commutating current falls below the scan current D2
switches off and we then get the flyback action as the magnetic fields
around the coils collapse.
Power Transferring ; during the forward scan Ct is charged via L1 and L2, its right-hand plate being held at little above
through
the conduction of D2 and then TH2. During the flyback, when TH1 and D1
conduct alternately, connecting the junction L1, L2 to chassis, Ct
supplies energy to the scan part of the circuit. The Practical Circuit
so much then for the basic circuit and its action. Turning now to a
practical circuit, Fig. 6 shows the thyristor line output stage used in
the Grundig SuperColor Models 5011 and 6011. Ty511/Di511 form the
flyback switch, T1 is the input/commutating transformer, C516/7/8
comprise the tuning capacitance, Di518 is the efficiency diode and Ty518
the forward scan thyristor. The scan -correction capacitor Cs is C537.
As can be seen, the line output transformer circuit is quite
conventional. The main complication arises because of the need to
provide width/e.h.t. stabilisation. In a valve line output stage it is a
simple matter to achieve stabilisation by using a v.d.r. in a feedback
circuit to alter the bias on the output pentode. We can't do this with a
transistor line output stage, so we have to operate this in conjunction
with a stabilised supply. There is a subtle but quite simple method of
applying stabilisation to a thyristor line output stage however. As we
have seen, the energy supplied to the output side of the circuit is
provided by the tuning capacitors when they discharge during the flyback
period. During the forward scan they charge via the input coil - or
transformer as it is in practice. Now if we shunt the transformer's
input winding with a transductor we can control the inductance in series
with the tuning capacitors and in consequence the charging time of the
capacitors and hence the power supplied to the output side of the
circuit.
EHT/Width Stabilisation:
The
stabilising transductor in Fig. 6 is Td 1, whose load windings are
connected in series with R504/Di504 across the input winding of T1. The
transductor's control winding is driven by transistor Tr506, which
senses the h.t. voltage (via R506) and the amplitude of the signal at
tag d on the line output transformer. R508 in the transistor's base
circuit enables the e.h.t. to be set to the correct voltage (25kV).
Other Circuit Details:
A fourth winding on Ti feeds the 1.t. rectifier and stabiliser which
provide the supply for the low -power circuits in the receiver. The
trigger pulse winding also feeds a stabilised 1.t. supply circuit
(21V).
EW
pincushion distortion correction is applied by connecting the load
windings of a second transductor (Td2) across a section of the line
output
transformer's primary winding. By feeding a field frequency waveform to
the control winding on this transductor the line scanning is modulated
at field frequency. There is a simple but effective safety circuit in
this Grundig line output stage. If the voltage at tag c on the line
output transformer rises above 68V zener diode Di514 conducts,
triggering thyristor Ty511 into conduction with the result that the
cut-out operates. C517 is returned to chassis via a damped coil (L517)
so that the voltage transient when the efficiency diode cuts off is
attenuated. Likewise L512/C512/R512 are added to suppress the voltage
transient when the flyback thyristor Ty511 cuts off. The balancing coil
L516 is included to remove unwanted voltage spikes produced by the
thyristors.

At
the end........This Grundig circuit is representative of the way in
which thyristor line output circuits are used in practice. There are
differences in detail in the thyristor line output stages found in other
setmakers' chassis, but the basic arrangement will be found to be
substantially
Servicing / Throubleshooting / Repairing Thyristor Line Scan Timebases Crt Deflections circuits:

It
was a very good system to use where the line scan coils require large
peak currents with only a moderate flyback voltage - an intrinsic
characteristic of toroidally wound deflection coils.
it was originally devised by RCA. Many sets fitted with
110°, narrow -neck delta -gun tubes used a thyristor line output stage -
for example those in the Grundig and Saba ranges and the Finlux Peacock
, Indesit, Siemens, Salora, Metz, Nordmende, Blaupunkt, ITT, Seleco,
REX, Mivar, Emerson, Brionvega, Loewe, Galaxi, Stern, Zanussi, Wega,
Philco. The circuit continued to find favour in earlier chassis designed
for use with in -line gun tubes, examples being found in the Grundig
and Korting ranges - also, Indesit, Siemens, Salora, Metz, Nordmende,
Blaupunkt, ITT, Seleco, REX, Mivar, Emerson, Brionvega, Loewe, Galaxi,
Stern, Zanussi, Wega, Philco the Rediffusion Mk. III chassis. Deflection
currents of up to 13A peak -to -peak are commonly encountered with 110°
tubes, with a flyback voltage of only some 600V peak to peak. The
total energy requirement is of the order of 6mJ, which is 50 per cent
higher than modern 110° tubes of the 30AX and S4 variety with their
saddle -wound line scan coils. The only problem with this type of
circuit is the large amount of energy that shuttles back and forth at
line frequency. This places a heavy stress on certain components.
Circuit losses produce quite high temperatures, which are concentrated
at certain points, in particular the commutating combi coil. This leads
to deterioration of the soldered joints around the coil, a common cause
of failure. This can have
a cumulative effect, a high resistance joint increasing the local
heating until the joint becomes well and truly dry -a classic symptom
with some Grundig / Emerson sets. The wound components themselves can be
a source of trouble, due to losses - particularly the combi coil and
the regulating transductor. Later chassis are less prone to this sort of
thing, partly because of the use of later generation, higher efficiency
yokes but mainly due to more generous and better design of the wound
components. The ideal dielectric for use in the tuning capacitors is
polypropylene (either metalised or film). It's a truly won- derful
dielectric - very stable, with very small losses, and capable of
operation at high frequencies and elevated temperatures. It's also
nowadays reasonably inexpensive. Unfortunately many earlier chassis of
thi
s type used polyester capacitors, and it's no surprise that they were
inclined to give up. When replacing the tuning capacitors in a
thyristor line output stage it's essential to use polypropylene types -a
good range of axial components with values ranging from 0.001µF to
047µF is available from RS Components, enabling even non-standard values
to be made up from an appropriate combination. Using polypropylene
capacitors in place of polyester ones will not only ensure capacitor
reliability but will also lower the stress on other components by
reducing the circuit losses (and hence power consumption).

