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Old 19th Nov 2012, 6:26 pm   #141
Herald1360
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Default Re: Building an oscilloscope at home

A simple schmitt trigger inverter will oscillate quite happily if output is fed back to input via a resistor and a capacitor connected between input and ground. When turned on, input is zero so output is high. This charges the input capacitor through the feedback resistor until the upper threshold of the Schmitt is passed at which point the output goes low. The low output now discharges the input capacitor via the feedback resistor until the lower threshold is reached causing the output to go high again. And so on with the input capacitor voltage cycling between the thresholds. Output from a cold start is thus a longer first pulse followed by a train of narrower pulses at about 1:1 mark space ratio.
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Old 19th Nov 2012, 7:45 pm   #142
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Default Re: Building an oscilloscope at home

I think that the cap I used is short-circuited. I simulated this circuit on Electronics Workbench and it works fine. Today I will replace the capacitor. Tell you tomorrow.
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Old 20th Nov 2012, 6:05 pm   #143
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Default Re: Building an oscilloscope at home

I solved the problem. It was the capacitor C5, it was not short-circuited, but opened. It oscillated at 50Hz (+/-) and it synchronised very well with a 60Hz signal when applied.

Sensitivity was good (approx. 2.5V AC)

I had to replace both anode resistors by 22K

Now I go for the sawtooth.
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Old 21st Nov 2012, 3:14 pm   #144
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Default Re: Building an oscilloscope at home

I'm having a problem here. I have always seen (in the document I have read) that the Miller timebase is triggered by a negative pulse applied to the supressor grid. In this case I see the triggering pulse applied to the control grid.

I would like to hear an explanation in this case.

Any help welcomed
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Old 21st Nov 2012, 8:59 pm   #145
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Default Re: Building an oscilloscope at home

Hi Miguel,

Take a look at this link;

http://www.radarpages.co.uk/theory/a...ec2ch8p138.htm

It explains how this version of the Miller run-down circuit works. The Phantastron circuit uses a special type of valve where the supressor grid g3 is wound very similarly to the control grid g1 (normally g3 has a much wider space between turns than the control grid g1) this allows switching of the electron stream from the anode to the screen grid g2.

Ross
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Old 21st Nov 2012, 9:02 pm   #146
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Default Re: Building an oscilloscope at home

Thank you very much Ross. I'm reading the page.

I found in the web that diode OA81 is a Germanium contact diode. I will replace it with a D223, D220 or D104 Silicon junction diode.

Can anybody give the characteristic of diode ZS70? Can't find a proper link.

Thanks.
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Old 21st Nov 2012, 10:29 pm   #147
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Default Re: Building an oscilloscope at home

Hi Miguel,

There is nothing special about the ZS70, any small signal silicon diode will work ok for this application.

Ross
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Old 21st Nov 2012, 11:37 pm   #148
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Default Re: Building an oscilloscope at home

Could you use the first and third grids of, say, the heptode section of an ECH81 frequency changer? This is designed as a mixer valve so G3 is more like a control grid. Even the triode grid is brought out separately from G3, so you get a spare triode to use for something else (though it's sharing a cathode with the mixer).
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Old 22nd Nov 2012, 2:14 pm   #149
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Default Re: Building an oscilloscope at home

I've got 6BW7 pentodes, thanks to my british friends . I just was a bit worry about the frequency response required for these diodes because the usual silicon rectifiers aren't fast. My main concern was with the reverse peak voltage as this circuit is operating with +315V and -600V

I was also thinking to use another 6X2P twin diode valve to replace these diodes.
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Old 22nd Nov 2012, 4:55 pm   #150
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Default Re: Building an oscilloscope at home

Quote:
I was also thinking to use another 6X2P twin diode valve to replace these diodes.
Watch out for heater cathode insulation and capacitance.
 
