Wanted - Insight Into Forbes Scope Clock Blanking Circuit

[Tom Katt]

I continue to do thought experiments with the easiest to accomplish crt blanking circuit for a vector display project.  The blocking capacitor design used on many scopes is insufficient for the blanking I want to do, as they only work well for regularly repeating short pulses and my needs are more random.

Anyway, looking at how Mr. Forbes approached this for his Scope Clock it seems to me that what's going on is a low voltage (maybe 35 - 40 volts) rectifier being applied to a high voltage line, basically summing the voltages together to raise the -1200V supply to about -1240V, which is enough to provide cutoff for the crt grid.

Is there anything special going on here that I am missing, or could I take virtually any low voltage and impose that on a high voltage?  It looks like caps C7-C8 in the schematic would block the high voltage source from the transformer windings.  So even if I did not have a special multicore transformer, I could potentially add a separate small 40 VCT (or 24 VCT?) transformer to my existing supply and end up with the same idea?  Or even maybe a small DC-DC board that provides about 40VDC with a series capacitor and sum that with the higher voltage?

Suggestions welcome!  Thanks!

 

[gregebert]

C7 & C8 are for a voltage-doubler. The isolation is thru U3 (TLP2361 opto-isolator) and from what I can tell it's a blanking circuit that goes all the way down to DC, unlike the Z-axis inputs on scopes that use a capacitor and have a lower-frequency limit.

[Tom Katt]

I get the voltage doubler - but that is creating the additional 40 volts that gets summed to the -1200 line and switched by the opto.

My question concerns creating the -1240 for the opto to switch.  The transformer windings on pins 4, 5 and 6 aren't creating the HV, but only the 40 volts.  I'm trying to come up with a way to add that additional voltage needed for cutoff to my existing -1100 volt supply, which will be switched in a similar manner.

[gregebert]

You just need to be careful about the isolation, because it's 1100 volts rather than the typical 120V for a typical transformer. Good transformer manufacturers such as Triad will provide isolation info in their datasheet.

[David]

Tom,

This circuit is rather complicated for a blanking circuit, because I wanted high performance without needing an expensive 1600V film capacitor as is used in high end oscilloscopes. 

It may be possible to achieve decent performance with a voltage multiplier driven from the CRT heater supply to drive the grid switch circuit. I recently did a test of lowering the voltage from 40V to 20V with decent results. I haven’t explored all the possibilities yet, but I do want to find a way to simplify it while keeping good performance. 

David Forbes, Tucson AZ

On Aug 23, 2026, at 9:37 AM, Tom Katt <tomk...@gmail.com> wrote:

I continue to do thought experiments with the easiest to accomplish crt blanking circuit for a vector display project.  The blocking capacitor design used on many scopes is insufficient for the blanking I want to do, as they only work well for regularly repeating short pulses and my needs are more random.

Anyway, looking at how Mr. Forbes approached this for his Scope Clock it seems to me that what's going on is a low voltage (maybe 35 - 40 volts) rectifier being applied to a high voltage line, basically summing the voltages together to raise the -1200V supply to about -1240V, which is enough to provide cutoff for the crt grid.

Is there anything special going on here that I am missing, or could I take virtually any low voltage and impose that on a high voltage?  It looks like caps C7-C8 in the schematic would block the high voltage source from the transformer windings.  So even if I did not have a special multicore transformer, I could potentially add a separate small 40 VCT (or 24 VCT?) transformer to my existing supply and end up with the same idea?  Or even maybe a small DC-DC board that provides about 40VDC with a series capacitor and sum that with the higher voltage?

Suggestions welcome!  Thanks!

<sctv-schem-1.png>

 

--
You received this message because you are subscribed to the Google Groups "neonixie-l" group.
To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+...@googlegroups.com.
To view this discussion, visit https://groups.google.com/d/msgid/neonixie-l/f5dbeada-8178-4cc0-a23a-e894f93df86cn%40googlegroups.com.

<sctv-schem-1.png>

[Tom Katt]

Thanks so much for weighing in here!  I'd say 'complicated is in the eye of the beholder - personally, I see it as very clever.  The issue I am struggling with is that I have a fixed neg HV supply for the CRT and need to find some way to increase the grid sufficient for cutoff on a logic level as you have done.  I'm working towards converting an old Leader 310A scope to have a proper Z axis.  

I do have access to a rather unique HV optoisolator that would be great if I could figure a way out to apply it - I have considered using it to switch the entire neg HV rail on and off for blanking (which would be cathode, grid and A2?), but some reading suggests that fiddling around with the cathode HV supply may affect the beam focus as well.  So if I switch the entire negative supply as my blanking that might result in some visible distortion.  But maybe worth a try.

The opto I have is an MSS-OPTO-150-RevB.  Too expensive for mass production, but as I have it seems like a possible solution.

I thought about the heater supply as well since it is already there and tied to the HV section - that would be a fantastic solution!  I may play around with that as well ;-)

Thanks again!

Tom