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Old 2nd Apr 2018, 11:09 pm   #41
FERNSEH
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Default Re: DC Restoration questions

Davewantsone wrote: "Would you say the picture is very much better?" Much better now. The picture isn't subject to those annoying black level variations, the blacks are at last truly black. Attachments show the original and modified Ultra V17-53 and V17-70 video and CRT coupling circuits.
The valve used was a Mazda 6/30L2. An ECC82 will work just as well.
The 1.8megohm resistor connected across the DC restorer diode must be present in the circuit.

DFWB.
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Old 2nd Apr 2018, 11:19 pm   #42
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Default Re: DC Restoration questions

Hi David,
It's great when somebody else does all the hard work and I can just build the end result!!
A circuit of the final version would be really useful. Also I would be interested in seeing the circuit of post 37 which didn't work as well. I would like to see why it didn't work.
Any idea why they took the brightness control to the cathode? It seemed a bit unusual. The grid just seems to be used for blanking.
Many thanks,
Nick

Ah, I see you posted it while I was typing!! Thanks

Last edited by 1100 man; 2nd Apr 2018 at 11:30 pm. Reason: more text added
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Old 2nd Apr 2018, 11:46 pm   #43
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Hi Nick,
Because the video from the slider of the high level contrast control is AC coupled to the CRT cathode it is quite in order to connect the CRT grid to a fixed potential which in the Ultra V17-70 is the direct to chassis and vary the DC bias of the cathode to effect control of brightness.
When I introduced the DC restorer into the circuit the brightness control became the reference potential but what happened when the brightness control was turned up the cathode follower triode was driven into cut-off as a result of having the high value load resistor, 10Kohms. A reduction of cathode volts increases the brightness So it followed that the DC restorer reference became a fixed potential and the slider of the brightness control is now transferred to the grid of the CRT.
In fact the V17-53 and V17-70 do have frame flyback suppression. A negative going pulse from the anode of the triode section of the timebase is supplied to the first anode of the CRT.

DFWB.
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Old 3rd Apr 2018, 12:22 am   #44
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Ah, I was getting my CRT electrodes mixed up! I had never noticed that the grid was connected to chassis and that the blanking was to G2!
I'm still trying to properly understand the action of the clamp diode. On it's anode, there will be an upside -down video waveform with positive going sync pulses. It will have no definable DC level due to the 0.22 mfd cap. Which part of the waveform gets clamped to the 85V reference and why? What is the purpose of the 1.8 meg?
Apologies for so many questions, but I do like to try and understand!!
Many thanks
Nick
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Old 3rd Apr 2018, 1:06 am   #45
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Hi Nick,
The sync pulses can be considered as positive going and the diode conducts only on that part of the negatively going video waveform so it is only the sync pulse tips that are clamped to the reference voltage.
The circuit will not work without the 1.8meg resistor, it's function is to equalise the voltage across the diode. The value isn't critical, anything between 1 to 2Megohms is OK.
The circuit can be likened to the operation of the sync separator valve, again, the sync pulse tips drive the valve into conduction and the valve remains cutoff on the negative going video by the negative bias present on the grid.

With a positive video waveform it is the negative going sync tips that perform the DC restoration function.

DFWB.

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Old 3rd Apr 2018, 1:16 am   #46
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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.
Yes it will, but if it is a diode clamp the tip of the sync will remain at the same DC level and if a black level clamp , then the black level will remain at the same DC level with contrast control adjustments.

This is the one reason why the black level clamp is superior, because regardless of the composite video signal amplitude, the black level is constant, so altering the contrast control does not upset the ideal level setting of the brightness control (making the two controls independent).

With diode sync tip clamping, the black level moves with changes in composite video signal amplitude, so if the contrast is tweaked, the brightness control requires a small correction too. But still, once both controls are set for any received TV station, they both remain correct with sync tip (diode) clamping.

The arrangement with the cathode follower wastes some heat in the load resistor. A diode clamp works nearly as well either in the grid or the cathode circuit of the CRT, but if the CRT had very high range beam currents and the cathode resistance needed to be very low then the follower would be needed.

