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Old 24th Apr 2018, 7:19 pm   #81
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Default Re: DC Restoration questions

Several posts moved to another thread by request: https://www.vintage-radio.net/forum/...24#post1037924
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Old 25th Apr 2018, 12:08 am   #82
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Default Re: DC Restoration questions

The problem that I saw with the cathode follower was not the follower itself, but the circuit that moved the brightness control from the cathode to the CRT grid, which is simply a less favorable place for it, though some sets had it there anyway.

In the set, if the CRT grid is grounded or at ground potential instead, the cathode can only be positive with respect to the grid (or the grid negative with respect to the cathode), except say in case of high amplitude video signals which could drive the cathode voltage lower than ground on peak white. If it does that it is usually obvious because the shades of grey in the high white levels get clipped off.

But the follower circuit, say if the follower failed or bad connection etc, the cathode of the CRT would go to ground potential and the Grid would have a positive potential from the brightness control. So the circuit modification helps with the 470k cathode resistor added helps a lot to limit this, but the CRT grid and cathode would still be conducting.

If a low capacitance protection diode is connected across the CRT grid-cathode connection, the situation remains safe and the CRT grid can never go more than about 0.7v positive with respect to the cathode which is ok. A 1N458A is a good diode for the application. Or a BAV20, two can be used in series too for an even lower capacitance and higher voltage rating.

Most of the time though , the only likely reason for the CRT grid going positive with respect the cathode is with experimental circuits. So I think if anyone is experimenting adding DC restorers to sets, best to put the diode in first. Once the experimental circuit is perfected, it can be removed with likely not any trouble after that, though if it is a low capacitance diode and it's not affecting the high frequency response if the image, it's ok to leave it there.

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Old 25th Apr 2018, 1:00 am   #83
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Default Re: DC Restoration questions

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Originally Posted by Argus25 View Post
Radio graham,

Unfortunately the polarity of a video signal drive to the CRT, either to the grid, or its cathode is opposite to what is required for DC restoration by the CRT gun itself. For example, to get the CRT grid to draw current and alter the charge on a coupling capacitor, it has to be taken positive with respect to the cathode, but the video drive at the grid has a negative going sync tip. High peak white levels might cause some grid-cathode conduction with high contrast signals and the brightness pot in some position, but that is not DC restoration, and probably not healthy for the CRT either. Also the CRT gun is a fairly resistive diode, unlike a good DC restorer diode like a silicon diode or a 6AL5. The grid of a CRT should at all times be run zero to negative with respect to the cathode anyway.

I had read that myth about the CRT doing DC restoration too, but I never believed it.
That is as I thought,I never really believed it either,The usual resistor/capacity coupling can be said to be a compromise I suppose for saving money,although I cant see that they would have saved much on the price of the set.
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Old 25th Apr 2018, 10:37 am   #84
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The usual resistor/capacity coupling can be said to be a compromise I suppose for saving money,although I cant see that they would have saved much on the price of the set.
Earlier in the thread, several people who were in the trade at the time, commented that some customers didn't like sets with proper black level!
Maybe manufactures were aware that black level clamping was not important or even a negative thing for many customers? After all, most people are unaware of the technicalities of a TV picture:- they just want to watch telly!
If you are making thousands of sets, having to fit an extra valve to deal with BLC would be significant in terms of cost.
Almost no UK sets of the time seemed to have BLC. Did American designs address the issue?

When the clamp circuit finally gets built into the TV, I will add protection diodes just in case.
I must say that having proper black level does make a world of difference to the viewed picture, so this has been a very worthwhile experiment.

All the best
Nick
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Old 25th Apr 2018, 10:50 am   #85
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Hi Nick,
The next set to receive black level correction will be my Pye V600A.
Like the Ultra V17-53 and V17-70 the chassis in the Pye V600A is one that was made on the cheap. The sound quality is poor so that fault has to be addressed as well.

DFWB.
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Old 25th Apr 2018, 12:04 pm   #86
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1100man,

Yes, They seemed to be very thorough with black level clamping in American TV's sets. The topic & the importance of black level clamps, sync tip clamps or DC restoration was well covered in the standard American training textbooks for TV, both immediate post war and in the early 1950's, which were those by Fink and Grob. Also most of their domestic TV sets had either direct coupling from the detector via video amps to the CRT or separate DC restorers.