Numerous circuit designs for completely transistorized television
receivers either have been incorporated in commercially available
receivers or have been described in detail in various technical
publications. One of the most troublesome areas in such transistor
receivers, from the point of View of reliability and economy, lies in
the horizontal deflection circuits.
As an attempt to avoid the voltage and current limitations of transistor
deflection circuits, a number of circuits have been proposed utilizing
the silicon controlled rectifier (SCR), a semiconductor device capable
of handling substantially higher currents and voltages than transistors.
The circuit utilizes two bi-directionally conductive switching means
which serve respectively as trace and commutating switches.
Particularly, each of the switching means comprises the parallel
combination of a silicon controlled rectifier (SCR) and a diode. The
commutating switch is triggered on shortly before the desired beginning
of retrace and, in conjunction with a resonant commutating circuit
having an inductor and two capacitors, serves to turn off the trace
switch to initiate retrace. The commutating circuit is also arranged to
turn oft the commutating SCR before the end of retrace.
Circuit Operation:
The
basic thyristor line output stage arrangement used in all these chassis
is shown in Fig. 1 - it was originally devised by RCA. The part to th
e
right of the tuning capacitance acts in exactly the same manner as a
transis- tor line output stage, with the scan thyristor Th2 replacing
the transistor. The thyristor is switched on about half way through the
forward scan, the efficiency diode D2 provid- ing the initial part of
the line scan (left-hand side of the screen). The scan coils and line
output transformer (used to generate the e.h.t. plus various other
supply lines and pulse waveforms as required) are a.c. coupled, via the
scan -correction capacitor C5 and C6 respectively. The problem with a
thyristor is that it can be turned on at its gate but not off. To switch
a thyristor off, the current flowing through it must be reduced below a
value known as the hold -on current. This is the main function of the
components on the left-hand side - the line generator, the flyback
thyristor with its parallel diode and the commutat- ing coil. During the
forward scan, the tuning capacitors are charged from the h.t. line via
the input and commutat- ing coils. The line generator produces a pulse
to trigger the flyback thyristor Th1- this occurs just before the actual
flyback. When Thl1 switches on, the junction of the input coil and the
commutating coil is momentarily con- nected to chassis. The tuning
capacitance and the com- mutating coil then resonate, producing a pulse
which draws current via the scan thyristor. Since this current flow is
in the opposite direction to the scan current flow, the two cancel and
the current flowing via the scan thyris- tor falls below the hold -on
current. Th2 is thus switched off, and the scan coils resonate with the
tuning capaci- tance to provide the flyback action. So much for the
basic action. A secondary winding coupled to the input coil produces a
pulse to switch the scan thyristor on, in conjunction with the
shaping/delay network Ll, C4, R1. The tuning capacitors are usually
arranged in the T formation shown to reduce the values required and the
voltages developed across them. In practical circuits the input
and commutating coils are usually combined in a single unit which for
obvious reasons is generally known as the combi coil. The main point not
so far mentioned is stabilisation. There are two approaches to this. In
earlier circuits a transductor was included in parallel with the
input
coil to vary the impe- dance in series with the tuning capacitance.
This was driven by a transistor which was in turn controlled by feedback
from the line output transformer. A more efficient technique is used in
later circuits, with a current dumping thyristor in series with the
input coil. Practical Circuit As a typical example of the earlier type
of circuit, Fig. 2 shows the thyristor line output stage used in the
Grundig 5010/5011/6010/6011 series. Td1 is the regulating transductor
which is driven by Tr506. Ty511 is the flyback thyristor (commutating
thyristor might be a better name), Ty518 the scan thyristor, Di518 the
efficiency diode and C516/7/8 the tuning capacitance. The scan coils are
cou- pled via C537, while C532 provides coupling between the primary
winding of the line output transformer and chas- sis. A transductor
(Td2) is used for EW raster correction. The combi coil also feeds 1.t.
rectifiers from its secondary windings.




Basic Fault Conditions: At
one time every engineer must have scratched his head and cursed the
new-fangled idea of the thyristor line output stage. That they are
awkward to service is a fallacy however. The usual symptom of a fault in
the line output stage is the cutout tripping. All chassis that use a
thyristor line timebase incorporate a trip of some sort. The type varies
from chassis to chassis. Early Grundig sets have a mechanical cutout;
the Saba H chassis uses a thyristor and solenoid to open the mains
on/off switch; a common arrangement consists of a thyristor in series
with the h.t, line and a control transistor which shorts the thyristor's
gate and cathode in the event of excessive current demand (this gives
audible tripping at about 2Hz). Some sets incorporate both excess
current and over -voltage trips, but most have just the former.


The aim of this article has been to provide a general guide to servicing rather than to list faults common to particular models. Much useful information on individual
chassis with thyristor line output stages has appeared in previous issues of Obsolete Technology Tellye !- refer to the following as required: Search with the tag Thyristors at the bottom of the post to select all posts with this argument on various fabricants.
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