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Old 17th Apr 2018, 12:30 pm   #1
FERNSEH
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Default Frame gated AGC modifications

An effective vision AGC system. Earlier Ultra TVs employed a frame gated AGC system. This is much easier to implement than line sync sampling. The black level is sampled during the frame blanking period which means a sample of much long duration can be obtained, and no knife edge timing required. Possibly the only disadvantage of the this system is the long time constants required to smooth the AGC voltage.
Models V16-60 and W21-60 employ this system of vision AGC. A Mazda 6D2/EB91 is employed as the APC gating pulse limiter and APC rectifier. The negative going gating pulse comes from the anode of the frame blocking oscillator.
I'll post up the circuit of this AGC system later today.
Information for the V17-60 can be found in the 1957/58 R & T servicing book.

DFWB.

Last edited by AC/HL; 24th Apr 2018 at 7:14 pm. Reason: Thread split
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Old 17th Apr 2018, 2:26 pm   #2
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Default Re: DC Restoration questions

I like the sound of the frame gated AGC. I was reading up about 'sync cancelled' (I think that's the correct term) AGC and it looked rather complicated to implement. It required line sync pulses and a feed from the LOPT.
A long time constant will be fine for this application as it only needs to respond to slow amplitude changes rather then rapid signal variations due to atmospheric conditions.
It would be nice to experiment with a circuit already used by Ultra, so will be interesting to see it.
Many thanks
Nick
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Old 17th Apr 2018, 4:59 pm   #3
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Default Re: DC Restoration questions

A slightly more complex frame gated vision AGC system is employed in the Ultra VT9-17 of 1955.
The gate pulse is derived from the anode of the frame output valve.
APC gate amplifier V8B (20L1) receives positive going video from the demodulator.
The 1956 model V17-60 employs a simplified version of this circuit.
Instead of using an APC amplifier (V8B) negative going video from the anode of video amplifier goes direct to the sampling diodes.

DFWB.
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Old 17th Apr 2018, 9:16 pm   #4
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Default Re: DC Restoration questions

The circuit of the vision AGC system in the Ultra V17-60. The negative going gate pulse comes from the anode of the frame blocking oscillator.

This is a development of the more complex circuit employed in the VT9-17 shown in the previous post.

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Old 18th Apr 2018, 6:05 pm   #5
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Default Re: DC Restoration questions

Seeing Argus25's Indian Head test pattern I was curious to see how the low frequency response horizontal lines below the circle displayed poor DC restoration. It is quite different from Test Card C's letter box.

The first image has poor corner focus but has DC response. The second image (Sony 9-90) has good overall focus but the black horizontals actually cause complete sideways displacement of the lines.

Peter
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Old 19th Apr 2018, 4:44 pm   #6
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Default Re: DC Restoration questions

Reference to my post No. 79. A negative going pulse coincident with the frame blanking period is required for the for the black level sampling.
In the Ultra V17-60 the 500uS pulse present at the anode of the frame blocking oscillator is ideal because the performance of the oscillator is not impaired by the loading of the vision AGC circuits.
The V17-70 and Pilot Spacemaker employ a multivibrator timebase. Today, I hooked up a simple differentiator consisting of a 22Kohm resistor and a 0.047mfd capacitor to the anode of the triode section of the 30PL1 frame timebase valve. The resultant negative going pulse has an amplitude of >100volts and has a sharp leading edge but unfortunately is not broad enough for sampling the black level, the spike is coincident with the broad pulses, not good, we're looking a pulse width of >250microseconds.
The ideal source for the AGC gate pulse is the flyback part of the waveform present at the anode of the frame output valve, but it's positive going so it's no use for the circuit we're considering for V17-63 and PT650.

