ScopeClock with teensy4.0

[jörg]

Hi, maybe a bit offtopic.

I've put some effort in using the teensy4.x to run the scope clock based on david forbes one.
The teensy3.6 based one is running on my own hardware for a while.

But because of CPU shortage and the use of the teensy3.6 internal DAC's, I was curious to get the stuff up and running on the faster teensy 4.x CPU.
In this approach I'm using a parallel driven 12 bit 4 channels DAC. The usage of a serial DAC was too slow and I did not get DMA up and running on teensy4.x.
Anyway, this works and I like it.

Video: https://youtu.be/HYwgyYnQUxw

[Paul Andrews]

That looks great

[Jon D.]

Jörg,

Do you plan to release your design and code?  I have a Teensy 4.0 I haven't used yet and would be interested in what you did.

Jon

[David Forbes]

I was able to change to a Teensy 4.1 with a 2 channel 12 bit SPI DAC in my latest version. I had to reduce the DAC rate by half. It looks just as good as the old version. Of course, the DAC I chise is already unobtanium.

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[jörg]

@Jon,
yes, I will share the code. I have moved it to the VS Code IDE with PlatformIO. I do not like the Arduino IDE much. It should run on david's hardware, too.
Do yo you have build the hardware already?

[Max Di Noi]

Hello, what transformer do you use for the power supply? I'm about to build that clock and I was thinking of winding my own transformer but if an off the shelf version can be used that's easier. Thank you. 

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[Jon Jackson]

@jörg,

No, I have not yet built anything. Just "scoping" out where this will take me.

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[Michail Wilson]

I am very interested as well.

 

Would love to have the code and pcb design and even a kit.

 

I read through the posts, but didn’t see anything other than the video.   Thumbs up on it too.   😊

 

Michail Wilson

206-920-6312

 

From: neoni...@googlegroups.com <neoni...@googlegroups.com> On Behalf Of jörg
Sent: Saturday, March 4, 2023 7:14 AM
To: neonixie-l <neoni...@googlegroups.com>
Subject: Re: [neonixie-l] ScopeClock with teensy4.0

 

@Jon,

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[Wo Tu]

Hi !

Really interesting and great work. Looks really good and its a „want to have“.

Would be interested in pcb and code too , if possible a kit too.

Thank you.

[Nicholas Stock]

Definitely not off-topic! I think everyone on this forum appreciates that post, well done. I really like the starfield effect. What tube were you using? Looks like it has very good linearity across the whole screen....

Cheers,

Nick

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[jörg]

The tube is a RFT B7 S4-01

It‘s from an EO174A Oscilloscope.

Nice green and pretty good to focus and linearity to the edges.

The tube and first version of PCB could be viewed at https://m.youtube.com/watch?v=zTyXJE2faXw

I do not plan a kit or selling like david forbes do.
First of all, it is initially his design and software. Second, I do not have the will and time to go through all the regulations EU/Germany has for selling such electronics.

I could share the PCB if I order the next  batch, but it is not finished, yet.

I plan to add ESP32 for NTP and web config.

[Paul Andrews]

Do you plan on publishing the software (on GitHub for example)?

On Mar 8, 2023, at 5:36 PM, jörg <pos...@gmail.com> wrote:



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[Richard Arndt]

Yep... put me down too for a Scope Clock kit of parts :-)  Making the H/W & S/W open source would be cool too!

I think selling PCB's & parts in kit form without enclosure, places FCC requirements on the user to not cause RF interference. 

Isn't that how SparkFun and Adafruit can sell assembled PCBs without FCC certification?

[Darrin Weiss]

I also have some interest in a kit 

[liam bartosiewicz]

The schematic and software would be good enough for me, personally 

On Mar 10, 2023, at 10:15 AM, Darrin Weiss <hip...@gmail.com> wrote:

I also have some interest in a kit 

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[SD]

David Forbes has the schematic on his website, and the firmware on Github.  I did not see that Jörg was making kits, in fact he said he didn't want to make kits, but perhaps make a PCB available, so I do not know why people are asking for kits. 

[Max DN]

True, but the schematic is SCTV rev. C which is for Teensy 3.6. Does anyone have the schematic for using Teensy 4.1? Just designed a clock based on SCTV-rev and custom transformer but of course I have now realised that Teensy 3.6 is impossible to get...

