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Old 28th Mar 2018, 9:59 pm   #21
FERNSEH
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Default Re: DC Restoration questions

"Without DC restoration and with mean level AGC, does the black level noticeably change with scene content? Is it annoying?"
Hi Nick,
Back in the days when the set was new I doubt if any viewer complained about the wandering black level. Probably accepted that's how television was. But in fact it is a horrible AC coupled video system employed in this set. When the contrast control is turned down the brightness level goes up, also the frame flyback lines become visible. I find it annoying and it's likely other television enthusiasts the same.

The construction of the 53 and 63 series models looks a right mess but in use the sets were very reliable.

DFWB.
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Old 28th Mar 2018, 10:12 pm   #22
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Default Re: DC Restoration questions

Quote:
Originally Posted by Studio263 View Post
Compare this to the TV9-90UB, which has quite a complex AGC setup but its still mean level.
"Without DC restoration and with mean level AGC, does the black level noticeably change with scene content? Is it annoying?"

Oh! Yes.

https://www.vintage-radio.net/forum/...8&d=1112782192

Peter

p.s. I should say that it works much better on UHF though.
https://www.vintage-radio.net/forum/...ead.php?t=2464 post 11

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Old 29th Mar 2018, 12:18 am   #23
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Default Re: DC Restoration questions

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Originally Posted by 1100 man View Post
Without DC restoration and with mean level AGC, does the black level noticeably change with scene content? Is it annoying?
When the contrast in a scene is high, the amplitude of the video part of the signal is high and at maximum represents about 2/3 of the total peak to peak amplitude and the sync about 1/3.

So with AC coupling this pushes the sync level and black level toward black, or blacker than black. With this signal, for a stable high contrast scene the user can set the TV's contrast and brightness controls correctly, the CRT's beam is just cut off at black level and the sync pulses driving the beam blacker than black, all seems well and the picture looks great, with the CRT's beam cut off during retrace too.

(This is why of course people don't notice a test pattern having any problem, even with a poorly designed RF modulator without sync or black level stabilization).

However if the scene changes and the contrast drops low, typically on on a night time or dark room scene, the video part of the signal can drop toward the sync pulse voltage or lower, then with AC coupling the average video signal voltage has a new DC axis or average value. The black level and the sync pulse tip move positive, lifting the background brightness and reducing the retrace blanking. So the picture now looks washed out and the retrace lines appear (if the set does not have internal retrace blanking). The fact that the set's AGC results in the sync pulse part of the video signal having a fairly constant amplitude is of no help.

So it is a horrible and noticeable defect and I find it very annoying.

The same problem occurs of course, even when there is perfect 100% DC Coupling from the video detector in a TV to the CRT, if the RF modulator has AC coupling without clamping or sync tip stabilization.

In many American sets it didn't actually matter if the DC axis of the video signal was unstable with varying contrast levels at the RF modulator, because they tended to reinsert the DC level with a diode restorer later at the CRT. So the advantage of this, if a set has it, or has it added, you can still get good results from an inferior RF modulator.

One trick mounting the 6AL5 used as a restorer, if you put the the hex mounting posts from the socket holes and beside the tube body, and make them about 15mm longer than the the 6AL5 itself, you can mount it to a flat surface and then get a spring from a battery holder and put it between the surface and the top of the 6AL5 to make sure it can't fall out of its socket and then it is really easy to connect to the socket pins too.
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Old 29th Mar 2018, 1:39 am   #24
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Default Re: DC Restoration questions

...I could also add to the post that the way the average DC level and sync tip level alter with AC coupling and scene contrast changes was described for a usual negative going video signal polarity, say AC coupled to a CRT's grid. The fact that the video signal gets inverted to a positive going sync signal, to dive a CRT cathode instead, doesn't alter the problem, because the cathode of the CRT is such if you take the voltage positive, the beam current decreases, opposite to the grid. So the visual effects of an absent DC restorer are the same when the CRT is driven by AC coupling at its cathode or its grid.
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Old 29th Mar 2018, 7:21 am   #25
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Default Re: DC Restoration questions

I recall that my dentist, a keen (Hasselblad level) photographer could never live with the picture quality of a typical monochrome TV set. However, even though his photography was all in black & white, colour TV was immediately acceptable to him. I think that he, like many of us here, was very sensitive to a wandering unstable black level which is corrected in a colour receiver.