Though despite that I still saw the occasional set without it. But most of the sets that I noticed it missing on were early transistor sets, not valve sets. But in this part of the world there were practically no vintage 405 line sets, I only saw one or two sets that people had brought here from the UK and then couldn't get to work.

The whole "black level clamp" issue though cropped up in the Broadcast TV industry. This was because quite a few of the cheap monochrome TV monitors from Japan didn't have it. But these monitors were an inexpensive option for some video production houses to buy. That is how I learned how to modify monitors for black level clamping. I did quite a few for some production houses in Auckland back in the 1980's by adding black level clamp circuitry to them. One of the production houses prepared a test video recording to test the black level clamp. It consisted of two broad white bars that varied from black to peak white slowly, on a grey (just above black) background. It was obvious if the clamp wasn't working as when the white bars got more intense the grey level would darken.
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Old 26th Apr 2018, 1:29 pm   #87
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Reference to post No. 79. Tried out the beam current limiter today. It's effective and will protect the CRT in the event of failure of the cathode follower circuit.
When the brightness control is advanced too far the voltage developed across the CRT cathode resistor will rise thus causing the diode to cease conducting.
The video signal will then pass through the capacitor which will result in the loss of the DC component.
The resistor can be reduced to 220Kohms and will still provide a degree of protection for the CRT.

DFWB.
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Old 5th May 2018, 6:44 pm   #88
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The designers of many Alba TV sets paid due attention to maintaining the DC component right up to the cathode of the CRT.
Notable models such as the popular T655 did not have vision AGC and had instead two preset gain controls, one for Band 1 and the other for Band 3. Alba considered this better than mean level vision AGC. The T655 always displayed excellent pictures.
The 14" portable model T909 of 1957 had a proper line pulse gated vision AGC system.
I haven't worked out fully how the vision AGC works, is it a back porch gating system or of the sync cancelled type?
Alba's first 110* CRT models T717 and T721 also have this type of vision AGC.

I have the service manuals for these sets but there is no description in them to how the vision AGC system works.
The gating pulse is derived from the line timebase multivibrator. We have to establish if these pulses are positive or negative going and coincident with the line sync or the back porch.

DFWB.
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Old 5th May 2018, 11:30 pm   #89
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Default Re: DC Restoration questions

That’s a tough one to figure out.

The right-hand triode of the ECC83 looks like a conventional AGC grid-controlled shunt rectifier. That is, it is rectifying line-frequency pulses received from the line timebase. The magnitude of the DC bias produced at its anode is controlled by the combination of the grid bias and the cathode bias. The latter is determined by the setting of the contrast control. The grid bias is determined by the video signal applied to it, effectively DC-coupled from the demodulator diode, and so varies with the video signal. But it is its value at the time of conduction, that is when the line frequency pulse arrives at the anode, which counts, and presumably that corresponds with the back porch, that is black level. Insofar as the grid-controlled rectifier provides DC gain, it is also a gated amplifier.

The left-hand triode is the mystery. It is in grounded grid mode, and it looks as if it is also receiving line frequency-pulses at its anode, via capacitor coupling from the anode of the right-hand triode. Its grid is obviously at zero volts, and its cathode is coupled, DC-coupled I’d imagine, to the cathode of the common IF stage, which one should expect to be the first IF stage. Unless the first IF stage is only partially decoupled (a technique used to reduce input capacity variation and so input tank circuit detuning with changing AGC bias), then the cathode of the left-hand triode is in receipt of a positive DC bias that varies according to the conduction of the first IF valve, and that in turn is affected by the AGC bias applied to it.

The left-hand triode thus looks like another grid-controlled rectifier, whose output is controlled by its varying cathode bias. I’d guess that the DC output from its anode is used as RF amplifier AGC bias. As the main AGC bias reduces the first IF stage gain, its current decreases and so its cathode bias becomes less positive. This causes the left-hand triode to conduct more and so its DC output becomes more negative, i.e. increases in magnitude. Presumably the details are arranged to provide the appropriate delay for RF AGC as compared with IF AGC.