[David Forbes]

Hi. The SCTV with the Teensy 4.0 uses an LTC2632 dual DAC. This uses SPI to load the X and Y values. It's about half the update rate of the built-in DACs in the previous Teensy 3.6, so I had to change the step size in the drawing routines.

Unfortunately this chip is tiny, so it's hard to solder. 

I am in the process of redesigning the transformer and circuit board to be more friendly for kit and DIY use. I want to make the PC board with through hole parts, since people enjoy soldering those together.

I will see if I can post the latest SCTV schematic diagram to my website today. I've been traveling a lot lately. 

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[Max Di Noi]

Thanks for your prompt reply David.

I have already printed my board for a Teensy 3.6. I have no issues soldering tiny smd components assuming that I can get an LTC22632! I may design a small PCB to super-impose the Teensy 4.1 over the existing footprint and layout. In doing so, I could add the dual DAC on this new adapter board. I can see CS is connected to pin10 but where are CLK and SDI of the LTC2632 connected? I don’t see them in the code. Yes, It would be great if you could post diagram for SCTV-E for Teensy 4.1 please.

Interesting that you are redesigning the transformer. I managed to hand-wire mine and seems to work well. The 6.3V for the heater is the one that I’m not too sure how to calibrate exactly, since the SMPS… It shines a bit too bright at the moment and voltage drops to 2V, so that’s not healthy for the heater filament. I can remove a few turns but not sure what is the ‘exact’ RMS V I’m looking for given the spikes of the SMPS at 100Khz, I’ll continue to experiment on that using a 6.2V 500mA lightbulb to avoid damaging the CRT.

Thanks again for updating the schematic when you get a chance, it’ll save me so much time to experiment.

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[David Forbes]

Here is the schematic of the SCTV rev E, which uses the Teensy 4.1.

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[Adrian Godwin]

I think I have an unused teensy 3.6. 

It may take some time to track it down but I'd happily swap it for a 4.1 if that simplifies things for you.

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[Max Di Noi]

You are a legend! Ok, so simple enough now that I see it. Thank so much.

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<SCTVE2schem.pdf>

[Max Di Noi]

Oh, that’s very nice of you. That would help, I’m only making a couple of boards, one for me and one for my parents as a gift.

I’m in the UK, Adrian. Would you ship it? Please PM me for details.

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[jörg]

Attached my approach of driving the tubes filament and blanking with an of-the-shelf transformer from würth.
Hopefully still available @wurth. I've got mine as sample.
The heaters current is configurable via resistor.
The HV insulation works pretty well. No arcing, which I had on my self wound one.
I'm driving the MAX25256ATBA with 12V, which results in roughly 6.3 volts 300mA.

[Max Di Noi]

Jörg, thanks so much, now, that transformer is definitely a good find! If I had known a while ago I would have used it. By now, I have hand-wound my transformer and it has been a somewhat fun experience testing ratios, RMS, checking phase shifts etc. I noticed that even if I’m careful with the winding ratio, it’s difficult to get an exact 6.3VAC under load and the output voltage would also depend on the overall load on the primary, being the regulated secondary winding on the same core (regulated to +250VDC). So I tried several turn ratios and different wire gauges for the 600mA that I need for a 3RP1A CRT but I still cannot hit exactly 6.3VRMS+/-10%. So now I’m thinking to use a +6.3VDC power supply that I had designed on the same board. I have rectified one of the secondary voltage to about +12VDC, then I stepped it down to +6.3VDC regulated. The step-down power supply works well this way. However when I connect the ‘IsolatedGround’ from the 6.3VDC power supply to the Cathode PIN of the CRT (internally connected on this tube), the power consumption increases and the focus pot gets hot very quickly, taking the a full 600mA load. So, clearly something isn’t right in this last step, given the huge voltage differential between CATHODE and IsolatedGround

My schematic attached.

>> Any suggestions on how to connect the +6.3VDC isolated power supply to the Cathode pin of the CRT welcome. I’m sure I’m missing something obvious, but my eyes are tired and frustration starts to kick-in, so time to seek help.

>> Also, any tips on what RMS voltage I should read (with and without load) if I power the heater straight from the secondary. This works well when connected to the Cathode pin of the CRT, I can even read 6.2VRMS but a test light bulb of 6.2V 500mA got quickly very dark when receiving 5.7V under 450mA load, not good.

So, not testing it on the CRT again until I get this right. I manage to get a green dot on the CRT, which is a good sign. But I need to get the heater voltage right before I plug the CRT in again.