DFWB.
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Old 20th Apr 2018, 5:39 pm   #7
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Default Re: DC Restoration questions

Hi David,
Many thanks for posting the circuit and the above comments. If another ECC82 needs to be employed to act as an inverter or anything else, that wouldn't be the end of the world. After all, I'm already fitting one for the black level clamp so two is not much extra effort!
As the set already uses a couple of solid state (I assume crystal?) diodes, if a 1N4148 or two were to find their way in there as well, I wouldn't tell anyone! After all, these are reversible mods we are doing.
Cheers
Nick
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Old 22nd Apr 2018, 11:31 am   #8
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Default Re: DC Restoration questions

"If another ECC82 needs to be employed to act as an inverter or anything else, that wouldn't be the end of the world. After all, I'm already fitting one for the black level clamp so two is not much extra effort!"
Hi Nick,
One of the triode sections of the 6/30L2 is diode connected, it can be replaced with a solid state device thus leaving the triode free for other duties such as a pulse inverter and shaper.
The difficulty is generating keying pulse for the vision AGC, easy enough if we are willing to fill the chassis up with many more valves and components but we must follow the rules of domestic TV construction, which is as it always has been "do it on the cheap".

DFWB.
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Old 23rd Apr 2018, 10:59 am   #9
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Default Re: DC Restoration questions

I've been studying the circuits for the V17-60 & VT9-17. The 17-60 has many interesting features- two sync separators being just one of them! They must have set out to design a really good chassis:- nothing like the paired down V17-70!
The VT9 AGC circuit looks a realistic possibility. The pulse feed comes from the anode of the frame o/p valve. The waveform here in the Pilot is the same as that shown for the VT9. The whole circuit only uses a triode & two diodes so is 'within budget'!
Can you see a reason why it won't work?

All the best
Nick
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Old 23rd Apr 2018, 9:07 pm   #10
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Default Re: DC Restoration questions

Hi Nick,
The V17-60 employs a blocking oscillator in the frame timebase. The sawtooth supplied to the grid of the output valve is negative going. This means that the waveform at the anode of the output valve consists of a positive going forward scan and the high amplitude flyback pulse is negative.
In the models V17-53 and V17-70 the sawtooth waveform supplied to the output valve is positive going so the inverted forward scan at the anode of the output valve is negative going and the flyback pulse is positive.
The high amplitude flyback pulse is a result of -V = di/dt.

DFWB.
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Old 23rd Apr 2018, 10:43 pm   #11
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Default Re: DC Restoration questions

Quote:
Originally Posted by FERNSEH View Post
In the models V17-53 and V17-70 the sawtooth waveform supplied to the output valve is positive going so the inverted forward scan at the anode of the output valve is negative going and the flyback pulse is positive.

DFWB.
Hi David,
I was considering using the AGC circuit from the VT9-17 (post 78). Like the V17-70 (& Pilot), it uses the positive going flyback pulse from the anode of the o/p valve.
Positive going video can be obtained from the demodulator, so as far as I can see, the most important requirements are met.
I take your point that the flyback pulse from the V17-70 is not suitable for the simplified circuit of the V17-60.

Many thanks
Nick
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Old 24th Apr 2018, 5:34 pm   #12
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Default Frame gated AGC modifications

Having settled on a black level clamp circuit for my Pilot TV, the difference in black level stability is quite remarkable.
However, doing that has shown the limitations of the mean level AGC system fitted to this set. On a bright scene the amplitude of the video waveform changes at the clamp diode and alters the black level slightly.
I could live with it but there is a possible solution I wish to explore. Fernseh has introduced me to the concept of 'frame gated AGC'. This has the potential to give a stable video amplitude irrespective of scene content.
So hopefully a workable circuit can be constructed to achieve this.

