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Old 25th Mar 2018, 1:14 pm   #1
1100 man
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Default DC Restoration questions

Hi,
I understand the desirability of maintaining the DC component of the video signal at the cathode of the CRT. The various text books I have explain this in detail.
All the TV's I own or have ever seen from the 50's & 60's don't seem to use any form of DC restoration:- the video signal is coupled via a resistor & capacitor in parallel. Looking through various circuits in the R&TV books, that seems to be the standard method although some sets also have the cathode connected to a potential divider which I suppose does keep the DC level at some form of average.
Was DC restoration thought to be unnecessary or was it just cost saving? Even when a cathode follower is used to drive the tube, an RC parallel network is still used.
Some sets seem to perform much better in this respect than others. Average scene brightness seems quite consistent in some sets and all over the place in others.
Could proper DC restoration be added simply to an existing circuit?
Many thanks
Nick
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Old 25th Mar 2018, 1:33 pm   #2
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Default Re: DC Restoration questions

Hi Nick,
There's a discussion going on about this matter on the VRAT Forum:
https://www.radios-tv.co.uk/communit...k-level-clamp/
The problem with domestic television sets with series heater chains is that the heater to cathode voltage ratings of the CRT have to be considered, one of the reasons why there is no direct connection between the CRT modulating electrode and the video amplifier. A potential divider reduces the DC voltage applied the CRT and a capacitor passes the AC component, hence the DC component of the video waveform is lost.
I remember back in the sixties a "aircraft flutter" capacitor was introduced between the video amplifier and CRT, which causes the DC component to be lost.

DFWB.
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Old 25th Mar 2018, 1:43 pm   #3
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Default Re: DC Restoration questions

"Could proper DC restoration be added simply to an existing circuit?
Many thanks"

The schools version of the Pye 368 featured a DC restoration circuit consisting of an ECC82 serving as a cathode follower which supplied the CRT cathode with DC restored video. The other triode section was diode connected and worked as the sync tip DC restorer. It is always desirable to supply a DC restorer from a low impedance source.
The circuit of the modified 368 can be found in the television section of this Forum. It was a few years ago.

DFWB.
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Old 25th Mar 2018, 3:20 pm   #4
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Default Re: DC Restoration questions

RBM in the middle 60’s introduced a Black Level Clamp on their BW range, it definitely improved the contrast. Scenes with no video were black.

Now we in the trade who were selling these sets all thought this was an improvement, not so much the customers, over the years they had become accustomed to a grey screen between video and had difficulty in adjusting the set or getting used to the blank screen.

When colour arrived shortly after that had to be clamped and we got used to the technology.

Mean level AGC did not help either with varying brightness levels.
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Old 26th Mar 2018, 1:09 pm   #5
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Default Re: DC Restoration questions

It is easy to get confused between the effects of AC coupling to the picture tube and mean level AGC. In my experience the latter is by far the worst offender.

Take for example the Sony TV-110UB. This has quite a sophisticated line gated AGC system but the video amplifier is AC coupled. They still give lovely dark blacks though, if the tube is any good that is. Compare this to the TV9-90UB, which has quite a complex AGC setup but its still mean level. Here one finds the familiar grey backgrounds, lack of contrast etc.

Dual standard sets need AC coupled video to get over the biasing problems of having to deal with both positive and negative video modulation without excessive complexity, so it wasn't really the maker's fault. Some of the early single standard sets with DC coupled video and proper AGC worked really well, I'd give the Beovision 600 and the Philips G17T320 top marks in this respect.
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Old 26th Mar 2018, 11:33 pm   #6
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Default Re: DC Restoration questions

Quote:
Originally Posted by Nuvistor View Post
Now we in the trade who were selling these sets all thought this was an improvement, not so much the customers, over the years they had become accustomed to a grey screen between video and had difficulty in adjusting the set or getting used to the blank screen.
Amazing that technical improvements don't always go down well with customers! Mind you, I never really got on with CD with its total silence between tracks!! I like a bit of noise

David, despite several searches, I can't find the circuit of the modified Pye 368 which I would be interested to see. Any chance of a link?

Studio 263, I had never considered AGC before! I guess the purpose of AGC is to compensate for changes in received signal strength. Presumably, in an ideal world, it would not take any notice of changes of scene content? I assume that is the main failing with mean level AGC?
As we are running these sets from a modulator with a constant signal level, would it not be better to feed the AGC line with a constant voltage from a pot? This would surely prevent gain changes with scene content?

Looking through R&TV servicing books, Philips (being Philips!) were the only manufacturer I found who used a DC coupled video amplifier. Everyone else used either just a cap or a cap and resistor in parallel.