Martin
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Old 29th Mar 2018, 4:02 pm   #26
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Default Re: DC Restoration questions

There is a difference between sets like the RBM A640 chassis and the B&O 600 in respect to how black level stability was obtained. The RBM design attempted to recover the black level in the video amplifier by using a clamp, where as the B&O preserves it throughout the receiver. In fairness to RBM, dual standard operation makes this compromise unavoidable.

The 600 worked well because it had line gated AGC, DC coupled video and a clever contrast control circuit which meant that adjusting the contrast didn't upset the black level - an unusual feature for a set of its era. For anyone fussy about this sort of thing, this is the set to have. More details here:

https://www.vintage-radio.net/forum/...d.php?t=106461

Its still working too!
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Old 30th Mar 2018, 11:00 am   #27
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Default Re: DC Restoration questions

A small sub-assembly will be constructed for the video DC restorer. This will consist of a bracket which is be mounted on the tuner unit. See attachment. The bracket will carry the B9A valveholder for the 6/30L2 double-triode and tag strips to mount the few extra components that will be used in the new circuit.
The circuit: I'll try the Pye 368 DC restorer first and if this doesn't prove satisfactory a proper driven clamp circuit will be considered . A positive keying pulse will be required. The line flyback could be considered if it is coincident with the sync back porch. Much better timing accuracy can be obtained by using the sync pulses. The negative going syncs present at the anode of the sync separator valve will supply a simple differentiator circuit, the positive going pulse at the trailing edge of the syncs will be supplied to the clamp triode. The output from a differentiating circuit depends on the rate of change of the leading and trailing edges of the pulse. The problem is that the syncs present at the anode of most valve sync separators have poor rates of rise and fall.

Let's hope this project isn't going to be a waste of time. It's not easy to create high performance circuits with valve technology.

DFWB.
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Old 30th Mar 2018, 9:09 pm   #28
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Default Re: DC Restoration questions

Scoped the waveform at the anode of the sync separator, -150 volts P - P. The leading edge of the negative going syncs is steep but the trailing edge is not. A simple capacitor and resistor differentiator was made up. As expected the leading edge negative going spike was about 70 volts but the positive going trailing edge of the waveform was only 15 volts. No point using this for the black level clamp.

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Old 30th Mar 2018, 11:07 pm   #29
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Default Re: DC Restoration questions

Hi Hugo,
Thanks for your detailed explanation of what's actually happening. Peter Scott's picture in the link in post 22 (White square on a black background). illustrates the problem well.
David, I'm following your experiments with interest. Looking at the circuit of the V17-53, as you say its very similar to the Pilot PT650. The Pilot lends itself to having an extra valve fitted as it has two spare valve base holes on the bottom chassis rail so could be done very neatly.
I wonder what circuit arrangements you will find work the best?
All the best
Nick
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Old 30th Mar 2018, 11:35 pm   #30
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Default Re: DC Restoration questions

Quote:
Originally Posted by FERNSEH View Post
A simple capacitor and resistor differentiator was made up. As expected the leading edge negative going spike was about 70 volts but the positive going trailing edge of the waveform was only 15 volts. No point using this for the black level clamp.
I agree and the much better method to create a pulse that is coincident with the back porch is to feed the sync pulse into a resonant circuit. Basically the sync pulse is used to shock excite an RCL series resonant circuit and the voltage developed across the inductor on the next half cycle is used to drive a clamp stage that is mostly cut off and you get the delay required that way, referenced to the the leading edge of the sync, without having to rely on the vagaries of the shape of the falling edge when you use a differentiator.

However, for the most part, there is very little to pick between the two scenarios using a diode to clamp at the sync tip or an active valve clamp on the back porch for a vintage TV application, that is if the diode restorer is designed properly.

The sync amplitude component of the video is fairly stable watching a program. It does mean though that if the contrast control gets adjusted the brightness has to be re adjusted.

With a back porch or black level clamp the brightness and contrast controls are more independent.

It is a different story for a professional studio monitor that ideally does have a black level clamp.

In other words the extra complexity of the active clamp over a diode restorer in this application won't change the viewing experience once the TV is set up compared to a black level clamp, so is the extra complexity worth it ?

Also, don't forget if you use a triode active clamp method it still requires some resistance in series with the anode to the capacitor to isolate the anode capacitance which will be higher than the capacitance of the 6AL5 (Which was one reason I recommended it for the restorer) or picture HF detail will be affected. The more resistance here, in either the diode restorer, or the active clamp, reduces its efficiency.

So I would really recommend the circuit that I added to the Admiral set (referred to in my post above) with the 6AL5 because this valve has the ideal properties for a clamp diode, low resistance when conducting and low electrode capacitances too.