To confirm this, we’d need to see where the DC output from the left-hand triode is going. But the fact that the whole of the ECC83 is labelled as a gated AGC amplifier on the schematic, confirms, or at least does not rebut the notion that both halves have this function.


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Old 6th May 2018, 12:13 am   #90
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Hi Synchrodyne,
Your assumptions about where the cathode & anode of the left hand triode go are correct. The cathode is connected, via a potential divider, to the cathode of the 1st common IF amp. The anode is connected via R18 to the tuner's RF amp grid.
I'll leave it to you and David to figure out how it actually works!!

All the best
Nick
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Old 6th May 2018, 12:41 am   #91
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Default Re: DC Restoration questions

Quote:
Originally Posted by FERNSEH View Post
The designers of many Alba TV sets paid due attention to maintaining the DC component right up to the cathode of the CRT.
Notable models such as the popular T655 did not have vision AGC and had instead two preset gain controls, one for Band 1 and the other for Band 3. Alba considered this better than mean level vision AGC. The T655 always displayed excellent pictures.
Hi David,
Now that is really interesting!! I've just dug out the relevant R&TV book and had a look. Such a simple circuit for the video amp DC coupled to the cathode of the CRT. Why did almost all other manufacturers insist on using a cap & resistor (or just a cap) to couple to the cathode?

I also love the simplicity of the preset gain controls which is sort of where I was going before heading off down the rather tortuous road of frame gated AGC. I decided this latter approach was needed as when the AGC was fed from a fixed voltage, the contrast gradually increased as the set warmed up.
However, having done much experimenting with the AGC circuits over the last couple of weeks, this problem gradually got worse to the point where the video would sometimes go negative!
It turned out that the 30F5 vision IF amp was incredibly low gain and kept altering over the course of an hour. On top of that, tapping the 30FL1 Video amp made the picture gradually increase in contrast until it went negative! Replacing both these valves has made things much more stable.

I see in the Alba, they get the negative bias for the first IF and tuner from the line output valve grid- a very neat solution.
So all that being said, I'm now tempted to try and utilise some of the Alba's design in my Pilot.

Many thanks
Nick
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Old 6th May 2018, 1:47 am   #92
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Your assumptions about where the cathode & anode of the left hand triode go are correct. The cathode is connected, via a potential divider, to the cathode of the 1st common IF amp. The anode is connected via R18 to the tuner's RF amp grid.
I'll leave it to you and David to figure out how it actually works!!
Thanks for that. I'd expect that the left-hand triode is biassed off until the first IF amplifier current has been reduced quite a bit by the main AGC bias, thus providing an adequate RF AGC delay.

Using separate AGC rectifiers for the RF and IF AGCs might be unusual. But if you need to add an extra valve envelope anyway, then the second triode is there for the taking. With a single AGC rectifier, the second triode might have been diode-strapped to provide the RF AGC delay.

An ECC83 seems unusual in the TV AGC role; I think that the ECC82 was more common.

Cheers,
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Old 6th May 2018, 12:12 pm   #93
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The redrawn circuit diagram of the vision AGC system employed in the Alba T717 and T909 might clarify matters on how the circuit works.
I'm sure it is the leading edge negative going transition of the differentiated line sync that triggers the line oscillator. Because of diode action of the grid and cathode of the triode section of the ECL80 the positive going transitions will have no effect because these will be passed to ground.
So it is most likely the gating pulse at the anode of the ECL80 triode is positive going and coincident with the sync pulses, not the back porch, so can we assume this is a sync cancelled vision AGC system?

DFWB.
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Old 8th May 2018, 1:39 am   #94
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Now I have seen more of the Alba circuit, I am not sure how to classify it.

With sync-cancelled AGC, I’d expect to see summation of negative-going video (i.e. sync pulses upwards) and negative going separated sync. The sum signal would have blanking level as its highest level, and would be followed by a peak rectifier to generate a DC output proportional to this level. With DC coupling from the vision demodulator through the summing circuit to the peak rectifier, the sync tip level is fixed, and the blanking level goes more negative as the video signal increases in amplitude.

