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Old 11th Mar 2013, 4:21 pm   #201
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Default Re: Building an oscilloscope at home

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Old 11th Mar 2013, 11:04 pm   #202
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Default Re: Building an oscilloscope at home

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Too easy
Now I asked for that didn't I
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Old 4th Apr 2013, 8:14 am   #203
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Tomorrow there will be a year since I started this thread. I have finally assembled the unit.

Results are modest by now, cause I have a noise coming from the EHT PSU affecting the Y amplifier. I have not been able to supress this noise yet, still working on it. I did not supresses the flyback trace neither cause you can see the noise better in this trace than in the signal
Hi Miguel

I do hope that the work on supressing the "noise" from the EHT PSU is bearing fruit and possibly working and that some form of flyback suppresion has been worked out.

My first oscilloscope build was with valves, ECC81, EF80, and 2 of ECF80, flyback suppresion was by via a diode and a few discret components. I just cannot find the diagram for this project but think it was from a Phillips circuit.

After I got this oscilloscope working I made a case to rehouse the project but hav never gotten around to this, a transistor oscilloscope "sort of" took presidence

Best wishes
Tony
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Old 5th Apr 2013, 1:16 pm   #204
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Default Re: Building an oscilloscope at home

I have been out of this thread because I have not solved the problem of the noise yet, but I'm working on it, with the help of good friends. I think I will succeed.

On the other hand, this week I had to dismantle my bench in order to move it to a new location. I'm building a new shed in my wife's house, so I will expand my space.

I keep working on the project.
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Old 5th Apr 2013, 10:04 pm   #205
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Thanks for the update Miguel
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Old 29th Apr 2013, 7:31 pm   #206
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Default Re: Building an oscilloscope at home

Hi all

I've been a little distanced from this thread due to several reasons. I was building a shed in my wife's house (in fact the builders were building it ), I had to disassemble my bench so I slowed down electronics to the minimal. Anyway I kept working on this project whenever I could.

My bench is assembled again and I have retaken this project at 100%.

I was having problem with noise in the Y amplifier. This noise came from two sources. The EHT PSU and the HT PSU. The noise from the EHT PSU is not solved yet, but I have not work on it.

The noise coming from th HT was some kind of lobes apearing in the circuit even with no signal and huge electrolitycs across the HT. It seems that is was a faulty diode in the bridge rectifier cause I soldered 4,7nF in parallel with the diodes and the noise disappeared. Then I de-soldered this capacitors one by one to identify the noisy diode but the noise did not get back.

Have to say that I was even thinking to switch to transistors cause that noise was really driving crazy. That would have been a shame. Thanks a lot, to all the friends who helped me with supporting words and lots of advices.

I changed the Y amplifier circuit to that of the Telequipment S51. It is a better configuration (at least for me) cause its input is not floating, it is referenced to earth. That was the part that I don't like in that of the Serviscope Minor. The circuit works fine. I have enough amplification to sweep the screen of the CRT. Synchronization is good too.

I will show some pictures soon. I'm working now on the EHT PSU. I don't have too many space to fit the circuitry, and I have taken into account lots of variants, including rectification of high vltage using high voltage diodes. Anyway, I'm working on the oscillator and the ferrite transformer.

I will show pictures soon.
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Old 30th Apr 2013, 1:48 pm   #207
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Default Re: Building an oscilloscope at home

I've been simulating some circuits to get high voltage EHT to power the CRT.

To get the -1,5kV, I'm thinking to rectify 600V AC 60Hz using a bridge rectifier. As I have -600V to supply the sawtooth generator. I will add in series this voltage with that one to get around 1,4kV. That's enough for the CRT according to datasheet. See Pic. 1.

This circuit will have a ripple. See Pic. 2.

For the +4kV I want to use a voltage quadrupler from an 850V AC winding See Pic. 3. The output will be around 4kV according to simulation.

This circuit will also have a ripple. See Pic. 4.