Also, when your clamp is finished, whichever way you go or however you implement it, it is easy to test how well its working. With the scope set on DC looking at the clamped signal at the crt and the beam position as offset far as possible to offset the DC there and the attenuator set to just recover a view of the video signal; then swinging the contrast control from a low to high signal, the sync tip with a diode restorer, or the black level with a back porch clamp will be stable in their DC position.

If the clamping is sub optimal (for example an unsuitable choice of clamp diode or an improperly working active clamp) you will see the DC axis shift. Also, as a final test closely inspect the picture's HF detail, have a test pattern with multi-burst on it, with the clamp circuit connected and disconnected from the coupling capacitor and look for any impairment in the frequency response with the clamp connected. Having been through the process of adding active clamps to various monitors and adding DC restorers to vintage sets I have fallen into all of the traps before and had to crawl out of them licking my wounds.
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Old 31st Mar 2018, 12:50 am   #31
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Default Re: DC Restoration questions

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Originally Posted by rambo1152 View Post
There was a Decca single-standard chassis that that maintained a good black level. Quite refreshing to see next to all the contemporary Thorns etc.

Channel 5's presentation graphics use a lot of mid-grey backgrounds, am I alone in thinking (in the words of Amy Winehouse) they should go back to black? Perhaps I should redo my avatar .gif for the same reason.
I agree. Some online video streaming services use a mid/dark grey background and it instantly makes me think the LCD panel in our Sony set has gone wrong!* It just looks bad, a proper black background would look far better.

*In fact our Sony has a very good black level, far better than the 'milky' effect of our previous Panasonic.
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Old 1st Apr 2018, 1:13 am   #32
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Default Re: DC Restoration questions

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Originally Posted by Argus25 View Post

Also, when your clamp is finished, whichever way you go or however you implement it, it is easy to test how well its working. With the scope set on DC looking at the clamped signal at the crt, then swinging the contrast control from a low to high signal, the sync tip with a diode restorer, or the black level with a back porch clamp will be stable in their DC position.
Hi,
I've just done a couple of quick tests on two TV's to assess their black level performance. Ideally, I would be able to vary the amplitude of the video signal from white to black without altering the sync level. This would simulate changes of scene content. I don't have a way of doing that so have to vary the contrast control instead. Obviously, this varies the sync pulse amplitude as well.

VT161.
The video signal is fed to the CRT cathode via a cap and resistor in parallel. There is also a dc voltage tapped off the boost supply via a 4.4Meg resistor, plus the cathode is also connected via a potential divider to the HT rail. I assume as the contrast decreases, beam current will fall and the boost voltage will rise. This will increase the cathode potential and reduce the brightness, thus compensating to some extent for the change in dc level caused by the reduction in contrast.
The black level at the cathode actually did stay remarkably constant as the contrast was varied. The black bar on the screen (stairstep pattern) got a bit lighter as the contrast was reduced but not too much.
When I used to use this set 10 years ago, I always felt it's black level was much more stable than other sets I had and always wondered why.

Pilot PT650 (Ultra V17-70)
The video is fed to the cathode only via a capacitor. The brightness control is also fed to the cathode.
The black level on the scope varied widely as the contrast control was altered. Sure enough, the black bar on the screen turned a very light grey as the contrast was reduced.
I will be interested to see how stable it is when David has perfected his DC clamp. It certainly will benefit from the modification.
This whole chassis really is an exercise in how few components can we build a TV with? The designers have gone to great effort to pare it down to the bone!!

Cheers
Nick
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Old 1st Apr 2018, 10:01 am   #33
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The designers have gone to great effort to pare it down to the bone!!
That was the name of the game for most manufacturers, selling price for many items then, like now is how things are made.

Deficiencies in performance judged against cost of improvements, Thorn for one were masters of this technique for their rental shops. The vast majority of viewers accepted that the performance was normal and concentrated on the programme content. Some had snow on the picture in fringe areas, ghost images in areas where reflections could not be cured, a slightly varying black level was not noticed.

In the 50’s it was still a novelty for many to have a TV so again picture defects were often accepted.

There were viewers who wanted the best available but not the majority.

Some sets got the performance better than others and still kept costs down, others didn’t. Pye/Ekco brought a TV out in the middle late 60’s and I told the rep it had been paired down too far, his comment was that due to the companies finance problems the engineering Dept had to cut £2 worth of components out of the set and still make it work, it was in my opinion a step too far.