This Mullard circuit is an example:

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Here the video and separated sync are summed at the grid of the left-hand triode. This triode is a cathode follower configured as an infinite impedance detector. Its DC output is fed to the cathode of the right-hand triode, which is configured as a grid-controlled shunt rectifier fed by line flyback pulses. The latter are not used for timing purposes in this case, but simply as the energy input to the grid-controlled rectifier which thus acts as a DC amplifier, to provide higher AGC loop gain.

In the Alba case, the right-hand triode is receiving negative-going video DC-coupled from the demodulator diode. This would appear as positive-going video at the anode. The triode anode is also in receipt of (amplified) differentiated line sync pulses from the ECL80 triode anode. These would need to be positive-going to cancel the sync pulses on the video signal, which means that they were negative-going at the ECL80 triode grid. So far, so good. But I think that the differentiated line sync pulses from the ECL80 triode would need to be broad enough to cancel the whole of the video signal sync pulses, given that the objective is cancellation not sampling. I am not sure that that would be the case with differentiated pulses. Also, they would need to coincide with sync pulses.

An alternative postulate is that in going through the ECL80 triode and associated circuitry, the amplified differentiated line sync pulses are sufficiently delayed to coincide with the back porch, and so the right-hand triode is acting as a conventional AGC grid-controlled shunt rectifier. Its pulse feed would probably be lower in amplitude than the case where line flyback pulses are used, so it would have lower gain. But modest gain may have been adequate in this case. The controlled IF valve is an EF80, which probably has a grid base of 4 to 5 volts. An AGC voltage of -5 V at maximum magnitude would probably be enough, whereas for an EF85, around -15 V, even more, might be desirable. I’d guess that the RF valve would be either a PCC84 or PCC88 with divider-type bias for the upper triode, and so also with a similarly short grid base.

Which is the correct interpretation I don’t know.


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Old 10th May 2018, 10:16 am   #95
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Refer to the attachment in the previous post. The 1960 range of Ekco TVs employed the Mullard sync cancelled vision AGC system.
The attached circuit diagram is from the Ekco T368 service manual.
A similar circuit is used in the Ekco T344, the companies' first TV set to employ a 110* CRT.

DFWB.
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Old 13th May 2018, 10:54 pm   #96
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Default Re: DC Restoration questions

Mullard also developed a sync-cancelled AGC system for negative modulation systems:

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This differed in that whereas in the positive modulation version, the two triodes were cathode-coupled, in the negative modulation case, the cathode of the first triode was coupled to the grid of the second triode.

Mullard noted that this version also provided for noise-gating, in that noise pulses would go through the sync separator and so cancel the corresponding noise pulses on the original signal.

Of course, that would not be the case where a noise-gated sync separator was used, and such were normal for negative modulation receivers by the time the Mullard circuit was developed, c.1957-58. Also appearing around that time were noise-gated AGC systems, often noise- and line-gated.

Nevertheless, given that the positive and negative modulation versions of the sync-cancelled AGC system were very similar, one might expect that a dual-standard version could have been derived with relatively simple switching.

But instead, Mullard’s offering for the UK dual-standard era was a combination system that was mean level on 405 (positive), and line-gated black level on 625 (negative):

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On 625 it was effectively a standard negative modulation single valve AGC circuit, in which the valve undertook the gating and gain functions. On 405 the AGC valve provided gain only, with integrated video signal providing a mean level input at its cathode. On 405 it was actually an amplified mean level AGC system. Doing it this way appeared to save half a valve as compared with having black level AGC on 405 also.


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Old 6th Jun 2018, 9:49 am   #97
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Quote:
Originally Posted by FERNSEH View Post
The designers of many Alba TV sets paid due attention to maintaining the DC component right up to the cathode of the CRT.
Notable models such as the popular T655 did not have vision AGC and had instead two preset gain controls, one for Band 1 and the other for Band 3. Alba considered this better than mean level vision AGC. The T655 always displayed excellent pictures.
It would seem I have been looking at this problem from the wrong end!! The focus of this thread has been to build a sync tip clamp to maintain a constant DC level at the tube's cathode. This has worked really well and the black level performance of my Pilot TV is now really good.
However, rather than restore the DC level after it's been lost by capacitive coupling, surely it's better to not loose it in the first place!