In the CRT's cathode the impedance is lower often in the range of 100k to 220k (because the cathode voltage would be too heavily affected by beam current if the resistor is too large, so in some sets the cathode return resistance is lower) vs 1M for the grid and the coupling capacitor to the CRT therefore is often larger for cathode drive (to maintain the video's LF response) than grid coupling , making it more difficult to charge or discharge on sync tips, but with a good restorer diode it is not a problem to perform the DC restoration (as noted in the Admiral TV/link circuit I posted) directly in the CRT's cathode circuit, without requiring the cathode follower.

It is important that the overall time constant of the DC restorer allows it to change quickly enough from frame to fame without any obvious delay.

Last edited by Argus25; 3rd Apr 2018 at 1:22 am.
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Old 3rd Apr 2018, 3:05 am   #47
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Originally Posted by FERNSEH View Post
The valve used was a Mazda 6/30L2. An ECC82 will work just as well.
The 1.8megohm resistor connected across the DC restorer diode must be present in the circuit.
David,

I think there might be a problem that could occur with your circuit (right hand image post #41), now that the brightness control has moved to the grid. If the output from the cathode follower failed for any reason (say just a bad socket connection, low emission valve heater slow to warm up or goes out etc) to develop a voltage across the load resistor, the grid of the CRT would be substantially positive with respect to its cathode which should never be allowed to happen for a CRT. (One way to help prevent this is to put a low capacitance high voltage silicon diode across the CRT's grid-cathode, 3 x 1N4148 in series works as a protection device).

It is probably better to keep the CRT's grid near earth potential and the cathode circuit with the restorer components at a more positive potential as shown in the Admiral article I posted on post 9 last page of article. This way, the CRT's grid-cathode potential, at worst is zero with voltage collapse or component failure in the restorer circuit, and at other times the average cathode potential always more positive than the grid and never the other way around.

Hugo.

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Old 3rd Apr 2018, 9:07 pm   #48
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Hi Hugo,
A diode wired between the grid and cathode of the CRT will certainly provide a degree of protection to the CRT in the unlikely event of failure of the cathode follower. Under the condition no cathode volts the diode will conduct and effectively place a short circuit between the two electrodes.

Meanwhile the new circuit is performing perfectly, the modified set is now a pleasure to watch. The EHT regulation is satisfactory, this was one of my concerns as the CRT beam current will be subject greater variations now since the introduction of the black level clamp.

DFWB.
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Old 3rd Apr 2018, 9:40 pm   #49
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Hi David,
I've been looking at and comparing your circuit with Hugo's. What would be the effect of using the brightness control to supply a variable reference voltage to the clamp diode's cathode? Or would there be some undesirable effect of doing this?
I will certainly incorporate these mods into my set just as soon as I have managed to sort out it's remaining issues: namely getting more then 10kv EHT!!
Many thanks to you and Hugo for your help and explanations!
All the best
Nick.
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Old 3rd Apr 2018, 10:36 pm   #50
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"I've been looking at and comparing your circuit with Hugo's. What would be the effect of using the brightness control to supply a variable reference voltage to the clamp diode's cathode? Or would there be some undesirable effect of doing this?"
Hi Nick,
I might well change the circuit again and make the clamp voltage variable. The problem I was experiencing was at low brightness settings the cathode follower triode was going into cut-off. The simple solution is to raise the clamp level potential from 85 volts to something like 120.
If this is done the variable clamp voltage alters the brightness level and the CRT grid can be returned to chassis.
However, it might be still necessary to supply the CRT grid with a fixed voltage to compensate for the increased cathode volts.

It is possible to operate the black level clamp circuit without the cathode follower but it was found that the variations in CRT beam current were effecting the performance of the circuit. The cathode follower works as an effective buffer between the DC restorer and the CRT. Provides a low impedance drive to the CR tube.