My doubt is about the ripple.
How do both ripple could affect to electron beam?
Will a see a dotted line in the screen instead of a hard line?
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Old 30th Apr 2013, 6:26 pm   #208
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Default Re: Building an oscilloscope at home

Hi Miguel.

Ripple in the EHT will velocity-modulate the beam, and the sensitivity of the plates is inversely proportional to velocity, so you get some wiggling of your traces, but in directions radial to the centre of the screen.

If you have various capacitors to ground in the grid/cathode area you may get some intensity modulation.

David
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Old 30th Apr 2013, 6:39 pm   #209
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Default Re: Building an oscilloscope at home

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If you have various capacitors to ground in the grid/cathode area you may get some intensity modulation.
Sorry David, I didn't understand this. Could you explain more.

Thanks in advance.

See the attached picture to see the circuit which power the CRT
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Old 30th Apr 2013, 11:03 pm   #210
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Default Re: Building an oscilloscope at home

Sorry, Miguel I was at work and typed only a short explanation.

The EHT voltage sets the speed of the electrons out of the gun. Fast electrons spend less time travelling past the deflection plates, and so are deflected less by the electric field of the plates, so the sensitivity of the plates is less in millimetres of deflection per volt on the plates. When the EHT is lower, the electrons are accelerated to a lower speed by the gun, and these slower electrons spend more time passing through the deflection area, and so the angle they are deflected through is greater for the same deflection plate voltages.

This is the same thing as the size of the picture on a TV "breathing" larger and smaller as the EHT on a poor set varies with overall picture brightness.

So if you have ripple on your EHT, the tube sensitivity in both X and Y deflection varies with it, so the trace grows a wiggle towards and away from the centre of the tube.


Depending on your design of bias circuitry, the ripple on the EHT may be filtered differently on the way to the cathode, compared to the filtering on the way to the grid. This can make a ripple on the EHT give a significant ripple on the grid-cathode voltage, so the brightness of the spot varies, and this can have the effect of making moving dots appear on your traces.

In your circuit the Z drive paths put pretty much balanced capacitances onto the grid and cathode of the tube, and the resistive impedances aren't too badly off-balance, so you should be OK.

In fancy oscilloscopes with HF oscillators and ferrite transformers, it is usual to have two matched EHT windings and two EHT rectifiers. This allows the cathode and the grid to be each biased by one supply, and as the supplies are floating, an amplifier near ground potential can be used to shift one EHT supply with respect to the other, This allows DC coupling of the Z modulation, it allows DC coupled flyback blanking for very low speeds, and it allows the brightness pot to be near ground potential.

I've a bit more time now. About two hours ago I got dumped by a rather tall horse, so I fell about 2 metres doing about 25kph, and landed on the riding school floor which is rather dirty sand. My protective hat saved my head from a hard knock. I'm aching a bit, but I'm not injured. I'm about to get out of some very dirty clothes and have a nice relaxing bath. My choice of pseudonym is because I spend a lot of my time in the saddle, although I spent less time in the saddle today than I was planning to!


Cheers
David
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Old 2nd May 2013, 2:20 pm   #211
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Default Re: Building an oscilloscope at home

Oh, David, Sorry about your accident. Whish you get better soon. I'm glad to hear you are no injured.

I decided to use a bridge rectifier to get the EHT that I need.

I will use a bridge rectifier with 2,3uF (4.7 + 4.7 in series) cap for the -1,5kV voltage. It will be obtained from a center tapped winding. I will use the center tap to assure that both capcitors will be at the same potential. This will give around 800V, and added in series with the -600V that I already have will give me -1,4kV (+/-) which is enough. In the simulation I got a ripple of less than 200mV with that values.

For the +4kV, I also intend to use a bridge rectifier from another center tapped winding. Same purpose for the center tap. This time I will use around 5 or 6nF to filter the EHT. With a 30uA current (which is the current for the CRT according to the datasheet), the ripple in this PSU will be around 300mV.