[Max Di Noi]

Hey, Adrian, let me knew if you find that Teensy 3.6.

I'm not having much luck with a teensy 4.1 + external DAC using SPI. Nor I can see any issues at all in the code. 

The connections are simple enough but it cannot draw a circle (as a test) on my CRT using the open source code that Cathode Corner published. Indeed, when I test the DAC output on the oscilloscope looks irregular, whereas the SPI lines seems to behave as expected. Odd. 

If I input two sine waves into the deflection amplifiers using my signal generator, then I can get a circle on the CRT, that proves that power supply, level shifters and deflection amplifiers should be fine. 

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[gregebert]

Sounds like the CRT-related stuff is fine (congratulations, BTW), and you have issues with the DAC. Have you checked the linearity of the DAC ? The best way to start is to run all DAC codes, starting from 0 and incrementing to the top, and verify the waveform is a stair-step. I suggest leaving each value for about 10usec to allow more than enough time for the DAC and amplifiers to settle. Which DAC are you using ? My only experience is with 8-bit parallel-input DACs, and I had no problems with it (ADCs, on the other hand, are a different story....). If you see erratic behavior, post the findings and we can figure out next steps.

If the DAC is operating correctly, then my first suspect is the range/offset of your DAC. You might not be able to drive it rail-to-rail due to the design of the level-shifters/deflection amps. In that case, try to narrow-down the range where the DAC is driving the CRT as-expected.

[Max DN]

Ok, I'm back. Been busy with work.

I managed to purchase a Teensy 3.6 and I have the scope clock running. For some reason, I am not able to get a crisp picture, no matter how much I try to balance focus vs astigmatism vs brightness.

Could it be that my hand-wound transformer is creating some noise? It's quite a complex transformer (at least for me). I guess I could check the voltages with the oscilloscope but I worry that I accidentally put 1200V on the oscilloscope.  I'm using the schematic published by David at Cathode Corner, I built the transformer by trial and error. For the heather I'm using a stabilised 6.4VDC using a buck-converter, which I thought should help, but no!

Any suggestions on how to stability the picture? Attached a couple of photos.

[Grahame]

Hi

Is that image of the face - showing the image offset twice in the X direction only - what you see with your eyes or is it a camera artefact?

If it is real, what happens when you rotate the tube?

It is interesting that it seems to occur in the X direction only. If you feed the X and Y data from the DACs into a scope in XY mode do you see a good image or this offset one.

Do you have any strong alternating magnetic fields near by? example soldering iron base station...

Grahame

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[Max DN]

Hi Grahame,

Those are the images that I actually see with my eyes, the camera is capturing it as I see it.

Nothing changes when I rotate the tube, only the X-Y centers seem to move, which is fine and it's easily adjusted.

I tried  shielding the CRT, no change. No other magnetic fields that I can tell other than earth.

I switched X and Y DAC output pins from the code, the issue persists (picture transposed and issue is vertical as opposed to horizontal, 3rd picture bottom right of the photo attached).

I didn't occur to me that the oscilloscope has an XY-mode. I tried but cannot tell for sure if the DAC outputs are ok, perhaps I don't know how to get a crisp picture on the oscilloscope (I'll look the manual).

I am using a switching power supply to feed the whole circuit and the Teensy is receiving a +5 from a rectified output of the same transformer that also provides +250VDC to a multiplier chain. If the issue was badly filtered power, I should see a very noisy picture, I'd assume. Instead of two sharp pictures almost overlapping. Somehow how don't seem to be able to balance focus vs astigmatism.

[Grahame]

Hi Max

To me its not a PSU problem as it is one one axis only. The deflection amplifiers are identical and swapping their X and Y inputs shows they are not the problem.

It seems to me that the problem is back in the DACs or the code, that the X DAC (originally) is drawing the image in one position and then drawing it displaced slightly in another, perhaps one refresh later.

If you try your scope in XY mode, then I find the images are not very good on digital scopes or at least the cheap ones I have access to :^( I have an old analogue CRT based scope for when the (cheap) digital one struggles.

Grahame

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[Max DN]

Thanks for your suggestions, Grahame.

I just made a new transformer (a bit of a tricky one to make given the many secondaries) and I no longer see a double picture. However I cannot manage to get a sharp and well focused picture either, no matter how I change the potentiometers.