DFWB.
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Old 4th Apr 2018, 11:08 pm   #51
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An example of a well designed TV receiver. The attachment shows an extract from the circuit diagram of the 1955 Pye V14 TV receiver. The designer has paid particular attention to preserving the video DC component from the demodulator to the cathode of the CR tube. No DC restoration circuits are required. Back porch black level sampling is employed for the vision AGC.
The gating pulses are derived from the transformer in the anode circuit of the sync separator valve. The transformer performs the differentiation function, the leading edge of the sync pulse is used in the line flywheel discriminator and the trailing edge pulses which are co-incident with the sync back porch are supplied to the vision AGC gating diodes.
This 14" set is capable of displaying superb pictures, I have the Pam version, the model 754.
Unfortunately, the line flywheel and oscillator needed three attempts by the manufacturer to get the circuits to work properly.

DFWB.
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Old 4th Apr 2018, 11:50 pm   #52
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Good grief:- I've never seen so many diodes- the whole circuit is liberally sprinkled with them!! They are everywhere: in fact, I've just counted 13 of them They wouldn't have been quite so free if they had to use valves!!
Interestingly, the cathode follower/ brightness arrangements follow your circuit meaning any failure of valve or wiring would mean huge beam current.
Must have been an early incarnation of flywheel sync and I don't doubt an expensive set capable of outstanding performance.
Did Pye have anything to do with Philips at this time? It's the sort of 'doing it differently' design you might expect from them.
All the best
Nick
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Old 4th Apr 2018, 11:57 pm   #53
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It is possible to operate the black level clamp circuit without the cathode follower but it was found that the variations in CRT beam current were effecting the performance of the circuit. The cathode follower works as an effective buffer between the DC restorer and the CRT. Provides a low impedance drive to the CR tube.
This can happen but it depends how well the diode conducts on sync tips (the particular diode) on a line by line basis, there is negligible voltage tilt, if the diode is conducting with a low forward resistance, the voltage becomes stable with changes in CRT beam current , even if the resistor across the diode is too large to be a satisfactory cathode return resistor on its own. This is why it is important to use a low on resistance diode for the job, a 6AL5 works well, it has a very small cathode-plate spacing and I think has an effective forward resistance of less than 1k (I'd have to look that up) unlike say a 6H6 which is a few k.

In normal operation, with the video signal present then, the source resistance of the divider feeding the diode's cathode (the brightness control and its associated two resistors) is the dominant resistance seen by the CRT'cathode, not the discharge resistor across the diode. Of course it is hard to beat a high voltage low capacitance silicon diode.

So you can regard the restorer diode as effectively shorting the CRT cathode to the brightness control center leg at line rate, and the voltage on the diode between sync tips being maintained by the video coupling capacitor. So its really just the source impedance of the voltage divider that sets the brightness that is the important value, unless your diode it too resistive.

If the video signal disappears and the diode stops conducting, then the only path for the CRT's cathode current is via the resistor across the diode and in this case the cathode goes a little positive as a result and pushes the beam brightness blacker than black which is not a problem.

Probably in the set I used it in the CRT's (7JP4) beam current would be lower than a bigger set, so the plate resistor (47k) could be omitted and the additional drive RC network (10k & 0.1uf) omitted, and just string in the 6AL5 diode with a parallel 470k resistor, plate connected to the cathode of the CRT & coupling cap, and 6AL5 cathode to the brightness control. A 0.22uF coupling cap is also fine.

Last edited by Argus25; 5th Apr 2018 at 12:18 am. Reason: clarify
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Old 5th Apr 2018, 12:06 am   #54
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Unfortunately, the line flywheel and oscillator needed three attempts by the manufacturer to get the circuits to work properly.
I'm really not a fan of flywheel sync:- only because it causes me so much grief getting sets to work with old video recorders!! Whilst I grudgingly admit there are some benefits with DVD, I always considered it a nasty format Video is so much more appealing!!
I guess with fringe off air signals, flywheel sync was a much better idea so I'll let them off with it!
Cheers
Nick
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Old 5th Apr 2018, 12:25 am   #55
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The 6AL5 (which I don't have) is listed equivalent to 6D2 & EB91 (which I do have) which are much more familiar numbers to me. It is listed specifically for 'DC restoration'.

