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Vintage Television
1946 RCA 621TS television restoration
By Dr Hugo Holden
The 621TS is a
remarkable television
set. It is RCA’s
first post-WW2
set, using pre-war
television technology
but introducing
several new ideas.
These include line
output efficiency
damping, FM sound,
complex line output
transformer core
metallurgy and
improvements in CRT
design.
T
he 1946 RCA 621TS set has a
7-inch (18cm) screen with a 7DP4
CRT. The 7DP4 has an 8kV maximum
EHT voltage (typically 6kV), uses an
ion trap magnet and provides a very
bright, high-contrast picture.
The cabinet was designed by the
respected industrial designer John
Vassos in 1941. WW2 meant a six-year
delay to get it to market. The model
was quickly replaced by a 10-inch
(25cm) set, the RCA 630TS, with a
10BP4 CRT.
The chassis of the set I acquired was
in very poor and rusted condition, typical for its age – see Photo 1. It required
a complete rebuild using similar techniques to those used in the HMV904
restoration (described in the November 2018 issue – see siliconchip.com.
au/Article/11314).
In the post-war period, it became
standard practice to enclose the line
output transformer and EHT rectifier
in a separate cage, in this case on the
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lower-right of the chassis. The power
transformer at lower left was mounted
on the chassis under-surface to keep
it as far as possible from the CRT, to
avoid magnetic interference with the
beam.
TV overview
WW2 somewhat delayed the
621TS’s development. One of its
An example image of the set taken
from an advertisement.
Australia's electronics magazine
important new design elements was
a 7-inch electromagnetically deflected
CRT with a high final anode voltage of 7.5kV. This gave a bright,
high-contrast image that could easily be viewed in good room lighting.
Most pre-WW2 TVs ran lower EHT
voltages and could not produce such
a high-contrast image.
The 621TS was designed to receive
the standard American VHF range of
TV station frequencies for channel 1
(45.25MHz video carrier, 49.75MHz
sound) to 13 (211.25MHz video,
215.75MHz sound).
The set’s oscillator runs above the
received channel frequencies. Taking
channel 1 as an example, the Kallitron
oscillator runs at 71MHz. The sound
intermediate frequency (IF) emerging
from the tuner is at 21.25MHz, while
the picture (video) IF is 25.75MHz.
The picture (video) carrier is amplitude modulated (AM) while the
sound is frequency modulated (FM),
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compared to pre-WW2 TV sets which
had AM sound.
The sound carrier wave was transmitted 4.5MHz higher than the vision
carrier; they had not yet moved to
‘inter-carrier sound’. In this system,
the two carriers beat together at the
video detector output to produce a
4.5MHz carrier wave, passing on to a
4.5MHz sound IF amplifier.
However, in the 621TS, the
21.25MHz sound IF carrier is taken
directly from the converter coil into
the sound IF. Only a few years later,
most American TVs had moved to
inter-carrier sound. The advantage
was that the sound did not drift out
of tune with variations in local oscillator frequency.
At the tail end of this amplifier is
the FM detector, in this case, a discriminator type where the driving
stage is designed to amplitude-limit
the 4.5MHz carrier. Later, many manufacturers moved to a ratio detector
design, which has the advantage of
inherent amplitude limiting.
You can find the service manual
with circuit diagrams etc at the Early
Television Foundation website:
siliconchip.au/link/abge
Tuner
The tuner is a separate assembly
very similar to the type of tuned box
seen in practically all TV sets after
1946. However, it does not use a rotating drum; instead, it has an array of
rotary switches.
The tuner is very elegant and is
based on three 6J6 twin triodes. Based
on one 6J6 dual triode (V1), the input
stage is a para-phase (differential
amplifier) that is neutralised by two
small 1.5pF capacitors from the plate
of one triode to the grid of the other.
This design became very popular later
in wideband oscilloscope circuits.
In this case, though, the anode loads
are broadly tuned in the region of the
received station frequency.
The received frequencies are then
passed to the converter (mixer) stage,
V2, using inductive link coupling. The
converter also receives the signal from
the local oscillator, again by inductive link coupling, and the signals are
mixed in the plate circuits of both the
triodes of the V2, which are connected
together in the converter stage to feed
the converter coil.
The converter stage has an astonishingly large converter coil with a
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Photo 1: the
chassis of the
621TS was
acquired with a
heaping of dust
and rust.
large tuning slug. The coil assembly is
close to 25mm in diameter and about
75mm tall.