Nothing like the very neat picture of the scope clock that I purchased from you a while back.

I'll keep on trying. The code should be fine, it's the same that David Forbes posted for the clocks he sells. I wanted to build this clock to learn about CRTs in a clock and I like the compact design.

[Grahame]

Hi

I should have asked, I had assumed you had modded the code.

It is a surprise that the transformer affected only the X axis.

With the astigmatism set midway and minimal brightness, does the focus pot produce the best focus at one end, or at some point inside the travel range?

If you connect the 4 deflection plates to the final acceleration anode, can you focus a really good spot?

Grahame

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[Max DN]

Hi,

With the previous transformer I couldn't get a single focused dot, only a small flat line. Changing with anode2 voltage would move the dot left or right too (while all 4 deflection plates where connected to A2). Very odd.

With new transformer and with the astigmatism set midway and minimal brightness, the focus produces an even focus across the travel range (expect the far ends, of course).

I have connected the 4 deflection plates to the final acceleration anode, and the spot isn't really focused. I changed the voltage on the astigmatism trimmer and I got a very focused dot. When I switch back to run the full code, the screen is blank and I can only see the picture if I increase brightness, which would then lead to a dot having a ring around it, hence I cannot get a focused picture.

I'm thinking that it may have something to do with glow delays etc. Any suggestions on best timings please? I'm using a 3RP1A CRT.

I have played around with these parameters, with only minor improvements if I increase 'glowDelay' slightly (which is odd, as David uses this same CRT with these parameters):

const int motionDelay = 15;  // how fast the beam will get there before enabling
const int settlingDelay = 8;  // let the beam finish its move before turning on
const int glowDelay   =  2;  //  usec to wait for beam to get bright before moving  [slightly better / brighter picture at 5]
const int circleSpeed = 200;   // angular step; bigger makes circles draw faster and more coarsely [slightly better picture at 150, worst/unstable at 100, not even focus/brightness across the range at 200]

const int lineStride  =  1;   // linear step; bigger makes lines draw faster and more coarsely

DAC update rate is about 1MHz on Teensy 3.6

[Grahame]

It is an odd problem. I need to think a bit more. Unfortunately I don't know David's software so I can't help with the draw factors...

Grahame

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[jörg]

Hi Max,

first of all, congrats to be that far with your project.
I think you are very close to success.

Could you please measure your voltage between the NEG1200V and GRID.
It should be, for testing, about 0V. In Blanking mode it should be about -40V.

In David's schematic it is R11.
Measurement NEG1200V and GRID, so measure across R11.

If you see a negative voltage, try to disable blanking with BLANK high.
Or, just disconnect the GRID connection from the tube and connect it with a 20k resistor to NEG1200V.
You should see something like the picture attached.

In my setup, I see your described behaviour, if I have GRID floating (not connected) or constant at negative 40V.

I mean the over-blurred image.

GRID floating or at -40V


GRID connected to NEG1200V via 20k resistor

[jörg]

Edit: GRID floating or at -40V and Brightness at full

[Max DN]

Hi Jörg, Grahame, 

Thanks. Yes, I'm very pleased of getting this far, especially since this transformer wasn't an easy one for me to hand-wind. It's just this one last step missing to get a sharp picture.

Between NEG1240 and NEG1200 I have -38V with the CRT connected or -42V with no CRT connected.

I tried connecting the GRID connected to NEG1200V via 20k resistor and I see smears as in your picture. 

If I leave the grid floating, I see a very blurred image as in your picture.

If I connect the grid to NEG1240, I see nothing on the screen.

In the code, the BlankPin is set high/low only in 4 place (DoSeg function): 

- if I set BlankPin always HIGH, I measure 0V across NEG1200 and GRID;
- if I set BlankPin always LOW, I measure 40V across NEG1200 and GRID (pic attached). As per David's schematic, I have a 10k resistor on R11.

I had thought about something going on with blanking as when the code displays a long test, I see what I have in the video attached (although the camera accentuates the effect).

[Max DN]

Another thing I don't understand is the following.

The left hand side of the display seems to be more blurry than the right hand side. I even tried to change CRT, same thing.

If I go back and use the previous transformer (hand-wound, so there will be some inconsistencies), the I do get double lines everywhere (and mode obvious on the right hand side of the screen).

I have tested again using a digital oscilloscope in XY mode and I don't see double lines, although it's difficult to tell for sure on a digital scope (Rigol DS1054Z).

