Class D & E amps

Then simply this diode and RC network is placed on a secondary winding, not the primary, and the position of the diode and RC network reversed as they’re in a series circuit then you end up with the following seen in Fig 5C:

This circuit, although looking a little similar to that for the TRK12, is in fact quite different.
Observe the transformer polarity.
As will be seen below this is in fact the basic circuit used in the RCA 621 TS, except that the voltage generated across the capacitor is added in series with the B+ voltage to create what we now know as B+ boost voltage.

This same basic circuit, with the diode and RC network is also shown in Fink’s Principles of Television Engineering 1940 page 152, fig 87, placed in the primary circuit

When the line output tube is cut-off at flyback, the first half cycle of voltage oscillation takes the damper anode negative (cutting it off during flyback).
The damper anode has the opposite polarity to the anode of the line output tube. Then on the first positive half cycle of the voltage oscillations at the damper anode, 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.
Before the current reaches zero however, the line output tube is driven into conduction and the process repeats.

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 degrees.
When the output tube 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 tube's anode.
These peaks occur in time within about the middle of the 10.16usec fly-back interval (American system).
At the time of this peak, the yoke's current value 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 at this time.
After that the secondary voltage returns to zero, at the end of flyback, and the current is at a negative maximum, now with the beam at the extreme left of the raster.
Then as the voltage at the damper anode attempts to oscillate in a positive direction, at the damper's anode with respect to its cathode, the damper diode conducts, damping the oscillations and resulting in a more linear current at the beginning of active scan time on the left side of the raster.
Moving on to the post war period we find Fig 6 below which really represents the Baird/Bush concept 1938,or the basic function outlined by Fink 1940, in the format seen in Fig 5C.

Damped current charges capacitor Cb and provides energy to a load R. Cb charges up and lifts the cathode potential of the damper diode.
This means that 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 fly-back 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.
The load resistor can now be replaced with the primary circuit. This is shown in Fig 7 below

This basic circuit was used by RCA in the 621 TS, and this, or a modified version of it became the "Modern Standard" for line output stage deflection, using tubes, ever since.
Cb's negative can either be returned to ground, or to B+ as shown, which is at ground from the AC perspective.
The recovered potential energy generated by the magnetic field of the yoke and transformer, which was in fact 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 larger primary supply potential, the B+ Boost voltage, which helps attain the required picture width from a smaller B+.
It should be pointed out that, 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 primarily wasted as heat, which it is in all cases of resistive damping.
One definition of a resistance is an energy wasting or heat dissipative device.
Moving on the Fig 8, we can see what happens if we simply re-draw the above circuit with Cb connected to ground.

This circuit, as deployed in the 621 TS, with small modifications is the basis for modern tube line scanning.
At turn on, DC 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.
The circuit is however “inconvenient” in that the transformer cannot be configured as an auto-transformer.
It is a small modification to introduce B+ directly at the anode of the damper diode and then the circuit comprising the secondary, damper diode and Cb can be rotated around to create the circuit of Fig 9 below.
The circuit of Fig 9 has the advantage that the Cb only needs to be rated to handle the Boost component of the total “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 1950’s, for example in their series 23 chassis.
In general, by the time efficient energy recovery line output stages had arrived, it had become the custom, as it is in the 621 TS, to derive the EHT from an OVER-WIND linked to the plate circuit of the line output tube shown in red in Fig 9, and the heater supply for this EHT diode derived from a small number of well insulated turns on the output transformer.

This is an account of some modifications I did to an ancient (circa 1980) Sanyo TV of ours to fix an annoying side effect of the so-called "Macrovision" protection of rental VHS video tapes.

NOTE: If you came to this page as a result of looking for Macrovision removal methods (eg: to enable your DVD player to run to your TV set via your VCR without having the picture go dark every 30 seconds or so), what you really need is a video timebase stabiliser.

This page doesn't provide that. You can buy kits for such items (for the Oz and UK PAL TV standard) from your nearest Jaycar Electronics shop and make your very own in one evening (if you aren't frightened of soldering). Just type "stabiliser" into Jaycar's search box and then look for "Dr video Mk II". They're about $100.

Okay - back to the story of our old Sanyo!

This old warrior has been kept in service over the past 22 years for the simple reason that I fitted a nice big new 63cm (25") tube back in 1985 after the green cathode of the original had gone low in emmision.
(Well, re-gunned actually ... you would give Thomas Tubes here in Melbourne your old CRT, and they'll supply you a re-gunned one of the same type for around $150.)

Anyway, in June of 2003, this old Sanyo had gone "bang" and stopped working completely.
I learned that when the set was later turned on once again, there was now a strong, acrid burning smell in the room and no picture or sound at all.

An old original electrolytic capacitor (4.7uF 350 volt) had shorted out, plus incinerating a 1 ohm series resistor which obviously hadn't enjoyed having 120 volts across it as a result (see Fig.2).
These were part of the overall 120 volt feed to the horizontal output stage.

An 8uF 450 volt electro and a 1 ohm 5 watt resistor soldered in, and sound and picture returned as good as new.