Hugo, in your Admiral circuit, the clamp diode is connected to the cathode via the 47K resistor. There is also another feed via the 10K/ 0.1Mfd from the video amp anode circuit. Could you explain why you settled on this circuit arrangement? I'm interested in the pro's & con's of all these different arrangements!
Many thanks
Nick
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Old 5th Apr 2018, 2:55 am   #56
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Its a standard design recommended by RCA. The 47k isolates the anode capacitance of the 6AL5 and its socket from the video and CRT cathode (added capacitance here has the potential to degrade the high frequency response and picture detail) The 47k passes the DC potential of the restorer, which becomes equal to the brightness control voltage, when the diode conducts on positive going sync tips, to the CRT's cathode.

The 0.1uF and 10 k resistor take the video drive for the restorer plate from the HT load resistor which is a less important place for any added capacitance affecting the high frequency response, than say from the video output valve directly on its anode on the opposite side of the anode choke.Added capacitance here will have more effect on the HF picture detail.

In practice as I mentioned you could dispense with (omit) the 10k and 0.1uF coupling cap, and short out the 47k instead or perhaps just reduce it to 4.7k or similar, to help isolate the plate capacitance of the 6AL5 and it would work as well.

Make sure your circuit doesn't exceed the max heater cathode voltage of the 6AL5, if it does, you might need a small heater transformer and connect the 6AL5 cathode & heater together. Or you could just make the clamp diode from 3 or 4 1N4148 diodes in series.

Last edited by Argus25; 5th Apr 2018 at 3:03 am.
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Old 5th Apr 2018, 10:00 pm   #57
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Attachment shows the sync separator circuits of the Pye V14. The primary of the sync transformer functions as a pulse differentiator. The leading edge of the sync pulses supply the line flywheel discriminator and the trailing edge pulses perform the black level sampling for the vision AGC, or APC as Pye call it.

DFWB.
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Old 5th Apr 2018, 11:06 pm   #58
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The V14 flywheel sync system: "Must have been an early incarnation of flywheel sync and I don't doubt an expensive set capable of outstanding performance."

Hi Nick, Pye employed a similar flywheel sync system in the 1952 models FV2C and FV4C. English Electric offered a plug-in flywheel sync unit for the 1950 models 1550 and 1650.
The American 1946? RCA 630 had flywheel sync.

"Did Pye have anything to do with Philips at this time? It's the sort of 'doing it differently' design you might expect from them."
Philips acquired the Pye Group in 1966/7 and even after that date Pye retained it's product design autonomy until 1970s.
Browsing through the 1956/57 I found the description of the sync cancelled vision AGC system employed in the Philips models 1458U and 1758U.
The CRT receives it's cathode drive direct from the anode of the video amplifier, the DC component of the video waveform is maintained.
There was still a good number of these sets still in use well into the sixties, picture quality was excellent.

DFWB.
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Old 6th Apr 2018, 11:02 pm   #59
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Hi David,
Thanks for your detailed reply. I thumbed my way through several volumes of R&TV Servicing looking at the video drive arrangements. The only sets I found that addressed the black level issue were either Pye or Philips. Everything else just used a cap & resistor in parallel or just a cap.
I connected a 6D2 diode (equivalent to a 6AL5) in a similar arrangement to the one in Hugo's Admiral. Like you, I couldn't get it to clamp the sync tips unless the brightness was reduced below the CRT cut off.
So I will add a cathode follower and undoubtedly end up with the same arrangement as you! It's fun to experiment though!
Many thanks
Nick

Last edited by 1100 man; 6th Apr 2018 at 11:03 pm. Reason: Typo
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Old 6th Apr 2018, 11:33 pm   #60
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I connected a 6D2 diode (equivalent to a 6AL5) in a similar arrangement to the one in Hugo's Admiral. Like you, I couldn't get it to clamp the sync tips unless the brightness was reduced below the CRT cut off.
Can you post the exact circuit you had with component values so I can see why it didn't work ?

(The CRT brightness should not affect the function of the restorer as there should never be CRT grid current and the CRT's cathode current should pass via the restorer diode to a relatively low Z brightness control network and not significantly affect the restorer either. The voltage on the diode's anode should not be able to exceed the voltage on the brightness control supply by any significant amount due to diode conduction, and therefore nor should the voltage on sync tips exceed that value either)

It is difficult to understand why it worked in the Admiral, but not in your set, there must be a reason..

Hugo.

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