Pictures attached (they are very close to what I actually see, no camera effects). 

I have tried several things with no lack over the past 3 days (wife starting to get impatience I'm spending too much time in my mini-lab, as it happens).

[jörg]

Could you please try that code:

I've got a Rigol DS1054Z too. So we can compare the scopes image.

void test_raw()

{

static int iniflg = 0;

long c;

if(iniflg == 0) {

digitalWrite(EnaPwsPin, HIGH); // hv & heater pws enable

iniflg = 1;

}

for(c=0; c!=3000; c++)

drawACircle(0, 0, 1000);

for(c=0; c!=7000; c++)

drawRadialLine(0, 2000, 1024, 0);

for(c=0; c!=7000; c++)

drawRadialLine(0, 2000, 360, 90);

}

// main()

void loop()

{

// main_loop();

test_raw();

}

[jörg]

The iniflg stuff is only to enable my HV and filament pws. You do not need it, I suppose.

[Max DN]

Hello, 

I have tested lines and circle only. They aren't indeed as neat as yours...

Pics attached.

[jörg]

Please measure with your scope the deflection board.
At base and collectors of the MPSW42's.
250V line measured with x10 switch set at the probe.
Attached some pictures of my measurement with the code from above. Hope that helps a bit.
Made with Rigol screencapture utility.

XOUT:

XOUTN:


POS250V line...
XOUT-250:


XOUTN-250:



YOUT:


YOUTN:


YOUT-250:


YOUTN-250:

[Max DN]

Hi,

Could I check please? Those measurements are GROUND-to-Base of MPSW42's and POS250V-to-collector of MPSW42's ?

[Postert]

All measured against GND.

Am 28.08.2023 um 22:36 schrieb Max DN <flata...@gmail.com>:

Hi,

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[Max DN]

Ok, for some reason I don't get a clear picture on the Rigol, maybe I have a bad ground. I'll try again.

However, I have noticed something important. As I probe (probe switch on 10x) DeflectorRight (that is just connector of Q5 on David's schematic at http://www.cathodecorner.com/sctv/sctv-schem-2.png ), I immediately get a very sharp picture (see picture attached).

I tried connecting a 10m resistor with a 27pf capacitor from DefectorRight to ground but made no difference.

Could it be a faulty MPSW42? I'll see if I can find another one. Or issues with capacitance, somehow (the wire connecting Dright to the CRT is just about 20cm long).

[Max DN]

And it even works if I unplug the oscilloscope probe from the Rigol as well as from Ground. The 10x probe is connected directly to DeflectionRight and nowhere else. Clear picture! How do I debug that, and if it cannot be fixed, could I balance by adding a 10M resistor + capacitor to simulate the probe?

[jörg]

Seems like ground loop problems. Pretty odd.
Please check the bypass caps of the op-amps. Maybe add in parallel a 10uF or so to the 100nF caps.
C31 in David's schematics is important, too.
Maybe the layout of the traces. Position of the bypass caps on the PCB.
https://sparkoslabs.com/power-supply-decoupling-capacitors/
If that will not help, you could provide your schematics and your PCB layout to get a clue.
I've attached my deflection board (Eagle). This is a standalone one. Could be self etched.

[Adrian Godwin]

I suspect a mechanical connection problem disturbed by the probe pin rather than electrical loading. 

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[Max DN]

ok, posted a longer message with more details but seems like it's gone lost. Odd. Short version here.

Tried several things, 10uF bypass caps in parallel of decoupling caps and directly on the pins of each Op Amps, made no difference. Tried a 47pF cap on the feedback resistors of the Op Amps, no change.

In the end, I put a 27pF ceramic cap between Deflector-Left and -5V, then had to add a 9pF ceramic cap between Deflector-Right and -5V (I tried caps with 1pF increments and 0.5pF error).

The picture is very good now, maybe just needs minor adjustments in the code.

Thanks all! Now I need to figure out why the power supplies needs 3.5A at 12V for the first 2 seconds at start-up, maybe a bad transformer...).