Oscillator
The 6J6 twin triode local oscillator
circuit around V3, shown in Fig.1,
is pleasingly symmetrical. On its
face, it could be regarded as an over-
neutralised (unstable) para-phase
amplifier which, with high feedback
from each plate to the grid of the other
triode via the 4.7pF capacitors, resembles a classic multi-vibrator circuit.
However, the load for each plate is
a split resonant circuit which generates a negative resistance. If a negative
resistance is applied across a resonant
or tank circuit, it will oscillate.
The arrangement is a “Kallitron oscillator” (sometimes
spelled with one L, but it
has two Ls in Terman’s Radio
Engineers’ Handbook).
adjustment) creates the bandpass characteristic of the video IF. In this set,
the bandpass characteristic is very
well described in the service manual
(Fig.2). The bandpass characteristic
is only in the order of 3MHz for the
video, which is enough to support a
fairly detailed picture on the relatively
small screen.
Later, as the CRTs got bigger, the
bandwidth had to increase to have
good high-frequency picture detail. In
RCA’s next TV, the 630TS, they moved
to a 10-inch CRT, and the video bandwidth was a little closer to 4MHz.
Video & audio IF stages
The video IF stage consists of
a string of four stagger-tuned
circuits based on 6AG5 pentodes V101, V102, V103 &
V104. Like the 6J6, these revolutionary small 7-pin types
would ultimately lead to the
demise of their larger octal-base
counterparts. A few years later,
the 6AG5 turned out to be an
excellent performer in the VHF
turret tuner units of many brands
of TV sets.
The stagger tuning (with correct
Australia's electronics magazine
Fig.1: the oscillator section of the
circuit, based on a 6J6 dual triode (V3).
December 2022 95
Fig.2: with the receiver RF
oscillator operating at a higher
frequency than the received
carrier, the intermediate frequency
relation of picture to sound carrier
is reversed as shown below.
picture signals. The FM sound detector has the typical S-shape required
for FM sound demodulation.
The audio stages consist of 6AT6
triode driver V117 and 6K6 audio output stage V118. The maximum power
output from a 6K6 is generally around
4W, similar to the more common 6V6
beam power valve, so there is plenty
of audio power.
Vertical scan
The curve shown is typical of the picture IF
amplifier response.
The output from the video detector, an octal 6H6 (V104A), passes to
video preamplifier triode V105, half
of a 6SN7. The other half is used for
video output to the CRT.
One interesting feature is that the
video DC restoration is done at the
grid of this video output valve. The
positive sync tips cause grid current,
with the grid-cathode acting as a diode.
This clamps the sync tip and the black
level to a stable point. The anode of
this valve is directly coupled into the
grid of the 7DP4 CRT.
The plate load of the 6SN7b has
both shunt and series peaking with
inductors to maintain the frequency
response required for the video signal. This became industry standard
for the video output stage. The video
background is unstable, depending
on the image contrast, without direct
coupling from the video detector and
amplifier to the CRT or DC restoration.
The sound carrier frequency of
21.25MHz is filtered out by T101 to
avoid any sound interfering with the
This is handled by another 6SN7
(V107). One triode is used as a combined blocking oscillator and discharge valve with a small transformer,
running at 60Hz. The other triode in
the 6SN7 is used for the vertical output.
‘Discharge’ in this case refers to rapidly discharging a ‘sawtooth’ capacitor, C130, which is then charged via a
high resistance source. This generates
the sawtooth wave required for the
scan. However, a trapezoidal wave is
needed to develop a sawtooth current
in an inductor with resistance, such as
the vertical yoke coils.
This is created by placing a small
resistor, typically less than 5kW (here
R149 = 3.3kW) in series with the sawtooth capacitor. The vertical output
transformer matches the output stage
to the vertical yoke coils in the usual
manner.
Horizontal scan and
EHT generation
The horizontal oscillator, running at
15.75kHz, is also half of another 6SN7,
V108. The oscillator is synchronised
on a line-by-line basis from the sync
pulses. By the late 1940s, this idea was
abandoned in favour of an automatic
frequency control circuit (AFC) with
better noise immunity, operating on
the same principles as a phase-locked
loop (PLL).
The other triode in the same 6SN7
is used as a separate discharge valve.
