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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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1st Nov 2016, 11:31 am | #1 |
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Electrostatic deflection CRTs in television sets.
In this thread "FERNSEH" is working on a pre-war TV that uses electrostatic deflection. By the time I started in the TV service trade in the early 1960s all sets used magnetic deflection and the few early post war sets I saw also used magnetic deflection.
Were there any post WW2 sets made with electrostatic deflection? I am only talking about sets made for retail to the general public for domestic use and not TV kits and the like. |
1st Nov 2016, 11:46 am | #2 |
Heptode
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Re: Electrostatic deflection CRTs in television sets.
The only one I have is a Pilot TV37 of 1949 (from the USA).
Jac |
1st Nov 2016, 12:23 pm | #3 |
Octode
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Re: Electrostatic deflection CRTs in television sets.
Some American DuMont sets used electrostatic deflection. This one
http://earlytelevision.org/dumont_180.html appears to have been strongly influenced by the Cossor 137T. John |
1st Nov 2016, 1:10 pm | #4 |
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Re: Electrostatic deflection CRTs in television sets.
Post WW2 there was no UK made TVs fitted with electrostatic CRTs. In fact by 1939 all the original adherents to electrostatic deflection tubes had moved over to magnetic deflection, Cossor, GEC, RGD and Murphy.
After the war many of the valuable electrostatic CRT sets were converted to magnetic deflection, RGD supplied the chassis assemblies from the 2347T and Murphy offered a modified V114 chassis. Seems they wanted rid of all the few remaining electrostatic CRT sets. In 1936 Pye made a TV which used a strange CRT which had both magnetic and electrostatic deflection, the model 4045 I think. After WWII in the USA a 7" electorostatic CRT was fitted to many low cost "entry level" sets. DFWB. |
1st Nov 2016, 1:53 pm | #5 |
Hexode
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Re: Electrostatic deflection CRTs in television sets.
I have a rather strange American made electrostatic deflection Motorola 9T1 set which, I believe, dates from about 1948. They were quite popular over there, so I've been told, as they were relatively cheap. It's on the pile to be made to work, but its 'ticket' has a high number on it. Picture attached. I have the missing knobs and the service manual. It's a very simple set having a very low component count. One day . . .
Paul M |
1st Nov 2016, 1:58 pm | #6 |
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Re: Electrostatic deflection CRTs in television sets.
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1st Nov 2016, 8:02 pm | #7 |
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Re: Electrostatic deflection CRTs in television sets.
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2nd Nov 2016, 10:36 am | #8 |
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Re: Electrostatic deflection CRTs in television sets.
Just to qualify my query.
By the time I had anything to do with TV, magnetic deflection was firmly established as the norm for both the frame and line deflection circuits. As a consequence it is not apparent to me why, in early TV, it might have been easier to do magnet deflection for frame and electrostatic for line. |
2nd Nov 2016, 10:46 am | #9 |
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Re: Electrostatic deflection CRTs in television sets.
There was a resurrection of electrostatic CRTs post war in the DIY market for home built TVs. Practical Television brought the Argos design using a VCR97 and I know a number of other people built sets using this tube and other electrostatic ones using ex WD components.
Focus control designs went the other way in the '50s from magnetic to electrostatic. I built one using a VCR138a with was I think a two and a half inch tube. Peter |
2nd Nov 2016, 2:29 pm | #10 |
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Re: Electrostatic deflection CRTs in television sets.
More logical that magnetic line scan and electrostatic frame combination. Usually scan coils get driven via power transformers, 50Hz frame is fine, but the materials available at the time would have probably been pretty poor at 10-and-a-bit KHz.
TTFN, Jon |
2nd Nov 2016, 2:43 pm | #11 |
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Re: Electrostatic deflection CRTs in television sets.
Thanks Jon, another bit of knowledge for the memory banks.
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2nd Nov 2016, 4:09 pm | #12 |
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Re: Electrostatic deflection CRTs in television sets.