Having studied the circuit of the Alba T655 (thanks Fernseh!) I was amazed by the simplicity of their approach.
The output of the vision detector is DC coupled to the grid of the video amp. The anode of the video amp is directly connected to the tube's cathode with a 10K anode load resistor to HT.
Thus DC level is maintained throughout. The brightness control is connected to the grid.

It was a simple matter to modify the pilot's circuitry to that of the Alba. All that was required was to short out the coupling cap between the detector and the video amp grid and the same for the cap coupling the video amp to the CRT cathode. The brightness control was moved to the grid.

So simple and it works perfectly and again raises the question of why didn't all manufacturers do this? It actually uses less components!
Maybe I'm missing something...?

Cheers
Nick
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Old 6th Jun 2018, 12:19 pm   #98
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Hi Nick,
In 1961 Alba introduced two new 19inch CRT 405 line receivers which employed a simplified version of the vision AGC circuit that was employed in the T717 and T909. Only one triode valve is used in the new circuit but like the vision AGC circuit in the T717 a positive going pulse at line rate is required for the gating function.

DFWB.
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Old 10th Jun 2018, 11:43 am   #99
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The circuit of the vision AGC system employed in the 1961 Alba models T866 and T877.
A simplification of the AGC circuits in the T717 and T909.

DFWB.
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Old 10th Jun 2018, 1:02 pm   #100
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[QUOTE=1100 man;1049558]
Quote:
Originally Posted by FERNSEH View Post

However, rather than restore the DC level after it's been lost by capacitive coupling, surely it's better to not loose it in the first place!

.....again raises the question of why didn't all manufacturers do this? It actually uses less components!
Maybe I'm missing something...?

Cheers
Nick
Nick,

This question is very well answered in Grob's book on Basic Television ( I have recommended this on the forum many times, it has a whole chapter on DC restoration issues) although the answers need to be flipped around a little when one compares the American video modulation where sync increases the carrier level to the 405 line system where syncs reduce the carrier level.

Firstly assume in the TV set that the brightness & contrast controls are previously set are about correct and initially assume American polarity video modulation:

Direct Coupled vs DC restorer:

In the direct coupled case the brightness control sets the CRT bias close to zero or full brightness (without a video signal) so, with no signal or no received station, the raster is visible (carrier level goes low so picture goes white).

On the other hand with a DC restorer or "DC re-insertion" acting on the sync tips, the brightness control biases the CRT to beyond cut-off with no signal and the raster blacks out between received stations.

This is the fundamental reason I think why many American sets used direct coupling from the detector (apart from the parts savings) the raster was visible with no station tuned in, rather than being blacked out making it look like the TV was off or not working and harder to find & tune weak stations.

However, when the modulation scheme is reversed to this, as in the 405 line transmission system and the setup is direct coupled (with no loss of the DC component) when there is no video (off station) the raster is not visible (without manipulating the brightness control). And in this case, the effect matches that pretty closely of the DC restorer.

In other words the DC restorer effect is the same regardless of what video modulation polarity is used. But the effect of direct coupling from the detector, in the signal vs no signal condition, depends on the modulation polarity.

The above applies regardless of whether the final drive is to the CRT's grid, or its cathode, its just that the applied signal in each case has opposite polarity.

So in early English TV sets there was nothing much to choose between the direct coupled and DC restorer system, except perhaps two things:

In the direct coupled case, especially to the CRT cathode, the connection of course was to the plate circuit of the video output tube. If the current in that tube failed or slow to warm up the CRT's cathode voltage could go very high and compromise the heater cathode insulation, which is why the DC component is often partially divided down, like it is , say in a TV22.

The restorer requires an extra valve and resistors & caps, which is more costly. So this was a factor. But if the CRT got damaged its more costly.

Then of course there is the cheap & safe option with poorer performance; capacitively couple it and don't bother with direct coupling or DC restorers, as many manufacturers chose to do.

Hugo.

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