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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 94 Silicon Chip 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), siliconchip.com.au 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 siliconchip.com.au 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. 96 Silicon Chip Australia's electronics magazine siliconchip.com.au 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. 98 Silicon Chip Australia's electronics magazine siliconchip.com.au siliconchip.com.au Australia's electronics magazine 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 100 Silicon Chip Australia's electronics magazine siliconchip.com.au 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 siliconchip.com.au 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 102 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 Australia's electronics magazine siliconchip.com.au 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. siliconchip.com.au 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 Silicon Chip Australia's electronics magazine siliconchip.com.au 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 siliconchip.com.au 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 Improved SMD Test Tweezers Complete Kit for $35 Includes everything pictured (now comes with tips!), except the lithium button cell. ● ● ● ● ● ● 106 Resistance measurement: 10W to 1MW Capacitance measurements: ~10pF to 150μF Diode measurements: polarity & forward voltage, up to about 3V Compact OLED display readout with variable orientation Runs from a single lithium coin cell, ~five years of standby life Can measure components in-circuit under some circumstances Silicon Chip SC5934: $35 + postage siliconchip.com.au/Shop/20/5934 Australia's electronics magazine siliconchip.com.au