Many thanks for the comments,
All the best
Nick
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Old 26th Mar 2018, 11:49 pm   #7
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Default Re: DC Restoration questions

Quote:
Originally Posted by FERNSEH View Post
A potential divider reduces the DC voltage applied the CRT and a capacitor passes the AC component, hence the DC component of the video waveform is lost.
The DC component is not lost, it is just reduced.You need to inspect the resistor values in the particular set to work out by what %.

Last time I looked at the resistor values in say the Bush TV22 the DC component, as a result of the resistive divider, was reduced to 80% of what it would have been with no divider. Which is still perfectly good to maintain a reasonably stable black level and prevent retrace lines appearing in the image, assuming that the brightness and contrast controls were correctly set and the transmitted signal has a stable black level at the RF modulator and there is DC coupling from the video detector to the video output valve.

As noted, in TV sets with series heater chains, the divider was required to keep the CRT's heater-cathode voltage at a respectable level.

In many American valve sets the fashion (unlike a Bush TV22) was to AC couple from the detector to the video output and AC couple to the CRT with the DC restorer diode at the CRT grid or cathode. But sometimes they used direct coupling too.

The main type of TV's I have seen with no DC restoration or direct coupling at all, are the transistor portables (typically Japanese ones, from the early 1960's) .

With no restorer or no direct coupling to maintain the DC axis of the signal, it is an unpleasant experience watching a set as when the controls are set for a good contrast image, they are incorrect for a low light scene and if there is no internal retrace blanking the retrace line will show up at times too.
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Old 27th Mar 2018, 12:18 am   #8
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Default Re: DC Restoration questions

Hi Nick,
The Pye 368 and the earlier Group 67 series employ a high level contrast control which is connected across the video amplifier load resistor. The video is coupled from the slider of the control to the cathode of the CRT via a capacitor. Not nice, a total loss of DC component. The add-on DC restorer module put's things right. I'll draw out the circuit.
An ideal candidate for the addition of the Pye DC restorer circuit is the Ultra V17-70 series which also includes the Pilot Spacemaker models. Mazda valves are employed in the Ultra so the DC restorer valve will have to be the Mazda 6/30L2 instead of the ECC82.

I remember back in the late sixties having to convince customers that there was nothing wrong with their new Bush TV. Many preferred the grey intervals between programs and adverts. Didn't like it when the screen went blank.

DFWB.
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Old 27th Mar 2018, 12:31 am   #9
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Default Re: DC Restoration questions

Nick,

I once added a DC restorer to the CRT's cathode drive circuit of a TV set that did not have one. A 6AL5 makes an excellent restorer diode.

As noted, if you look at the circuit on about page 12 of this article:

http://worldphaco.com/uploads/Admiral_TV..pdf

There are two important tricks when you do it (These were outlined in Grob's Television book)

Firstly the drive comes ideally not directly from the video output valve's anode load choke but from the resistor near the HT, this reduces the capacitive loading on the video opt valve anode, say compared to the signal being taken directly from the anode(plate) and secondly there is a series 10k resistor that helps. Also the capacitance of the restorer diode's anode is isolated from the CRT cathode by the 47k resistor. This is the way the Americans did it and it works very well without the addition of the restorer degrading the high frequency detail in the picture.

Also if you look at the circuit you will see I threw in some retrace blanking too.

PS: A 6AL5 is easy to mount under a chassis on its socket on two hex posts.
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Old 27th Mar 2018, 1:13 am   #10
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Default Re: DC Restoration questions

Circuit diagram of what I assume the Pye 368 schools receiver DC restorer might be like.

DFWB.
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Old 27th Mar 2018, 8:28 am   #11
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Default Re: DC Restoration questions

Hi
Have a look at post 11 in Adding black level clamp to a Thorn 1500
http://www.vintage-radio.net/forum/s...ad.php?t=35563
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Old 27th Mar 2018, 9:35 am   #12
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Default Re: DC Restoration questions

Thanks for the comments & links. I'll do some reading when I've got more time tonight. The 1500 thread looks interesting!
Cheers
Nick
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Old 27th Mar 2018, 10:06 am   #13
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Default Re: DC Restoration questions

For some reason Thorn stuck to AC coupling of the video output to the CRT cathode on the 1500 as well as the smaller screen 1580 and 1590. Both the smaller screen sets used Line gated AGC.
I remember our first black and white television which had a proper black level, it was a 20" 625 single standard Pye 169 chassis. It took us quite a while to get use to a good black level after years of watching dual standard TV's with no black level. The picture on this set was far superior to most of the dual standards and the 1500's of the day.
I am a big fan of the RBM A640 chassis which used a black level correction circuit plus DC coupling from the video output to the CRT cathode. Even using mean level AGC these sets gave good pictures with good black level on both 405 and 625.
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Old 27th Mar 2018, 11:52 am   #14
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Default Re: DC Restoration questions

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.
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Old 27th Mar 2018, 12:27 pm   #15
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Default Re: DC Restoration questions

Quote:
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.
I think that would be the MS2001 series, using the single PCB with massive dropper.

