Help with CRT intensity hack for Z axis

[Tom Katt]

Hello all - 

I've been a bit preoccupied with life stuff for that past few months but finally found some time to continue working on a scope-clock type display.  I located an old Leader LBO-310A scope in mint condition (even original box and accessories) that I'd like to use for my vector display.  Although it has an Intensity Modulation input, that is basically just a series capacitor to the grid that is a bit too sloppy for the timings I'd like to use.  I've looked around at other designs like David Forbes clock but I'd like to minimize bastardizing this scope too much if possible.

I recently traded some parts with a friend and obtained an OPTO-150 15KV optoisolator module...  I'm thinking that I could use this to allow a low voltage logic signal to basically tie the scope's intensity pot wiper to the minimum brightness end - in the schematic below, that would be the CRT cathode R104 100K to the left side of VR102 (which I believe is the side that would cut off the beam).  Does that sound correct?  Or do you see a better method that doesn't involve too much surgery in case I want to revert the scope to original condition.

TIA

[gregebert]

The first thing I would do is connect a pulse generator to the Z-axis jack and characterize it's operation. The amplitude affects the brightness and dimness, and the duty cycle will affect the contrast between the 2. There will be a lower frequency where the capacitor's impedance will limit the amount of intensity modulation, as well as an upper cutoff frequency. Once you get a feel for how the tube behaves, you can look into the next step. Hopefully you will find a way to use the scope as-is.

I'm not entirely sure where to connect the opto-isolator, because it will also affect the focus and intensity. When I was setting-up my scope clock kit I had to do a lot of back-and-forth between focus, intensity, and contrast.

[Tom Katt]

Thanks for the suggestion.  I did try using the existing modulation input as-is, but I think my timings are much too fast.  At least with the existing capacitor value.  I was considering experimenting with different capacitors to see if that would help when I stumbled on this fancy optoisolator - I didn't know anything like this existed (too bad it turns out to be very expensive about $90 new).

I thought about the relationship to the focus, but was hoping that might not matter since the beam will (hopefully) be fully off during blanking so any focus issues would be moot.

I don't mind adding a few components to the scope - I just don't want to inflict anything that cannot be undone.  The scope looks like it was never used.

[gregebert]

...or your amplitude is too low at the Z-axis. A google search showed -20 to -50 V is required on vintage CRT-based scopes, though I recall seeing somewhere that -10V should get noticeable results. I haven't done anything with the Z-axis since I was in high school, which was back in the 1970s.

I would make sure it works thru the Z-axis connector before tinkering with the scope's HV section.

[Tom Katt]

The only thing I was thinking about is that I'd need to find or design a -20V or so supply for the existing input, vs the plug & play optoisolator solution.

But I have a bench supply I can use to check if higher potentials offer better results.

[gregebert]

Because of the capacitor at the Z-axis input, you dont need a negative supply voltage....it's AC-coupled. If the output of your gizmo is normally around +20 to +30V, when it pulses to GND it will cause a negative signal on the scope, resulting in blanking.

[Dekatron42]

In the instruction manual for the scope they show how you can connect a pulse generator to get the blanking of a sgnal to work.

/Martin

[Tom Katt]

I will play around with that more this weekend...  I was hoping the opto would be easier because it takes the low voltage logic signal I already have rather than having to add another 30V supply rail to the project.