The drive then passes to a substantial power output valve (6BG6, V109)
with a 0.9A heater. It is specifically
designed to be a ‘sweep valve’ for
horizontal output stages, withstanding very high peak anode voltages in
the order of 6.6kV. Peaks of a few kV
appear on the anode during flyback
in this set.
The flyback circuit uses energy
recovery damping (with 5V4 damper
diode V111). See the following panel
on “The evolution of the damper
diode” for details.
This was a revolution in TV design,
providing highly-efficient horizontal
scanning using the stored magnetic
energy from the right half of the scan to
scan the left side. At the same time, it
created the high voltage flyback pulse
that could be stepped up to many kilovolts and be rectified, in this case with
a 1B3 rectifier (V110) to run the CRT’s
final anode.
Before this idea of using the energy
recovery diode, the scanning was less
efficient, and the required damping
wasted energy in resistors and sometimes diodes as heat.
Generally, because pre-WW2 sets
had no high voltage spike in the horizontal scan output stages from which
to derive EHT, they simply used a line
transformer. Large filter capacitors
were needed to remove the ripple, and
the supplies were a lot more dangerous
Photos 2 & 3: the line output transformer (shown disassembled at left, and whole at right) has an advanced moulded iron
core made of three parts. This was around the time most manufacturers were switching to ferrite-cored transformers.
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Photo 4: the
unrestored chassis
without the CRT. The
photo above shows a
close-up of one of the
valves with a lead
shield.
as they could source higher currents
and store more energy.
It is safer to have a flyback supply
with a relatively high internal resistance to generate the few milliamps
needed. A supply that can deliver
more than 30mA at several kV is hazardous.
Contrary to what some believe, the
charge stored on the bulb of a CRT after
turn off is low and generally cannot
harm a person as a one-off discharge.
This is why, even with the set running, very few if any TV technicians
have received a fatal shock from a flyback EHT supply for a CRT, as they
can mostly only source relatively low
currents.
Line output transformer
The line output transformer in
this set is very interesting. It has an
advanced moulded iron core made of
three parts (it’s visible disassembled
in Photo 2 and assembled in Photo 3).
The core is intermediate in appearance
between a ferrite dust core and an
iron core.
Laminated iron cores struggled to
work well at the 15.75kHz horizontal scan frequency. However, some
UK-made TV sets still used iron-cored
line output transformers even in the
post-war period. Within a decade after
the 621TS was released, most American TV manufacturers had moved to
ferrite-cored horizontal output transformers.
This basic design set the standard
for practically all line output transformers to follow, complete with the
two-turn winding for the EHT rectifier.
A dirty and rusty chassis
Photo 4 shows the unrestored chassis with the CRT removed, with a
close-up after I had removed the superficial dust and dirt removed. One valve
has a lead shield, with a spring clip
holding it in place.
As is standard practice, I hollowed
out the original wax paper capacitors
and fitted new polyester types with
double the original voltage ratings
inside. I then poured polyester into
each end to seal them up on alternate days.
After that, I had the chassis finebead blasted to remove all the rust,
re-plated with 20-micron electro-less
nickel, and lacquer coated. This helps
to avoid corrosion and finger marks.
I rebuilt the tuner first. The tuner in
this set is ‘space age’ sophisticated for
1946. Its features include a differential
input RF amplifier with neutralisation
based on a 6J6, another 6J6 Kallitron
oscillator and the spectacular large
mixer coil driven by the combined
anode signals from another 6J6. The
use of a combination of both ferromagnetic materials and brass slugs to tune
the coils is also advanced.
The idea behind the large mixer coil
(seen on the top of Photos 5 & 6) is to
create a very high-Q, loosely coupled
selective sound take-off. The large
sound IF coil is spaced away from
the former to avoid it being tuned by
distributed capacitance; instead, it is
tuned mainly by the ‘high-Q’ 62pF
dog-bone ceramic capacitor across it.
The mixer anode coil for the video
is broadly tuned and loaded by a 10kW
resistor and the plate impedance of the
6J6 mixer valve.
While restoring the tuner, I replaced
most of the bypass/coupling capacitors with silver mica types, except
the local oscillator feedback capacitors. I replaced those with 500V 4.7pF
mil-spec dog-bone ceramic capacitors
with the same temperature coefficient
as the originals. The same goes for the
...continued on page 100
Photos 5 & 6: the disassembled (left) and assembled (right) RF tuner. The tuner knob has dual-control with the more
protruded section providing station selection while the rest is used for fine tuning.
siliconchip.com.au
Australia's electronics magazine
December 2022 97
The complete circuit
diagram for the RCA
621TS TV set.
