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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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29th Sep 2006, 10:41 pm | #1 |
Octode
Join Date: Jan 2003
Location: Birmingham, West Midlands, UK.
Posts: 1,268
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Early Flyback EHT
I displayed my D18T at the last NVCF. After a couple of hours it was definitely not happy, being somewhat defocussed, and after five hours I switched it off as it looked very poor (a shame as it is capable of a very good picture). The fact that it was also very hot in the display area probably didn’t help too much either.
BTW, the D/B18T was not the first British set to derive EHT from the flyback – but it was the first to use the combination of flyback EHT plus a boost diode. TTFN, Jon |
30th Sep 2006, 7:55 pm | #2 | |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
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Re: Pye D18T Focus Problem
Quote:
Now that is very interesting. I always thought that the B18T was the first set to use flyback EHT [and an AC/DC circuit] as opposed to the separate R.F. system. The post war Philips using 2 HVR2s in a very dated circuit was released after the B18T. The Wireless World gives the release date for the B18T at 1948. Who made the first one? Regards John. |
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1st Oct 2006, 8:32 pm | #3 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
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Re: Pye D18T Focus Problem
Hello again Jon,
Since meeting you at Harpenden today and having a chat I realize that there are a couple of sets that predate the B18T, but use flyback EHT. K.B.'s first post war set [1947?], the CV30 and that Philips I mentioned with the voltage doubler HVR2s. I thought that set arrived after the Pye even with its very dated circuit and odd valve line up. The K.B. is a very rare set and I have never seen one. The PYE on the other hand was made in huge quantities and was Britain's first AC/DC set [I think!] Nice meeting you today, I enjoyed the whole swapmeet. Regards John. |
2nd Oct 2006, 8:49 am | #4 |
Retired Dormant Member
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Location: North London, UK.
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Re: Pye D18T Focus Problem
Flyback EHT with energy recovery was invented by Blumlein in the 1930s. It wasn't used until after WW2. Perhaps because the efficiency diode was an awkward thing needing excellent HK insulation, perhaps because the benefits were not sufficient with narrow angle CRTs.
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2nd Oct 2006, 12:47 pm | #5 | |||
Octode
Join Date: Jan 2003
Location: Birmingham, West Midlands, UK.
Posts: 1,268
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Re: Early Flyback EHT
Someone has just reminded me - the Bush TV1 pre-dates the Pye B18/D18 by some 4 months and that uses flyback EHT.
The B18T still remains a most important design as it was the first to utilise both flyback EHT and a boost diode which within a few years was used almost universally. It was also the first true AC/DC design, but I think Pye didn't necessarily consider this, the circuit was designed purely on cost grounds. Pye very soon realised that the set was capable of operation on DC mains and soon modified the HT smoothing choke so that a section of it could be bypassed for DC mains, just to squeeze those extra few vital volts through to the HT rail. Quote:
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Incidently if the LOP valve warms up quicker than the boost diode then the LOP valve's cathode bypass electrolytic gets reverse biased. TTFN, Jon |
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2nd Oct 2006, 1:10 pm | #6 | |
Retired Dormant Member
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Re: Early Flyback EHT
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It's worth remembering how much energy is being flung around when scanning a wide angle CRT. I can't lay my hands on the exact numbers but for a first generation delta gun shadowmask CRT I think the peak volt x amp product was around 3500VA. Whether you did it low voltage with transistors or high voltage with valves that's an awful lot of energy. Without energy recovery you would be dissipating over 3kW of heat to do the line scan. With energy recovery you only have to supply enough power to replace the energy lost on each stroke due to copper, iron and switching inefficiency. It's safe to say that without energy recovery, the scanning of wide angle CRTs would not have been practical. |
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2nd Oct 2006, 1:51 pm | #7 | |
Dekatron
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Location: Gateshead, Tyne and Wear, UK.
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Re: Early Flyback EHT
Quote:
The 1946 RCA 630 series of 1946 used a flyback EHT system. DFWB Last edited by Kat Manton; 2nd Oct 2006 at 2:11 pm. Reason: Fixed quote |
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2nd Oct 2006, 2:42 pm | #8 |
Rest in Peace
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Re: Early Flyback EHT
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2nd Oct 2006, 4:12 pm | #9 | ||
Octode
Join Date: Jan 2003
Location: Birmingham, West Midlands, UK.
Posts: 1,268
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Re: Early Flyback EHT
Quote:
TTFN, Jon |
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2nd Oct 2006, 10:34 pm | #10 | |
Dekatron
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Location: Gateshead, Tyne and Wear, UK.
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Re: Early Flyback EHT
Quote:
However, the EMI studio monitors did have an energy reclaim diode, actually called a "resonant energy reclaim diode" (V8). According to text in the Black Book the reclaim circuit permits the use of a less powerful line output valve. (V7). Also, there was a flyback damping diode across the scan coils (V9) DFWB. |
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3rd Oct 2006, 7:34 am | #11 |
Retired Dormant Member
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Re: Early Flyback EHT
Odd sort of arrangment, almost schizophrenic. The damping diode simply dumps stored energy into a resistor while there is also some attempt at energy recovery.
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