Okay - so far, pretty straightfoward.

At this point, I wondered to myself
"Should I just take the set back to the living room now, or try to do something about it's Macrovision susceptibility?"
(see Fig.4) Because, whenever we'd play rented tapes over the years, every 30 seconds or so, we'd get all these bright white dashed lines coming up over the top half of the picture for 9 or 10 seconds. A most annoying effect.

Finally, I decided to have a shot at getting rid of the problem.
Basically, it seems Macrovision sneaks in a series of high amplitude white-going pulses during the vertical retrace interval. Aha, I thought,
"okay then, let's have another look at that Sanyo circuit
(see Fig.8).
What could be wrong with its vertical retrace blanking?"

One guess.

Sure enough - there was no evidence of any CRT-retrace blanking circuitry at all! One could only assume that more recent (or better designed) receivers must include such blanking, because our old chassis was one of the few on which I'd ever noticed this effect. So - how to get rid of the problem?

It seemed obvious that what was needed here was some additional circuitry that would clamp the video signal down to black (or below) during the vertical (field) retrace interval.
I quickly sketched up a circuit for a single NPN transisor, some resistors and a pair of coupling capacitors, grabbed the components out of the drawer and soldered myself up a little rats-nest for the job. I then connected the transistor input circuit up to the vertical output sawtooth (via a 100k resistor and a 470pF cap) and experimentally tried hooking its output up to various spots in the video-signal chain.

I eventually managed to get this single transistor switch system working to a greater or lesser extent. I could clean out a certain amount of the effect - but it was still disappointingly short of the mark.
For one thing, it wasn't staying on for sufficiently long. For another, because I was taking my driving signal off the 120 volt analogue sawtooth vertical output and differentiating via an RC network into the transistor base, the transistor was only acting as a switch as it turned ON. The turn-off process was really slow and sloppy.

At this point, the idea of some digital logic went through my mind.

Over the following weekend, I put the CRO on the set once more and checked out the waveform and gave it some more thought. "Why not a bistable flip-flop? That should allow some control over the pulse length, if nothing else, and it'll be absolutely "on" or "off", ie: a nice simple, predictable switch. Why not give that a try?"

So once again it was out with the pen and paper, now drawing up a bistable. I then wired it up and tried it, and this time things looked much more promising. Initially it would turn off too early, but this was soon traced to my poor choice of sampling point (on the vertical output circuit) and a coupling capacitor which at 470pf was too low. So I fixed that, moved the input to the actual vertical oscillator circuit (which is an 18 volt square wave instead the 120 volt sawtooth wave I'd been sampling) - and now the timing was much closer to what I wanted. Spot on, in fact.

My only other problem now was that there seemed to be no convenient spot in the circuit to which I connect the output of this little rat's nest circuit to completely blank the video. I could reduce it, sure - but not completely kill it. It was frustrating in the extreme. All I wanted was a spot where I could ground the video signal or pull it up to 18 volts for black, and yet there seemed to be no such point! I could hardly believe it - every obvious output connection point produced problems of one sort or another. So once again I seemed to be going around in circles.

It occurred to me that I should just try running the flip-flop output into a high voltage NPN transistor and use that to ground the CRT control grid.
I'd already checked that circuit and the control grids (all wired together) were normally sitting around +100 volts, with the cathodes on about +250 volts (see Fig 6 and/or Fig 8). In other words, the control grids were sitting around -150 volts with respect to the cathodes during normal running, but grounding them would increase this negative bias by a further 100 volts.

I checked the effect of grounding the grid circuit, and going that extra 100 volts negative certainly blacked things out. So I grabbed an MJE340, wired it up to the CRT board, and connected the output of the flip-flop to it. And hey - bingo - success at long last!

I finally had reason to feel pleased with myself.

Here's a few pictures.

Fig.1 The old 65cm Sanyo on the workbench with the back removed.

Fig.2 The original fault was caused by this little electrolytic capacitor on the left. The poor resistor then died as a result.

Fig.3 Having fixed the above fault, I decided to have a look at our Macrovision problem. Looking at the video signal coming out of the video detector with my old Telequipment D66 CRO, I could clearly see the Macrovision interference signal

Fig.4 And here's the delightful effect on the screen. Believe it or not, not only had the family been tolerating this for 10 years with the more recent rented tapes, they told me not to worry about it! (I know that it severly annoyed our friends)

Fig.5 This is what the signal should look like, as seen between Macrovision interference periods

Fig.6 This is the final version of the add-on circuit to provide vertical retrace blanking - a bistable flip-flop (using discrete transistors, no less).
When the vert oscillator waveform swings positive (ie: at far left of lower trace on CRO screen above), the flip-flop switches the MJE340 on and pulls the CRT grid down to 0 volts. When the vert osc swings negative (about half way across the screen), the bistable flips back and the MJE340 turns off again.

Fig.7 I couldn't be bothered making up a nice circuit-board for something as small as this, so I simply wired it onto the back of the Sanyo's main video processing board.

(The protection diode on the RHS is uneccessary, of course - I only included it because I was feeding signal to that side in one of my earlier experiments)