[jörg]

So you got it finally. Congratulation!
If you want some chaos on your screen. Drop me a mail, I'll send you the code.

https://www.youtube.com/watch?v=xHd83pm7ea4

[Toby Thain]

On 2023-08-30 3:30 a.m., jörg wrote:
> So you got it finally. Congratulation!
> If you want some chaos on your screen. Drop me a mail, I'll send you the
> code.
>
> https://www.youtube.com/watch?v=xHd83pm7ea4
>

It's also possible to build an analog computer for the Lorenz attractor.
I have built Horowitz' version, e.g.
https://www.chaotic-circuits.com/10-creating-lorenz-butterfly/

Video by Horowitz at https://youtu.be/DBteowmSN8g

--Toby

[Max DN]

Just to take a quick victory lap to celebrate how beautiful scope clocks can be!

And to thanks all of those who has helped me to get there, especially Jörg and Grahame!

https://youtube.com/shorts/cXWgcxxiqfA

Enjoy!

[Anders Mikkelsen]

I recently started playing with scope clocks again, after seeing David's circle graphics clock the better part of two decades ago and having had it in the back of my mind since. This was prompted by Teensy 4.1 becoming available again on the market, and the official port of the SCTV codebase for this platform. I got it up and running, and explored some aspects around the design, I thought I'd share some info with the group in case anybody else can benefit from it.

Firstly, off-the-shelf flyback transformers for low power auxiliary supplies work very well in Baxandall oscillators to make the required voltages at minimal cost and complexity, without using any custom parts. I used a Wurth 750310787, but many others are available. A buck converter can be wrapped around the circuit to get good line and load regulation, and the resulting BOM cost is less than 5 bucks. An off-the-shelf gate drive transformer can be used to drive the filament, providing the required isolation and power handling for a few bucks more, making a very simple and cheap design.

Secondly, the LT DAC in the design can be replaced by a much more affordable (and easy to solder) MCP4922, without taking a hit on the DAC update rate. A comparison of the two parts would suggest that the 20 MHz maximum SPI rate of this part would significantly impact performance, but in practice it ends up working at around 500 KSPS as well. The issue with the original part is that it relies on 24 bit SPI transfers, which are again implemented as an 8 bit and a 16 bit transfer, and the overhead from reconfiguring the SPI peripheral and waiting for the receive buffer to fill is significant. The MCP allows native 16 bit operation of the peripheral, and use of the transmit FIFO to make the transfers without any intervention from the firmware once a set of XY coordinates are passed. I also experimented with porting the code to run on an RP2040, which works well so far. I haven't assembled the RTC part yet, but everything else seems to work with minimal changes to the code. The biggest effort was figuring out how to do efficient SPI transfers, which required some raw register access, and clock tree reconfiguration for synchronous operation of the SPI peripheral (which is not the case with the Arduino default clock frequency for this board).

Thirdly, I did struggle a lot with getting a sharp image without having a lot of interaction between focus, brightness and astigmatism. Part of it came from feeding the cathode from a high impedance point in the divider, but a major factor turned out to be the source impedance of the anode drive. Most circuits I've seen drive the anode from a potentiometer fed from the deflection design. Most CRTs seem to draw an anode current that's significantly higher than the screen current, often by an order of magnitude, and this current causes the anode voltage to drift around as the brightness is adjusted. I initially used the high value of 1 Mohm for the pot, which exaggerated this issue a bit as well. In electrostatic CRTs, the final anode together with the first set of deflection plates forms a cylindrical lens, that affects both image geometry and beam focus. I looked at some oscilloscope schematics, and found some that use an emitter follower to buffer the output of the astigmatism pot to deal with this, and I'm just waiting for some parts in order to try this out in my design.

I tried it with a range of old CRTs, and most of them struggle with getting a sharp dot near the edges of the screen. I suspect this is partly due to my sagging voltages, but I'll see if the improvements address it. Does anybody know what to expect in terms of corner spot size for typical non-PDA electrostatic CRTs in the range of 30 - 100 mm screen diameter? This is at 1200 V, and with enough beam current to make the scope clock image readable in a room with normal indoor lighting.

Some pictures attached in order to compensate for all the words.

