Displaying video on oscilloscope

[aussie_bloke]

G'day all.

I have an old Dick Smith Q-1280 oscilloscope that I don't use anymore as its inferior to my modern Tektronix scope I now use but I kept it as I know I can find a good use for it and now I am wanting to turn it into a monitor for standard 4:3 video and hopefully also NBTV. I have watched some videos on YouTube on converting a scope to a video monitor and seen some schematics but it doesn't quite help me with converting this scope as I don't know where X, Y and Z inputs are, well particularly Z input. The scope uses a NEC C312P31B tube which I can't find any specification data or information on except that its equivalent is a 75AWB31 which I also can't find any data on either.

So I was wondering if anyone here can help me with a suitable schematic to convert this scope into a video monitor for standard video and if possible NBTV?

Here below are pics of my scope:

[Harry Dalek]

G'day all.

I have an old Dick Smith Q-1280 oscilloscope that I don't use anymore as its inferior to my modern Tektronix scope I now use but I kept it as I know I can find a good use for it and now I am wanting to turn it into a monitor for standard 4:3 video and hopefully also NBTV. I have watched some videos on YouTube on converting a scope to a video monitor and seen some schematics but it doesn't quite help me with converting this scope as I don't know where X, Y and Z inputs are, well particularly Z input. The scope uses a NEC C312P31B tube which I can't find any specification data or information on except that its equivalent is a 75AWB31 which I also can't find any data on either.

So I was wondering if anyone here can help me with a suitable schematic to convert this scope into a video monitor for standard video and if possible NBTV?

Here below are pics of my scope:

Hi troy looks like mine from a google ..my z connection is at the back like a antenna screw on connection it should say Z connection in the metal next to it .
The other 2 are at the front you also have a switch to by pass the scopes time base..

If your scope does not have a Z connection you would have to try spot wobble which i think is modulating the line sawtooth ,i have never done this apart from a mistake converting a scope to 625 line many years ago and doing this by a mistake which had me wondering why i was seeing a type of video with out modulating the z connection so have to ask others doing this right .

Troy just saw your picture up loads looks like not looks like my scope but your missing it !

[Steve Anderson]

Hi Troy,

The X,Y and Z inputs are references similar to those on a paper graph, the Y is the vertical axis, which is the input on the front panel. The X is the horizontal axis which is usually driven by the timebase/sweep circuit. This appears to be quite a simple instrument so doesn't appear to have an external X input. I would guess if you look at your Tek version you'll find one on the front panel. When in use the internal timebase is disconnected.

The Z axis is usually implemented as intensity modulation of the CRT, as Harry says, if it exists it's usually found on the rear panel. Most users never use it so it gets relegated there. Again have a look at your Tek.

For either 625/525 and/or NBTV you can probably use the internal timebase in external trigger mode. For the vertical timebase you'll have to build one, but they're quite simple, basically a ramp generator.

The problem arises with the Z-mod. Even if there is a connector on the rear panel, the circuit configuration usually isn't ideal for any type of TV, it will 'work' but not too well. It's intended function is primarily for blanking, especially when using the external X input...if it exists. (Both of 'em in this case).

Usually all it's just a capacitor coupling into the grid circuit of the CRT so the video would only be AC coupled, as I said, it isn't ideal, but it 'works'. Both an all-white picture and an all-black picture would show as a mid-grey. An attempt at DC restoration can be done with nothing more than adding a diode in the grid circuit...it's better...sorta.

Having a guess from the photos and the nature of this instrument I would imagine that there's something like -600 to -1.2kV on the grid/cathode circuit, so this capacitor would need to be rated for this voltage. This tends to make it expensive and perhaps they eventually omitted it from your example for that reason.

You can add said capacitor yourself...as long as you don't mind poking around in circuits with these sort of voltages. Harry may well be able to tell you the value fitted to his version, the best thing to identify the CRT grid pin is to follow the wire from the wiper of the 'brightness' front-panel pot.

But there is another method that doesn't need a Z-axis input at all. This is covered in Vol. 31 No. 4 of the NBTVA newsletter by Jeremy Jago...best source is the back-issue CD from the NBTVA. This applies to NBTV only, to use the same method on FSTV would require bandwidths beyond what a simple device like this can deal with.

Steve A.

P.S. There doesn't appear to be a front-panel brightness control, which is unusual, I can only assume it's an internal pre-set pot somewhere.
P.P.S. I too don't have nor can I find any data on those two CRT types. They're probably 'house numbers' allocated for a specific customer. Heathkit were frustratingly good at this. In their parts list...Q1 = 117-4075, no such thing ouside Heathkit, but it probably was BC107!

Much later...how embarrassing! I got the X and Y functions swapped in the first paragraph! Now corrected...but no-one said anything for quite a few days!

[Steve Anderson]

Troy,

I've just remembered this site where several small scope type CRTs have been used for (US) TV display, simple mods for 625. These are built from the ground-up, but the schematics may give you a few ideas...he's also a member here, but hasn't been seen in a while...

http://www.freewebs.com/cameramanlink/e ... ojects.htm

Steve A.