I guess this levels of ripple will be negligibles. What do you think of this arrangement and ripple level?
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Old 2nd May 2013, 8:49 pm   #212
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Default Re: Building an oscilloscope at home

Thanks, Miguel,

The only damage was to my ego. I've got a few sore muscles, but that's all. It took three goes in the washing machine to get the muck out of my clothes! And yes, I did get back on.

David
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Old 3rd May 2013, 8:17 am   #213
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Default Re: Building an oscilloscope at home

Good morning Miduel, just had a good sleep and awoke thinking of valve oscilloscopes

Whilst I like the theory offered by David there is the reason as to why the "interference" is present and modulating the CRT beam in the first place. A gram of prevention is worth a ton of cure is one thing to keep in mind when designing and creating anything in the electronics field.

Here are a couple of suggestions based on my very limited knowledge of such subjects and should perhaps be used as "food for thought" and perhaps not as a recipe for a cure to the problem, I am just a dabbler.

1) Might I suggest that a Faraday cage be constructed to surround the EHT transformer and driver components including valves, transformers etc. (this was often found on older television constructions, there was a reason for it.) capacitors to damp out the ripple are good but they do not limit the cause of the problem the "eht unit" itself.

2) Perhaps A filter on the supply lines into the EHT unit possibly in the form of a torroid with windings to both conductors wound in opposite directions like in the input to most better quality switch mode power supply units? This way ripple will cancel itself out.

Hope these suggestions make some form of sense, I am not very good at explaining somthing that I know so little about. They should hopefully give thought towards a solution I believe that those EHT transformers and assosciated components radiate interference quite well.

Hope that the house move goes smoothly and that you are back in action again soon, its very stressfull moving house.

Best Regards
Tony

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Old 3rd May 2013, 4:24 pm   #214
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Default Re: Building an oscilloscope at home

Thank you all.

@David
Good to know you are OK.

@MotorBikeLes
Would you blame the horses or your speedmeter? ......

@Anthony
Advice one: Taken. The two transformers, HT and EHT, will be enclosed in a ferromagnetic cage as well as the EHT circuitry.

Advice two: Taken too. I supply the HT transformer throught a line filter from an old microwave oven. I will use another for the EHT transformer.

I'm doing some "meal electronics" right now (those that I do for living) but I will re-take the scope at 100% soon.

Bless you all
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Old 3rd May 2013, 5:37 pm   #215
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Default Re: Building an oscilloscope at home

Tony, A faraday shield is no good for screening magnetic fields. It is a special sort of screen designed to pass magnetic fields and to stop only electric fields.

Ordinary screening will be much better.

David
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Old 3rd May 2013, 10:04 pm   #216
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OK, Thanks for the explanation David.
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Old 4th May 2013, 1:33 pm   #217
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Default Re: Building an oscilloscope at home

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My choice of pseudonym is because I spend a lot of my time in the saddle
There's me thinking it was just 'cos you seem to know your radios!

Academic Wranglers are stars in their speciality.

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Old 5th May 2013, 8:33 pm   #218
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Default Re: Building an oscilloscope at home

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1) Might I suggest that a Faraday cage be constructed to surround the EHT transformer and driver components including valves, transformers etc.
I stand corrected on calling a shield a Faraday cage, showing my ignorance again. Thanks for the explanation RW

Tony
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Old 15th May 2013, 6:48 pm   #219
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Default Re: Building an oscilloscope at home

This is the circuit that I'm building for the 4kV PSU. In the simulation, it has no more than 4V ripple.
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Old 22nd May 2013, 3:13 pm   #220
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Default Re: Building an oscilloscope at home

I finally did it!!!!

The scope is working OK with the 60Hz EHT PSU. I got -1300V for cathode and +3500V for anode, on the CRT. No noise present. See pictures bellow. It istill a bit unfocussed but i guess this due to some lack of negative voltage in the focussing plates. I hope to solve that next weekend. Synchonization is perfect, and the trace is steady in the screen.
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