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December 2022 99
Photos 7, 8 & 9:
the chassis as it
progressed through
restoration. All
resistors were
changed to 2W
metal film types,
and the wiring
cleaned up.
1.5pF neutralisation capacitors in the
6J6-based RF amplifier.
I mainly used metal film resistors
throughout. That helps to keep the
noise down a little. The valve sockets
are NOS (new old stock), including
the original push-on shield type for
the local oscillator valve. I replaced
the push-on shield, identical to the
rusted original. I also replaced the rivets and original screws with 4-40 and
6-32 stainless steel screws (to avoid
future rusting).
Although I used stainless locking washers, applying varnish to the
threads never hurts, so I did.
Restoration well underway
Photos 7-9 show the chassis’ progress throughout the restoration. By
Photo 9, the underside of the chassis
was re-wired and fitted with all-new
resistors, wiring and valve sockets.
The resistors are now all 2W metal film
types, yet the same size as the original
1/4W or 1/2W types.
After replacing the innards, I
cleaned the wax off the cardboard
shells of the wax paper capacitors and
varnished them with marine grade varnish. This way, they look excellent,
but the surface is not tacky to touch
and won’t pick up as much dust as
wax does.
I replaced the octal sockets with
American mil-spec brown phenolic
sockets with wrap-around pins and
stainless steel saddles. Similarly, I
replaced the 7-pin sockets with American phenolic sockets from Antique
Electronic Supply (AES).
AES (USA) supplied all the new
capacitors, including the micas, electros and polyesters, several NOS
valves for the set, the 300BX power
transformer and new tag strips. They
always send me excellent valves and
parts at competitive prices.
The adjustable IF coils had very
rusty spring mounting clips, so I
replaced them with new ones, as they
are a common part of many NOS coils.
The originals were soldered to the
chassis on the top, presumably to prevent capacitive effects from affecting the IF tuning. I simply soldered
them to the nearest Earth lugs under
the chassis with a short link wire to
avoid soldering to the top surface of
the chassis.
The new wiring is medical-grade
silicone rubber covered hook-up wire,
which looks just like old-fashioned
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Photo 10: the
top of the
chassis once
nearly finished
being restored.
rubber-covered wire, but is extremely
heat resistant, and this insulation
never melts back near the solder joints
(even if the iron is set to 480°C). The
wire is pleasant to handle and flexible,
but stays where it is put on the whole.
It is about 2.5mm outside diameter and
has 16 strands.
Once you have used superior wire
like this, it is tough to go back to
PVC-covered wire or anything else.
Silicone covered wire is available
from Jaycar, Altronics and RS components.
I stuck to the colour scheme on
the wiring diagram where possible.
Fabric-
c overed wire is available,
although I suspect it would meet the
same fate as the original wire over the
next 60 years. The silicone rubber wire
will outlast it, I’m sure. Ideally, I want
the restoration to look about the same
in 50 years.
Photo 10 shows the top of the chassis close to the end of the restoration
process.
Photo 11: I
designed this
support to
allow the CRT
to be mounted
when the
chassis is out
of the cabinet.
The support
sits on top of
the speaker
brackets and
is shown in
greater detail
in Fig.3 below.
Mounting the CRT for testing
The 621TS chassis design only
allows the CRT to be mounted properly when the chassis is in the cabinet. This is very inconvenient when
the chassis is out of the cabinet, so I
designed the support shown in Fig.3.
It is attached by lengthening the two
upper speaker screws and adding two
spacers, and it sits on the speaker
brackets (see Photo 11).
The screw holes are best marked out
after the bracket is in place. The CRT
sits on it, and you can strap the CRT
to the bracket with a large (industrial-
sized) Nylon cable tie with the chassis
out of the cabinet. The added timber
bracket can stay put when the chassis
is re-fitted to the cabinet, and the CRT
is mounted in the usual way.
The radius of curvature of the cutout in the timber is 93.5mm. This is
reduced to 90.5mm when the rubber
cushion is added to hug the CRT curvature. The bracket geometry ensures
the CRT neck is very close to level
with the chassis surface. No extra
holes need to be drilled to fit it. Photo
12 shows the CRT fitted to the chassis
with the assistance of the bracket for
testing and adjustment.