It was clearly a very different design to its predecessor chassis, as Decca's TV design team seems to have been "taken over" by former Radio Rentals (Bradford) TV design staff (who were probably made redundant after Thorn took over Radio Rentals).

A very nice chassis, producing a crisp picture and good sound.
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Old 27th Mar 2018, 1:56 pm   #16
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Default Re: DC Restoration questions

There is a sequence of black level control systems in TV & video technology,

1) DC coupling from the detector to the CRT electrode (grid or cathode)

2) AC coupling with diode DC restoration on the sync tips of the video signal.

3) AC coupling with clamping on the on black level just after H sync. This was popular in many color TV designs . The clamp pulse charges or discharges the coupling capacitor feeding the crt at line rate.

4) Gated negative feedback loop or the "feedback DC Restorer"

This latter system, as it tuns out, was invented by Tektronix. When I found this out it didn't surprise me much because Tektronix were the masters of discrete component analog circuit design. (If you want to do good design in this area look to their examples)

Tek had to do well here, for their oscilloscopes.

This video black level stabilizer circuit is little known to most television engineers (I tested this hypothesis by posting it on forums and nobody recognized the design).

Few have ever heard of it including myself (except Richard Russell who knew of the principle and understood it) or even know of its existence.

This design totally surpasses all other methods to stabilize the black level voltage at the CRT grid or cathode and leaves all other methods for dead. It has excellent noise immunity.

I first discovered this circuit concept in a vintage 1987 Conrac avionics video monitor. Initially I had thought that Conrac Engineers must have designed it. But, it turned out that Tektronix Engineers thought it up.

It is a highly ingenious and creative system to stabilize the black level of a video signal presented to a CRT. It requires anti-phase pulse voltages at black level timing to switch a diode bridge in an amplifier's feedback loop. It works spectacularly well and it also irons out any AC mains ripple and other interfering signals on a line by line basis in the video signal.

The principle of it is outlined on page 25 of this article:

http://worldphaco.com/uploads/The_19...o_monitor..pdf

It also goes by the name "Keyed DC Restorer"

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Old 27th Mar 2018, 10:14 pm   #17
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Default Re: DC Restoration questions

Quote:
Originally Posted by FERNSEH View Post
The Pye 368 and the earlier Group 67 series employ a high level contrast control which is connected across the video amplifier load resistor. The video is coupled from the slider of the control to the cathode of the CRT via a capacitor. Not nice, a total loss of DC component. The add-on DC restorer module put's things right. I'll draw out the circuit.
An ideal candidate for the addition of the Pye DC restorer circuit is the Ultra V17-70 series which also includes the Pilot Spacemaker models. Mazda valves are employed in the Ultra so the DC restorer valve will have to be the Mazda 6/30L2 instead of the ECC82.
Hi David,
Yes, you guessed correctly, the Pilot Spacemaker was in my mind when I started this thread. Knowing that the cathode is fed solely via a capacitor, I wondered what the black level performance would be like? I've not had a chance to assess this yet with moving pictures as I still have EHT issues to solve first.
The various links, especially the Thorn 1500, make interesting reading so thanks to everyone who has contributed.
All the best
Nick
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Old 28th Mar 2018, 10:46 am   #18
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Default Re: DC Restoration questions

Hi Nick,
I still have the Ultra V17-53 which is electrically similar to the Pilot Spacemaker and Ultra V17-70. The cathode of the CRT is supplied though a capacitor from the slider of the high level contrast control. The results of this arrangement is a total loss of the video DC component.
Brightness control is effected by varying the cathode voltage of the CRT.
The Ultra "53" will benefit from the inclusion of the Pye 368 DC restorer circuit.

But not the circuit I submitted on post No.10. The diagram is drawn incorrectly, the DC restorer diode and cathode follower should be transposed.

DFWB.
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Old 28th Mar 2018, 9:02 pm   #19
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Default Re: DC Restoration questions

Showing the busy interior of the Ultra V17-53. The DC restorer unit can be fitted at the rear of the tuner.
Rather than a sync tip DC restorer it might be possible to employ a proper back porch black level clamp circuit. The line flyback pulse from the line output transformer is co-incident with the sync back porch.
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Old 28th Mar 2018, 9:13 pm   #20
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Default Re: DC Restoration questions

Hi David,
That's one grim looking design of chassis!! Looks like they just threw a big handful of components into the box! At least the V17-70 (and Pilot Spacemaker) is fairly neat & tidy if rather strangely designed and laid out.
Without DC restoration and with mean level AGC, does the black level noticeably change with scene content? Is it annoying?
All the best
Nick
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