AM

[GDR GDR]

wow! while the technical details are all "greek" to me, the images look amazing. I love the "artifacts" in them. I dont know how you took these, but it looks awesome! I like it as it is! not sure how this looks like in "motion" but looks great in the stills... can you post some more? cheers

[jörg]

Pretty nice photos.
I like it too, to disable blanking and to see the painting beam.
I did some tests with the MCP4922, too, but it was too slow for me.
Mabe you've found a better way (code) driving it. If you don't mind, I'm intersted in a code snipped and which lib you have used.
I ended up in a AD5344, which works pretty well for me on the teensy40/41). So maybe I will give the 4922 a second try.
And I'm curious about your powersupply approach. Especial the +-5V and noise. I'm using a switched one, which is too noisy.
Last, I'm in the same problem, getting a sharp image an tweaking with focus, astig etc.
So, if you get good results with the emitter follower, please share your experience.
Cheers
Jörg

[Max DN]

Jörg,

This is the code that I am using with DAC 4822 (similar to 4922 but with internal ref.). I can generate some nice sinewaves with 1V or 2V amplitude with the Teensy 4.1 powered by USB data cable and output displayed on my Rigol oscilloscope.

And of course, I can draw a circle.

I think the code is as slim as can be, see below. However when I plug it in the scope clock code, It doesn't seem to work well... I need to figure it out.

//code

#include <SPI.h>

//typedef unsigned int mcp4xxx_register; // 16-bit MCP48XX/MCP49XX register for a single channel and 12-bit data
//
//const mcp4xxx_register mcp4xxx_channel_a =  0B0000000000000000; // for all MCP4XXX
//const mcp4xxx_register mcp4xxx_channel_b =  0B1000000000000000; // for MCP4XX2 only
//const mcp4xxx_register mcp4xxx_buffer_off = 0B0000000000000000; // for all MCP4XXX
//const mcp4xxx_register mcp4xxx_buffer_on  = 0B0100000000000000; // for MCP49XX only
//const mcp4xxx_register mcp4xxx_gain_two =   0B0000000000000000;
//const mcp4xxx_register mcp4xxx_gain_one =   0B0010000000000000;
//const mcp4xxx_register mcp4xxx_output_off = 0B0000000000000000;
//const mcp4xxx_register mcp4xxx_output_on  = 0B0001000000000000;
//
//
//const mcp4xxx_register mcp4822_channel_a = mcp4xxx_channel_a |
//                                           mcp4xxx_buffer_off |
//                                           mcp4xxx_gain_one |
//                                           mcp4xxx_output_on;
//
//const mcp4xxx_register mcp4822_channel_b = mcp4xxx_channel_b |
//                                           mcp4xxx_buffer_off |
//                                           mcp4xxx_gain_one |
//                                           mcp4xxx_output_on;

const unsigned int  steps = 512;

unsigned int sines_of_steps[steps]; // sine values scaled to 0 ... 4095 (2^12 - 1)

const byte          DACCSPin   = 10;

void setup()
{
   int i;

   for (i = 0; i < steps; i++)
   {
      sines_of_steps[i] = round((sin(TWO_PI / steps * i) + 1) * 2047.5);
   }
   
   pinMode(DACCSPin, OUTPUT);
   digitalWrite(DACCSPin, HIGH);

   SPI.begin();
}

void loop()
{
   unsigned int  i;
   for (;;)
   {
         for (i = 0; i < steps; i++)
         {
            SPI.beginTransaction(SPISettings(20000000, MSBFIRST, SPI_MODE2));

            digitalWrite(DACCSPin, LOW);
            SPI.transfer16(0B0011000000000000 | sines_of_steps[i]);
            // bits: DAC channel A, buffer off, gain 1x (Vout=VRef*D/4096), output on, 12-bit data (D)
            digitalWrite(DACCSPin, HIGH);
           
            digitalWrite(DACCSPin, LOW);
            SPI.transfer16(0B1011000000000000 | sines_of_steps[steps - 1 - i]);
            // bits: DAC channel B, buffer off, gain 1x (Vout=VRef*D/4096), output on, 12-bit data (D)
            digitalWrite(DACCSPin, HIGH);

           SPI.endTransaction();
         }
                   
   }
}

[Max Di Noi]

Actually, that code works well! I had a typo on a pin config. Seems to work well! Very pleased. I hope you find that snipped of code useful.

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[Anders Mikkelsen]

I know I'm a bit late in following up on this, but better late than never! I got caught up in life and other projects as usual.

I've put up some more photos and videos, plus some preliminary schematics in this google album: https://photos.app.goo.gl/dzu7QLQbDr4ameMr8

Best regards

[Anders Mikkelsen]

To give some more context around some of the design decisions, I'll do a quick summary of the power supply design process here:

A main goal was to only use off-the-shelf magnetics. The transformer was chosen to have the highest available HV winding magnetizing inductance, to limit the reactive power in the Baxandall oscillator. Flyback transformers are deliberately wound to have a low magnetizing inductance, so they are not perfect for this application, but the chosen part works pretty well.