Power transformer
Photo 13 includes the original
power transformer. I took the copper
flux band and covers off it and blasted
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Fig.3: the support bracket helps the neck of the CRT reach close to level with
the chassis surface, it’s designed so that no extra holes need to be drilled into
the chassis to fit it. It is attached by lengthening two screws from the speaker
brackets and adding two spacers. The screw holes can be marked out by hand
after the bracket is in place.
Australia's electronics magazine
December 2022 101
Photos 12 & 13: the CRT shown fitted to the chassis (left) and the original & new power transformers (right)
the original brackets free of rust, then
had them powder-coated black. The
finish looks very similar to the original
and is corrosion and scratch-resistant.
I then added the restored brackets to
a new Hammond 300BX transformer,
discarding the Hammond covers as
they are pretty different. The two transformers have very close to the same
geometry stack, just with the holes
placed a little differently. The two
wires for the 120V configured primary
windings have to exit via an additional
hole in the top bracket.
The reason for doing all this is that
the original power transformer is not
safe to run again, especially in Australia with our 50Hz supply frequency.
The transformer has very aged and
cracked insulation and draws an
excessive magnetisation current at
50Hz.
For example, with no load, the RMS
current at 115-120V 50Hz is 1.5A,
compared to 47mA for the Hammond
300BX transformer configured for
120V, which is designed for 50/60Hz
operation. In general, old American
60Hz transformers run very hot on
50Hz. There are also significant stray
magnetic fields generated. Switching
to the Hammond transformer solved
the problem.
The windings on the Hammond are
close to an exact match for the original.
I connected two 5V 1.2A windings in
parallel to run the 5V4G damper diode,
used one 5V 3A winding for the 5U4G,
one 6V 6A winding for the main heaters and one 800V centre-tapped winding for the HT supply.
There is only one winding ‘missing’,
a small 6.3V one for the CRT heater, so
I added a small auxiliary transformer
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Silicon Chip
for that. There is a convenient place
to locate it, and only one hole needs
to be drilled to mount it. I made this
by winding a small 1:1.17 ratio isolating transformer, which gives a separate 6.3V output at 0.6A to provide
the CRT heater supply from the power
transformer’s 6.3V winding.
Note that the data sheet on Hammond’s website says the 300BX has
only one 5V 1.2A winding when, in
fact, it has two.
At switch-on from cold, the heavy
loading of all of the TV’s low-resistance
cold heaters results in a slow rise in
the initial heater currents due to the
limited current handling ability of the
power transformer. So, in a sense, the
larger valves protect the smaller ones
at switch-on.
But in series heater chains, resistors or thermistors (Brimistors) are
needed as the internal resistance of
the mains power supply is very low
and the initial surge current in the
cold heater chain is very high. The
smaller valves warm up first (due to
lower thermal inertia) and more voltage is developed across their heaters
without current limiting.
It is interesting to note that the same
problem described above will occur
within any indirectly heated valve
if you connect the heater pins across
a power supply of very low internal
resistance. The part of the heater close
to its internal connections warms up
first, as there is less thermal inertia
there than the part in contact with
the cathode or the weld to the pin
connection.
So you will get an initial bright
flash from that area of the heater at
turn on. In fact, you can get this effect
if you unplug nearly all of the large
valves in a TV, except for a small one.
At switch-on, you’ll also get a bright
flash, as the large power transformer
is, under these circumstances, able to
maintain 6.3V across the single small
valve’s pins without the voltage collapsing under load.
I had to replace the two-turn heater
winding for the 1B3 rectifier as the
original had degraded insulation. I
found some identical geometry wire
Photo 14: the original cabinet had
been enlarged around the CRT.
Photo 15: I cut a piece of oak to
reproduce the original CRT window.
Valve heater inrush currents
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Photo
16: the
restored
cabinet
with the
newly
made
CRT
cutout.
inside the red sheath of some modern
25kV anode wire.
Also, all of the large Allen Bradley resistors in the focus chain were
open-circuit. I replaced them with
10kV-rated Philips focus-grade resistors.
I also replaced the doorknob capacitor with a 1000pF 15kV type (the same
physical size as the original 500pF
capacitor), allowing for CRTs without
external Aquadag. I’m not 100% sure
if the original doorknob capacitor is
OK; it only reads 375pF, and I’ve read
reports of them failing in the 621TS.
Electrical alignment
I aligned the set according to the
manufacturer’s instructions but also
with the aid of a sweep generator.
Scope 1 shows the overall response
from the antenna to the video detector.