And not just 10.125c/s. The line output stage has to handle up ten times the fundamental frequency in order the sawtooth waveform is rendered without introducing distortion. As Jon rightly states an electrostatic scanning system can meet all these requirements.
The Fourier series was well known in 1936. DFWB. Last edited by FERNSEH; 2nd Nov 2016 at 4:25 pm. |
7th Nov 2016, 12:17 am | #13 |
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Re: Electrostatic deflection CRTs in television sets.
How about the GEC pre-war sets using their electrostatic CRTs (the BT8161 havibf a huge 16" tube!). Both the line scan and frame scan output stages used transformers to drive the deflection plates. Presumably, the main reason for them was to up the amplitude of the deflection waveforms without increasing the scan HT supply.
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9th Nov 2016, 10:24 pm | #14 |
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Re: Electrostatic deflection CRTs in television sets.
Hi,
The classic American electrostatic CR tube was the 7JP4, used in many brands of sets post war. There is an interesting story here also, about the cleverest horizontal deflection circuit every designed used for the 7JP4, peculiarly, this ingenious circuit is little known by television engineers and rarely discussed in textbooks. The problem with electrostatic deflection is the amount of deflection is inversely proportional to the final anode voltage, so if you double the EHT you need twice the deflection voltage to get back to the same picture size. In magnetic deflection, it is inversely proportional to the square root of the EHT, so if you double the EHT you need 1.41 the peak deflection current to get back to the same sized picture. As EHT's increased with larger tubes & brighter pictures and CRT aluminization requiring it, it would mean the linear scan deflection amplifiers would require very high (>1000V) HT supplies and special tubes. So magnetic deflection won out over electrostatic. This is also why you will hardly ever see an early aluminized electrostatic CRT. though I have one, a 5CP11A where they got around the deflection issue with a high EHT, with post deflection acceleration. At least that what we were coached to believe, however there is a circuit attached. Astonishingly this circuit runs from a 250V DC supply, only has the one triode, that acts as the line oscillator and output stage, and produces two anti-phase 450V pp sawtooth waves for the H def plates. It does this by having a blocking oscillator nested inside a 2kHz resonant circuit. The first 25 degrees or so of a sine wave is fairly linear, the blocking osc chops that up, and the voltage magnification comes from the Q of the resonant circuit. I think it originated from a genius level engineer working at Motorola in the immediate post war period. It would be a difficult proposition to do it with the frame stage though, due to the large size of the required transformer. Hugo. |
10th Nov 2016, 10:15 am | #15 |
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Re: Electrostatic deflection CRTs in television sets.
And a 6SN7 is not exactly a power triode!
Peter |
10th Nov 2016, 9:27 pm | #16 |
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Re: Electrostatic deflection CRTs in television sets.
Ingenious indeed! As Peter N remarks, the 6SN7 is not a 'power' triode.
Looking at the waveform on the cathode, I would think there would be quite a stress on the heater/cathoe insulation. I wonder if there was a special "floating" or D.C. bias arrangement for the heater supply. If I've got it right, the H/K voltage would be way above the normal recommended maximum, even if it is negative. Maybe specially selected 6SN7s were used. Tony. |
10th Nov 2016, 10:28 pm | #17 |
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Re: Electrostatic deflection CRTs in television sets.
I have a TV set with this circuit in it, an Admiral 19A11S, 1948. The original schematic was drawn in a very awkward manner obscuring the beauty of it. I'm pretty sure this circuit is also in the Motorola VT71 1948 set also using a 7JP4 crt. They never seem to give trouble in the heater cathode area, the heater runs from the same supply as all the others.