All the best
Nick
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Old 24th Apr 2018, 7:21 pm   #13
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Default Re: Frame gated AGC modifications

Posts1-11 moved from https://www.vintage-radio.net/forum/...=145083&page=2 by request.
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Old 25th Apr 2018, 10:36 am   #14
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Default Re: Frame gated AGC modifications

Hi Nick,
Somehow we have to generate the keying pulse for the vision frame gated vision AGC system. This is easy if modern semiconductors are employed but the rules must be followed which means only the components and technology of those times must be adhered to.
There is an explanation about the workings of the vision AGC system in the Ultra V17-60 in volume two of the Spreadbury book "Television Receiver Servicing".
The Ultra V17-60 is unusual in having vision AGC supplied to the 30C1 tuner mixer valve. Four stages amplifying stages are controlled by the vision AGC system, the 30L1 RF amplifier, the 30C1 mixer and the two common vision and sound IF amplifiers.
In the Ultra V9-17 only the RF amplifier and common vision and sound IF amplifier are controlled by the vision AGC.

DFWB.
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Old 25th Apr 2018, 10:57 am   #15
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Default Re: Frame gated AGC modifications

Quote:
Originally Posted by FERNSEH View Post
In the Ultra V9-17 only the RF amplifier and common vision and sound IF amplifier are controlled by the vision AGC.
Hi David,
I noticed in the VT9-17, there is also a connection from the AGC line to the grid of the line o/p valve. Is this some attempt to regulate changes in picture size caused by high beam current on bright scenes?

Last night,I built the VT9-17 circuit as it is shown in the R&TV book. It seems applicable to the Pilot as the various pulse feeds are available with the correct polarity. The frame o/p anode waveform & also the video from the detector are the right way up!
I used the triode section of a 30FL1 and 1N4148's for the diodes just for testing.
Conveniently, the circuit shows expected waveforms throughout the circuit.
Initial tests show that an increase in video signal makes the resulting AGC control voltage more negative as you would expect.
I've not had a chance to do more involved investigation yet to see if the circuit is actually working correctly.
Hopefully, I can have another play tonight!

Cheers
Nick
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Old 25th Apr 2018, 2:50 pm   #16
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Default Re: Frame gated AGC modifications

From the Spreadbury book, the redrawn circuit diagram of the frame gated vision AGC system used in the Ultra V17-60.

DFWB.
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Old 25th Apr 2018, 8:56 pm   #17
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Default Re: Frame gated AGC modifications

Quote:
Originally Posted by FERNSEH View Post

There is an explanation about the workings of the vision AGC system in the Ultra V17-60 in volume two of the Spreadbury book "Television Receiver Servicing".
That would explain why I haven't come across it:- I've only got volume 1!! I would be interested to read the circuit description though.
The V17-60 looks like a really well designed chassis. Despite all the design frills where expense was obviously not an issue, there is still no black level clamp!

Cheers
Nick
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Old 25th Apr 2018, 9:11 pm   #18
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Default Re: Frame gated AGC modifications

Hi Nick,
Old timers like me remember well how after acquiring a Bush TV161 many viewers became alarmed when the picture went blank between program breaks. They found the grey screen more reassuring.
It was difficult to convince many viewers that the down to black level condition was correct and nothing to worry about.

The Ultra V17-60 employed an interesting tuner unit, manufactured by Ultra Electric it's a device call a "sliding turret". There's a picture of it in one of the Quarrington books.

DFWB.
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Old 25th Apr 2018, 9:59 pm   #19
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Default Re: Frame gated AGC modifications

The attachment shows the chassis of the Ultra VT9-17. Note the "sliding turret" tuner.
CRT is the Mazda CRM171, no dag coating on the bulb.

DFWB.
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Old 25th Apr 2018, 11:06 pm   #20
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Default Re: Frame gated AGC modifications

The Ultra V17-53 is on the work bench. The oscilloscope is connected to the secondary of the frame output transformer. At the correct height control setting the flyback pulse is about 60 volts negative. The leading edge has a rapid rise time and the trailing edge has a slope which takes about 300microseconds to come down to zero.
I'm not sure if -60 volts is sufficient for the gate function. We have to consider that a delay circuit has to be introduced so that the black level sampling takes place outside the frame sync eight broad pulses sync sequence.

The positive going flyback pulse at the anode of the frame output valve is something like 500 volts.

DFWB.
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