Screen 1 is an un-retouched image
taken via a camcorder on still frame
with an RF modulator. The broad grey
vertical band at the top is an artefact
of the camera’s exposure time versus
the scanning frame rate of the picture.
Cabinet restoration
One big problem I had with the set
was that the cabinet section over the
CRT face had been cut away to enlarge
the viewable area of the CRT. Perhaps
one previous owner wanted a bigger
picture! So some timber was missing,
as shown in Photo 14. I cut out a square
area and glued in some Tasmanian oak
to repair this.
I then varnished it and shaped it
to match the original design and fit
the curved CRT face. Applying varnish initially helps with getting the
geometry right as one files the timber
away by hand.
Finally, I lacquered it to match
the original part and got the result
shown in Photo 15. Photo 16 shows
the restored cabinet, while the lead
photo is the final result.
Summary
The 621TS is an extraordinary television set, marking a major milestone
in commercial TV manufacturing. The
entire design is futuristic, and the performance is outstanding for a set put
on sale in 1946.
FM sound became the gold standard
Scope 1: the overall response from the antenna.
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for television audio after WW2, and the
line deflection energy recovery system
did too. Any similarly-sized monochrome television set made decades
later would not have outperformed it.
The design of the 621TS, except for
the absence of the inter-carrier sound
system and a horizontal AFC system
(both of which would come in later
TV designs), set the ‘modern standard’
of what a monochrome TV would be
right up until the mid-1970s.
Finally, from the perspective of
industrial design, Mr Vassos created
yet another Art Deco masterpiece.
↪ see panel overleaf
Screen 1: an image of the 621TS screen from a camcorder.
Australia's electronics magazine
December 2022 103
The evolution of the damper diode in TV line output stages
Very basic coupling of the yoke to the line output valve
via a transformer is shown in Fig.a. At flyback, the valve
is cut off and the magnetic field in the transformer and
yoke collapses, resonating due to the self-inductance
and distributed capacitance of these structures. The
oscillatory voltages and currents are due to relatively
undamped oscillations.
These oscillations, visible in the scanning raster, decay
away and become damped out when the line output
valve is again driven into conduction by the drive voltage. These oscillations must be eliminated for satisfactory raster scanning.
Fig.b shows the same circuit but with resistive damping. Damping occurs over the entire duration of the sawtooth current scanning waveform, on both the positive
and negative excursions, ie, it is bidirectional damping.
This is wasteful of energy, lengthens the flyback period,
and reduces the opportunity to utilise the positive-going
high voltage spikes generated at the line output valve’s
anode, or via an overwind coil, to generate EHT.
Fig.c shows an improvement to resistive damping using
a snubber network. This technique is used in the 1939
HMV Marconi 904. The RC network is frequency-selective,
applying the most damping to the parts of the waveform
with the highest rates of change.
This reduces the oscillations (shown in red); however,
because the flyback period contains high-frequency (Fourier) components, it is also damped. Again, this wastes
energy and lengthens the flyback period.
Fig.d shows what might appear to be the introduction
of an efficiency diode as in the RCA TRK9 (and TRK 12),
but it is not. This circuit has the damper conducting only
during flyback and is actually a spike suppressor. A true
efficiency diode conducts during the active scan time
on the left-hand side of the scanning raster, and recovers energy from the yoke and line output transformer
magnetic fields.
The circuit of Fig.d damps the flyback voltage oscillations and absorbs energy when the output valve is cut
off. This arrangement can’t be used in a system to generate EHT from the flyback voltage spike.
In 1938, the Baird/Bush TV and radio company in the
United Kingdom used the circuit shown in Fig.e (on the
left side). This is probably one of the first examples of
energy recovery scanning.
When the magnetic field in the line output transformer
collapses, the diode conducts on the first negative half-
cycle of voltage on the diode’s cathode to produce a
more linear rate of change in current. This damps the
oscillations and also returns energy to the power supply.
This was the precursor of the typical transistorised
line output stage in early transistor televisions in the
early 1960s, depicted on the right of Fig.e.
Although the circuit in Fig.f looks a little similar to
that in Fig.e, it is actually quite different, with the diode
on the transformer secondary side. Observe the transformer polarity.
The current from damping the oscillations charges
capacitor Cb and provides energy to load R. Cb charges
up and lifts the cathode potential of the damper diode.
Fig.a
Fig.b
Fig.e
104
Fig.f
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So the plate potential has to rise to a higher value to
establish conduction.