With regards to a 6sn7 for TV scanning output applications; the RCA 621TS uses a 6sn7 (one triode) as the vertical scan output amplifier ! It has more than enough steam to fully vertically deflect a 7DP4 picture tube with around a 7kV eht ! I have this set. The anode load presented to the tube by the 10:1 matching transformer and vertical yoke coils is about 6.2k. They used the other half of the 6sn7 as the blocking oscillator. Now there is economy too. Though in the 630TS set, with a 10bp4 crt they went to a triode connected 6k6 for the vertical output stage as obviously the 6sn7 wasn't up to it at that level. The power demands for electrostatic deflection are much lower than magnetic, but even lower in this unique case.The 6sn7 has no trouble at all in this application, and when it conducts, at flyback, it has no difficulty discharging the the resonant circuit tuning capacitors to give a brisk flyback. Remember, since in this combined blocking osc output application, the tube is not actually conducting most of the time either, only at flyback, its easy to forget this point, so the tube duty cycle is so low it runs as cool as a cucumber (unlike magnetic deflection). The saw-tooth waves from this circuit are almost textbook perfect and better than a Miller-Transitron where in the H scan there is the small hook at the start of scan. The circuit, in my opinion, rates as one of the most beautiful,simple,economic, effective,efficient and clever vacuum tube circuits I have ever seen. Yet as I say its hardly talked about or or even well known. Has anyone ever seen it published in a textbook? |
10th Nov 2016, 10:59 pm | #18 |
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Re: Electrostatic deflection CRTs in television sets.
So does the circuit act like an integrator? My take of the circuit is that anode current through the triode is only during the flyback period. A sawtooth waveform is developed across the 0.01microfarad capacitor and in usual blocking oscillator practise the valve is in a cut-off condition during the ramping up time.
There is a 7" CRT Motorola TV in the shop, perhaps the set uses this clever circuit? DFWB. Last edited by FERNSEH; 10th Nov 2016 at 11:06 pm. |
11th Nov 2016, 6:10 am | #19 |
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Re: Electrostatic deflection CRTs in television sets.
No,
Its not an integrator. Both the windings on the output transformer are identical and tightly coupled. It forms a sine wave resonant circuit which is roughly equivalent to a single winding with twice the value of tuning capacitance that tunes each winding. The resonant frequency is around 2kHz. The part of this sine wave around +25/-25 about zero degrees is fairly linear over the H scan time interval. For example, if say energy is injected, by the 6SN7 conducting briefly, for a single event, then coming out of conduction, the waveforms seen would merely be a decaying sine wave of the form V(t) = Vpeak.e to the -kt.sin(wt), in other words just an exponentially decaying sine wave of a shock excited resonant circuit. However, the 2kHz resonant circuit never gets to oscillate a full cycle, not even close. Due to the repeated condition of the blocking oscillator at horizontal rate, what happens is that the voltage of the decaying oscillation only rises to about 25 to 30 degrees of the 2kHz sine wave (which is substantially linear ) before the 6SN7 conducts again resetting the resonant circuit to a phase angle of about - 25 to 30 degrees. So the blocking oscillator effectively chops out about +/- 30 degrees of a sine wave with rapid resetting (flyback).This makes a beautiful sawtooth. The circuit perhaps could best be described as a "blocking oscillator chopped sine wave circuit" and the part of the sine wave chopped out is substantially the linear section. And as mentioned, the voltage magnification comes from the Q of the resonant circuit. I have only encountered one other circuit that I think is smarter than this, but it is not in the field of vintage radio or television. |
11th Nov 2016, 10:31 pm | #20 |
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Re: Electrostatic deflection CRTs in television sets.
boxdoctor, I looked at the heater-cathode voltage situation: Probably the 450v pp waveform for the most part would be around 400V with the width adjusted. Since there is a low average current in the resonant circuit's two windings with not enough winding resistance to have any significant DC component, the cathode voltage will then be about + and minus about 200V which is just in spec for the 6SN7. I'd have to put the scope on to confirm that. This probably explains why they don't fail, at least that I have seen, most of the time RCA's spec's were fairly conservative too.
(by the way a lot of modern component specs are not conservative, for example one IC manufacturer now thinks its reasonable to run a 30A current via two pins on an SOIC IC package, in the 1940's it would be a conductor at least 2mm in diameter for that current. Also, a modern 2W resistor is the same physical size as a 1/4 watt one from the 1940's, but admittedly the modern materials are more temperature resistant. But it does appear the Engineers of yesteryear were conservative in the area of maximum allowed ratings for electronic parts than they are today) |