Home built modulation scope, and using LT Spice

[Opcom]

Things are moving along on the mod scope project. It's now worth its own topic because of all the work that goes in to the power supplies and figuring out how to get signals in and out. Rather than continue discussion about this add-on mod scope in old threads about the transmitter or threads on cutting A 5" diameter hole, high voltage gaseous regulators, or RF sampling methods, here is some info on the scope itself, that is, arranging the power supply, voltage dividers, and the like.

There is also no reason not to have a home-made mod scope topic, because they are the most important tool for AM transmission.

The most useful tool in this project has been LT Spice but some workarounds have had to be done.
1.) Apparently it does not have Zener diodes but it has voltage sources that can be used instead for this work.
2.) Its "scope" screen only seems to show 1 second's worth of time. It can take a bit longer for capacitors to charge in high voltage, high impedance circuits. That was worked around by minimizing the capacitor values in the rectifier circuits, and ignoring any hum.
3.) Potentiometers for scope functions like brightness and beam position had to be reduced to two fixed resistors. I just split the pot and went with that.

Otherwise the tool works great even calculating tiny currents through 1-meg resistors.

Attached are the .asc file from LT Spice and also a GIF of the schematic as of today.
The .asc file is in reality a text file and I renamed it to .txt to upload it, so you will have to rename it to .asc to use it with LT Spice.

Most of the voltages are pretty close to where they should be but the high voltage supply is a bit short on voltage, and there may not be enough bias to cut off the CRT. It is not that critical, but some resistor values can be changed to adjust that, or the LV supply (500V) can be changed to a doubler instead of a bridge and the HV supply can be 'stacked' on it.

The nice thing about scope tubes is they only use 200-500microamperes. The voltage dividers used with them consume more than the tube. Makes the simulations easier.

The schematic for this particular project is more complex than for most mod scopes because of the special tube being used. Later I would like to draw up a diagram with LT Spice simulations for a commonplace CRT requiring only a few voltages as well as much lower voltages. That will be more illustrative and also be a more popular project.

I am off the whole week of June 10-16. I hope to put this thing together then if I can get the peace and quiet necessary.

I already went to the hardware store and bought some aluminum angle and also to Tanner Electronics for the rectifiers and 250K-2M resistors, etc. I don't have the pots yet for adjusting V and H position but if they do not come in, I'll just use two 1M resistors in series to make a divider for the 'wiper' and 'fix' the situation as if the pot were centered for now. Those pots need to be dual, linear taper, 2 watt, 2 megohm types. A-B brand is best and pots are not the place to skimp in even a cheap modulation scope.

Then the issue of insulated mounting and insulated shaft on the brightness and focus pots, because they are at -1000 to -1400V, and this is true of most scope circuits-  the cathode is very negative so that the deflection plates can be near ground.

As one member here wisely advised, the 1/4" fiberglass shaft from a "home depot" driveway reflector stick will do and it is only $3.50. I already have that item.

[W4AMV]

LT Spice has over a dozen ZENERs in the library. If you can't find the one your looking for let me know. I may have it in my library,

Alan

After you pick a diode... scroll throw the library list past Schottky and others... Zener will begin to show.

[Rob K2CU]

LtSpice does have Zener diodes.

The Zener is the last of the symbols in the "Select Component" window.

many are available on the web.

You are seeing only one second of time in the simulation graphic window because the simulation is set to transient for a tiem of one second. The transient (as opposed to AC frequency response, or DC operating point) mode is from the spice command for doing same. it is on your schematic, just to the right of the 5400V power source, V7. It is in the form ".tran 0 1 0"

You can either edit it or use the pull down menu "Simulate" and select "Edit Command". You are presently simulating for 1 second, the top box called "Stop time" in the "Edit Command" pull down menu.  You can change this to a longer time for example, and then if you like, change the "Time to Start Saving Data" entry to other than zero.

I have a potentiometer model. It requires that you set the total value and the position of the wiper. But, in your circuit adjusting the pots seems of little consequence.

You should note that if you hold down the "Alt" key while placing the cursor over a part, you will see a thermometer and a click will give you a plot of power dissipation. That is, after you have run the simulation. You can also plot differential voltages by pressing and holding down the mouse button while on one node, Probe cursor red, then, while still holding the mouse button down, move cursor (probe) to second node and when it (probe) turns black, release button.

Anyone on this forum is welcome to any models or help I can give.

[Opcom]

Thanks! I did find them. strange they are sort of buried there but looks good!

[Opcom]

Some resistor values had to be changed once I found the Zener diodes and put them in. (as mentioned, the files have to be renamed to .txt to be attached and to .asc to be used)

Attached is the update (modscope_version2012_0606_2206_plus_crt_loads_ltspice4 - Copy.txt and a PDF as well.). The available current from the low voltage transformer (353VAC p-p, V1) is a little on the short side. The LV B+ winding is rated only 20mA so it may have come from a small o-scope.

I need a doubler to get the 500V and the charging of the caps makes for high peaks. I also tried the dual full wave doubler, from the "4x8" power supply scheme but the current didn't change. A tripler is not usable. The LTspice file for that is attached (modscope_LV_tests)

The reason for using that transformer is that it also has the 2800V 2mA (V2) secondary necessary to power the cathode side (-1400V) of the CRT. It might be necessary to try a different transformer.

Next is to figure out how to create a component so the CRT can be a drop-in.

[Rob K2CU]

Do you have specs or part number for your CRT? It might be easiest to just gin a model up. See attached for a generic symbol for the schematic. You will have to change the name to "xap1.asy".

[Opcom]

Thank you for the symbol!

The tube is from a tek 545A. the part numbers applying to it are:
154-0758
154-175-xx
T65-P2

The voltages for it were taken from the schematic. the voltage divider and the high and low regulated voltages are also from the scope. a clear chart is attached.

I do not know how much current each element of the CRT requires and Tektronix does not list it anywhere. 73uA is used in the cathode voltage divider, leaving 176uA of rectifier capacity for the CRT cathode current. - but this has not been measured off a real 545A. I don't have one.

The CRT's HV rectifiers for -1450V and +8650V in the 545 scope are all type 5642 rated 0.25mA DC maximum and are supplied by a square wave. I will be using little solid state HV rectifiers.

By keeping the same resistances and CRT circuit as in the 545A, I can hope that when the same voltages as in the 545A are supplied, the circuit will work properly.

Tektronix regulated these voltages and they were all separately generated. The theory of my circuit is that the three high voltage sources (-1350 cathode, -1450 grid, and +8650 anode) are supplied by a 2mA HV transformer, and the lower voltages for deflection and focus control, etc., are supplied by a zener+resistance voltage divider off a 20mA 600V transformer supply. The two will be connected at an appropriate place, so they properly reference each other (that is as easy as a tap on a high value divider at the output of the 2mA HV supply).