I originally looked at using the transformer in flyback mode, but this is not ideal for running voltage multipliers as the turn-on current in the switch (from charging the multiplier stack) is only limited by transformer leakage inductance, which interacts badly with peak current mode control. There are workarounds like using non-PCMC-controllers (as done in the original SCTV design), using fixed-on-time or hysteretic controllers, or filtering the current sense signal, but I wanted to explore something different. I had worries about startup behavior and control range in a self-oscillating design like this, but it behaves very well in my testing so far.

A cheap buck pre-regulator is used to control the secondary voltage. This part has not been fully tested yet, but the board has provisions for mixed-mode feedback to stabilize it if this turns out necessary. 

All voltages are doubler-derived to improve cross-regulation, to minimize focus variation when the cathode current (brightness) is adjusted. If a single secondary is used, this means more stages are needed for the PDA and cathode voltage multipliers, since the deflection voltage is limited to 300 V in order to manage dissipation in the deflection amplifiers while maintaining acceptable slew rate. Luckily the transformer has a tapped secondary, so I can get 300 V for acceleration, and 550 V per stage in the other multipliers. 

One issue I found in my original design is that focus, brightness and astigmatism interact to a large degree. This turned out to have two main causes. One was sagging of the anode 2/4 voltage from the screen current, here I ended up buffering the astigmatism voltage with an emitter follower to make this supply stiffer while not having excessive dissipation in the potmeter. The second issue was that the cathode voltage moved around a lot relative to VG1 and focus due to the cathode being driven from a resistor chain. I originally expected that most of the cathode current would end up as beam current, but most of it actually goes to the control grid, as much as 90+ percent. This means that the cathode current was about 10 times higher than I expected. The trick here is to drive both the cathode and control grid from low impedance sources. Here I solved it with deriving the control grid voltage from a separate multiplier running off the filament supply, instead of the common divider chain that some other designs use.

Filament power is derived from another parallel transformer, I chose a gate drive transformer for its good isolation rating and low cost. I went with inductive current limiting on the secondary side for a few reasons. 1: it provides current limiting behavior during start-up, giving gentle warmup of the filament. 2: It allows easy selection of filament voltage by just changing the inductance without modifying the transfomers. 3: it allows the secondary voltage to be higher than the filament voltage, requiring less stages on the voltage multiplier in order to reach the target -Vg1 voltage. 4: It provides some tank VARs to keep the Baxandall oscillator from dropping below the minimum required Q during startup with a cold filament.

The last trick is the -12 for the deflection amplifier constant current sinks. I played with different designs for the deflection amps, and found the simple long tailed pair with a CCS to work exceedingly well for its simplicity. The cheapest way I found of making the required negative rail for this was to make a capactive divider as part of the primary tank capacitance, and rectifying the voltage across the middle cap, which works fine since it's isolated by caps on both sides. It looks a bit funny, but works very well.

[Michail Wilson]

Careful.

 

https://www.ebay.com/itm/196312588768

 

I asked…

Have you done any testing? Example with a plasma ball? Visible cracks? You willing to accept returns if they don’t work if opened and tested on video?

 

His response was:

Can you please Read? Not tested! UNTESTED! AS IS for display ONLY and Absolutely no return. For parts or not working.

 

 

I was hoping he would at least check for cracks, but instead he added text to his auction saying…

Final Sale and NO Returns. Tubes are for display ONLY in Untested condition.

 

 

Michail

[Robert]

Did anyone here buy them?

Rob

On 27 Mar 2024, at 12:37, Michail Wilson <M...@michail.com> wrote:



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[Peter Hall]

I went too $75 aussie,each...hope he has a win👍

From🚨Peter Hall😁 insidiousnixies🦘Utube

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[Paul Andrews]

Seems like this is off-topic/cross posted?

[Peter Hall]

O" I am so Sorry Andrew..forgive us Rookies..I thought we supposed too answer questions...Its what I see on most posts.Going in many directions...I've retramanded myself and will Strickly stick tooooo the Subject....Sorry Everyone,Sorry Moderator👍 Unfortunately, I am handicapped in certain things🤔

From🚨Peter Hall😁 insidiousnixies🦘Utube

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[Clarke Payne]

I dropped out at 60 each.

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