Nice to look back and think how it should have been made but the makers built to what they could sell.

Mine own thought, probably many will disagree.
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Old 1st Apr 2018, 10:59 am   #34
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Hi Frank,
The whole subject of mass producing consumer goods I find fascinating. Especially the way the British did it compared with the Japanese and Americans.
So when I'm restoring a TV, I'm interested in the design decisions that were made and the corners that were cut. After all, it would be easy to design the perfect TV if the only criteria was picture quality!
My Pilot has been a very good case in point. They have gone to great efforts to get rid of any component which is not absolutely essential. It's like the design team's wages were inversely proportional to component count! It's only got 13 valves including the CRT!
I guess it was around this time (1960), that manufacturers realised that the public would buy anything if you told them it was great!! So it paid to put more effort into marketing than into design & picture quality.
I was constantly amazed at the abysmal pictures customers would be perfectly happy with!
All the best
Nick
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Old 1st Apr 2018, 11:05 am   #35
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Hi Nick,
Not TV but I think one item that comes very close to winning the title for minimum components for a working commercial result is the All American Five Radio.
You probably know about it but if not not well worth checking at the shortcuts taken.

Checking some Thorn circuits will show similarities with RCA.
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Old 1st Apr 2018, 4:46 pm   #36
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The problem with discussing the standard AA5 is that the vast majority of them deliver quite excellent reception, if not necessarily excellent sound due to the cheap speaker. The number of decoupling caps and resistors is zero.
But that does not hurt the operation. The major problem is the 35W4 rectifier and its tap for a dial lamp. Its not wise to run them without the correct (or lower resistance) dial lamp. The low B+ is harmless. The tubes (valves) are perfectly suited for their tasks (except 35W4). 12BE6, 12BA6, 12AV6, 50C5, 35W4. The loop antenna, be it a coil on the paperboard back or a ferrite loop, picks up so much signal that the AVC is always well activated. The IF response is optimal for the cheap speaker. The Q of the loop is so high that image rejection is just fine unless you live next door to a 50 kW cloud burner.
And the FCC did not assign stations so that that would cause problems unless you wanted to listen to a truly far DX station, assuming standard 455 kHz IF.
They are made of plastic in such a way that unless you take them in the bathtub with you they can't possibly electrocute you.
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Old 2nd Apr 2018, 3:48 pm   #37
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Testing the black level clamp. Well it works correctly insomuch as the clamping action is perfect, the sync tips of the negative going video are referenced to a voltage level determined by the settings of the brightness control. However, at higher brightness control settings the peak whites are clipped, the problem is caused by the cathode follower being supplied with low grid volts. I could try with dispensing with cathode follower but previous experiments proved that the operation of the sync tip clamp was impaired by the cathode current of the CRT. The cathode follower functions as a buffer between the clamp circuit and the CRT. Another solution is to move the brightness control from the cathode of the CRT and connect the slider of the control to the CR tube's grid. The sync tips will then be referenced from a fixed HT voltage divider, exactly as the Pye 368 circuit.
Nevertheless, the set is all the better for having video DC restoration so I believe it is well worthwhile continuing with the modifications.


DFWB.
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Old 2nd Apr 2018, 5:20 pm   #38
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Hi All
This D.C. restoration cct comes from a Philips 1953 data and circuits of television receiving valves book. It R20 is adjusted the black level can be made independent of the position of the contrast.
Keith
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Old 2nd Apr 2018, 8:05 pm   #39
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Success! The DC restorer is working as well as one can expect from this type of circuit. The re-wired brightness control now supplies the grid voltage to the CRT and the clamp diode reference is now a fixed voltage provided by an HT potential divider consisting of an 18Kohm and a 15Kohm resistor. The resistor values were chosen simply because they were to hand.
So besides providing a steady black level irrespective of picture content the circuit also serves as a level shifter to reduce the heater to cathode voltage applied to the CRT, less risk of h-k insulation breakdown.
Playing by the rules of only employing the technology of the times all the components used for the DC restorer are of the type available at the time the set was made, 1958. However, for those who wish not to abide by this rule the double-triode could be replaced by a silicon diode and a high voltage video transistor. A 90volt zener diode used to set the clamp reference voltage.

All these modifications can be applied to the Pilot Spacemaker and Ultra V17-70 series.

DFWB.
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Old 2nd Apr 2018, 9:57 pm   #40
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Well done!
Is it possible to provide a circuit diagram of your mod.
Would you say the picture is very much better?
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