Old 22nd Nov 2012, 5:11 pm   #151
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Default Re: Building an oscilloscope at home

Quote:
Originally Posted by merlinmaxwell
Watch out for heater cathode insulation
I'm a bit concern about that, even with the diode used in the original circuit. It has one cathode to earth and the other to a voltage near HT+, and the heater is the same for both of them. In the datasheets it is stated the max voltage between cathode and heater is 330V for EB91 and 350V for 6X2P, so it's OK. They are signal diodes so, I guess there will be no problem with capacitances (frequency response)

I drew the schematic in a way that I can understand it better, using the symbols that I usually use. Now I understand some things.

For instance, I don't think that the -600V rail should be really necessary. I think they just used the rail that was really needed for the CRT, and that was already there, so they used it. I think that I only required a negative voltage which allows me to adjust -100V in the pentode supressor grid.
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Old 22nd Nov 2012, 6:04 pm   #152
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Default Re: Building an oscilloscope at home

Use an SMPSU diode.
I would use UF4007 a zippy fast 1N4007.
I use UF5407 if i do not recognise the part number on a TV fly-back diode. This is it's big brother.
I have never had a job bounce with one of these fitted as a replacement.
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Old 22nd Nov 2012, 6:17 pm   #153
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Default Re: Building an oscilloscope at home

Sorry Ref

There's nothing like that here.

I'm trying to use what I have at hand. I guess the UF5407 would be the fast equivalent to 1N5407, isn't it?
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Old 22nd Nov 2012, 6:35 pm   #154
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Default Re: Building an oscilloscope at home

That is exactly right.
If the lads send you another parcel and they do not have any to hand i could send a few down to be added to the package for you.
I use them as a "one fits all" fast diode.
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Old 22nd Nov 2012, 6:51 pm   #155
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Still, getting the thing going is the first and biggest step, improvments can be done later.

I wait with anticipation...
 
Old 10th Dec 2012, 3:18 pm   #156
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Default Re: Building an oscilloscope at home

This weekend I was working on the scope. I haven't finished yet but I'm on the way. You can see how it is looking by now. It do not still work but the heaters already glow

I have a doubt about the capacitor C11 (30pF). I need to know how critical is this value. I don't have a capacitor of such value and high voltage but instead I have several 47pF high voltage capacitors. Any advice regarding this issue most welcome.
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Old 10th Dec 2012, 8:19 pm   #157
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Default Re: Building an oscilloscope at home

Hi Miguel, 2, 47pF caps in series will give you 23.5pF with twice the voltage rating.

If insulated wire is twisted together it will give about 2pF per cm with the voltage rating of the wire insulation.
It will also act as a variable cap as it can be twisted tighter or cut off to vary the capacitance.

From a very quick look at the circuit I think C11 controls the width of the blanking pulse to the CRT. This is used to reduce the trace brightness while the timebase is in flyback. This is so no "return" trace is seen on the screen. If the cap is too small this effect may be seen.

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Old 11th Dec 2012, 2:56 pm   #158
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Default Re: Building an oscilloscope at home

Thanks Ed. I will try with a simple 47pF cap first. I will see what happen, and then I will see if it should be adjusted. I thought to use a bigger capacitor in series to get an equivalent capacity close to 30pF.

Now, here is the wiring i have done. The flip-flop pulses are fine and the negative peaks which should start the ramp are there too. I will make a time to follow with the tests soon.
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Old 14th Dec 2012, 3:02 pm   #159
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Default Re: Building an oscilloscope at home

I did it!!!

You can see the sawtooth waveforms that I got. I did some minor changes in the circuit that I will explain later.

You can down load a short video showing how the sawtooth amplitude varies by moving the pot RV47.

http://www.ziddu.com/download/211150...N2087.AVI.html
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Last edited by Miguel Lopez; 14th Dec 2012 at 3:28 pm.
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Old 14th Dec 2012, 4:08 pm   #160
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Default Re: Building an oscilloscope at home

hi Miguel,

Well done! you have a free-running time base, the next step is to get the time base triggering to work. You did a nice job on the wiring.

Ross
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