This helps ensure that the diode is not conducting
until the start of active scan time, so there is negligible
damping during the flyback period.
This system is “recovering energy” from the magnetic
field of the yoke and transformer, which was stored at
the end of active horizontal scan time, and delivering
it to a load.
This is the basic circuit used in the RCA 621TS, except
the voltage generated across the capacitor is in series
with the B+ voltage to create what we now know as B+
boost voltage.
When the line output valve is cut off at flyback, the first
voltage oscillation half-cycle takes the damper anode
negative (cutting it off during flyback). The damper
anode has the opposite polarity to the anode of the line
output valve. Then when the oscillation brings the voltage positive, the damper conducts.
This damps the oscillations and results in a near-
linear scanning current at the left side of the raster, as
the magnetic field in the yoke and transformer now collapse in a controlled (damped) linear way toward zero.
However, before the current reaches zero, the line output
valve is driven into conduction and the process repeats.
The yoke and transformer circuit is equivalent to an
inductor with series resistance tuned by parallel distributed capacitance (or a tuning capacitor if fitted). The voltage you see across the transformer or yoke’s terminals
represents the voltage across the capacitive component,
which lags behind the circuit current by 90°.
When the output valve is cut off, the circuit current
during the flyback period is associated with a negative
peak voltage on the damper anode and a positive peak on
the line output valve’s anode. These peaks occur around
the middle of the 10.16μs flyback interval (for the American system). At the time of this peak, the yoke’s current
is zero (but has its greatest rate of change) and the rate
of change of voltage on the diode’s anode, although at
its peak, is zero.
After that, the secondary voltage returns to zero after
flyback, and the current is at a negative maximum with
the beam on the left of the raster. As the voltage at the
damper anode attempts to oscillate in a positive direction, the damper diode conducts, damping the oscillations
and giving a more linear current at the beginning of active
scan time on the left side of the raster.
The load resistor can now be replaced with the primary
circuit, as shown in Fig.g. RCA used this basic circuit in
the 621TS, and this, or a modified version of it, became
the ‘modern Standard’ for line output stage deflection
using valves ever since. Cb’s negative end can either be
returned to ground or B+ as shown, which is at ground
from an AC perspective.
Now the recovered potential energy generated by the
magnetic field of the yoke and transformer, which was
provided by the primary circuit at the end of the scan
(right side of the raster), is used to generate a boost
voltage to help supply the primary circuit. This gives a
higher primary supply potential, the B+ Boost voltage,
which helps attain the required picture width from a
lower-voltage B+ supply.
Fig.c
Fig.d
Fig.g
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Fig.h
Australia's electronics magazine
December 2022 105
As is always the case, no additional energy is created
that was not already supplied by the power supply in the
first place.
The circuit is simply more efficient because overall,
the damped current is not wasted as heat, which it is in
all cases of resistive damping.
Moving on the Fig.h, we can see what happens if we
redraw Fig.g circuit with Cb connected to ground. This circuit, as deployed in the 621TS with slight modifications,
is the basis for modern valve line scanning.
At switch-on, a direct current flows via the secondary winding and the damper diode to charge Cb to B+
potential and to initially supply the B+ to the primary circuit. During operation, the voltage across Cb charges to
B+ Boost. Therefore, Cb needs to be rated to handle this
higher voltage.
This circuit is inconvenient because the transformer
cannot be configured as an auto-transformer. But it is a
minor modification to introduce B+ directly at the anode
of the damper diode. Then, the circuit comprising the
secondary, damper diode and Cb can be rotated to create the circuit of Fig.i.
This circuit has the advantage that the Cb only needs to
be rated to handle the Boost component of the B+ Boost
voltage, rather than the total amount. Also, the primary and
secondary can be one tapped winding, with the yoke coupled across any part of it, in an efficient autotransformer
configuration. Admiral used this basic configuration in
the early 1950s, for example, in their series 23 chassis.
By the time that efficient energy recovery line output
stages arrived, it had become the custom, as it is in the
621TS, to derive the EHT from an over-wind linked to
the plate circuit of the line output valve shown in red in
Fig.i. The heater supply for this EHT diode was derived
from a small number of well-insulated turns on the output transformer.
Other variations of damper diode circuits from the
post-war period include a triode pair used as a controlled
damper diode, which gives additional control over the linSC
earity of the sawtooth scanning current.
Fig.i
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