7BP7 flying spot scanner project

[Danny]

Hi
I've been toying with the flying spot scanner idea for a while, and have started with the scanner.

My first plan was to make it all myself from scratch, but as I had a spare
Bush chassis, I converted this for the scanner.
The old PT mag did an article about the 931A multiplier tube and its insensitive to yellow, so the afterglow won't matter.
But this is hardly noticeable anyway; only from rapid bright to dark on a TV picture.

The next bit will be the photomultiplier preamp and power supply.

Has anyone got a spare anode cap for a 7BP7 tube as I think the croc clip
may start brushing on damp days?
Also has anyone got any diagrams for flying spot scanners?

I will still need a master sync genny, so any ideas would be fab.

Thanks Danny

[oldeurope]

I am sorry to tell you that a P7 will give a very bad resolution in a flying spot scanner. Maybe a blue filter helps.

Kind regards,
Darius

[ppppenguin]

Originally Posted by Danny

But this is hardly noticeable anyway, only from rapid bright to dark on a TV picture.

Unfortunately this is not quite the whole truth. If you are scanning a vertical white line the video signal must go from low to high (and back to low again) very quickly. Within 100s of nanoseconds. The main limitation here is the afterglow of the phosphor. Even in professional equipment with special CRTs there is a substantial correction for afterglow.

The BATC has published a fair bit about flying spot scanning in its magazine CQ-TV. Mainly in the earlier days fo the club. Some of this is available online, the rest is available on a CD to buy.

http://www.batc.org.uk
http://www.cq-tv.com/

I think I wrote an article in CQ-TV many years ago about using an EVR player as a flying spot slide scanner. EVR was an ill fated domestic video replay system using special film.

[Sean Williams]

Looks like fun - I still have a few tubes here........

Damn - another project!

Cheers
Sean

[Danny]

Hi
Thanks for the links, Jeffrey.
Looks interesting - there is a mention of the 5BP7 in there; it's also used in the PT articles of July 54 and Nov 54.
Thay say 3meg is possible, so I will keep this tube in at the moment.

There is also on one of the sites I visited, a suggestion of using a blue filter as Darius has also suggested as a possible help with the afterglow.

Thanks Danny

[grindrod]

I used 7BP7's for flying spot scanners, they work fine because you run them at a low beam current to minimise the spot size and at low levels the long persistance phosphor does not appear to be excited, conversly when I used a 7BP7 as a slow scan receiver you need loads of beam current to get any usable afterglow!!

[oldeurope]

Hi, for SSTV it is no problem because the spot goes slow. fh=16Hz.
But for fh=10000Hz or more the persistance is tooooo long.

Darius

[Trish]

Originally Posted by ppppenguin

I wrote an article in CQ-TV many years ago about using an EVR player as a flying spot slide scanner. EVR was an ill fated domestic video replay system using special film.

Interesting device that was. I'm not sure who developed it. Was it Technicolor or Motorola? I don't know what tube it used, but I seem to recall that it was a blue short persistence tube. Unlike normal telecine and colour slide scanners that used tubes with a P46 phosphor, which has a large green output.

Afterglow has always been a problem and required around seven different time constant correctors for each channel. Even then you could see noise patterns produced by the whole process. Normal CRT's with a white output are not really suitable for flying spot work, as the phosphor is a mix of different phosphors, each with its own decay characteristics, which makes it impossible to get rid of all the afterglows.

The photomultiplier you are using does have a peak response in the high blue range. But it will still have some output at other wavelengths. So you might have to use some optical filtering. My concern is that the CRT that you want to use has little output in the blue part of the spectrum and the signal to noise ratio isn't going to be good.

Trish

[ppppenguin]

Originally Posted by Trish

I'm not sure who developed it. Was it Technicolor or Motorola?

Motorola? http://www.labguysworld.com/Motorola_EVR.htm

Or RCA/CBS? http://www.cedmagic.com/history/cbs-evr.html

I think my EVR machines were made by another company, possibly Hitachi.

Originally Posted by Trish

Afterglow has always been a problem and required around seven different time constant correctors for each channel. Even then you could see noise patterns produced by the whole process. Normal CRT's with a white output are not really suitable for flying spot work, as the phosphor is a mix of different phosphors, each with its own decay characteristics, which makes it impossible to get rid of all the afterglows.

I've worked on a couple of different types of afterglow correctors. Because they boost high frequencies they definitely hit the signal to noise ratio.