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Old 23rd May 2013, 12:08 pm   #221
Aub
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Default Re: Building an oscilloscope at home

Well done Miguel!

I'm not sure I would have had the perceverance to do it.

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Old 3rd Jun 2013, 1:54 pm   #222
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Default Re: Building an oscilloscope at home

This is the oscillogram of a TTL signal. The upper level shows a peak that is not present in the original signal. Any idea on how to solve this ?

Thanks
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Old 3rd Jun 2013, 2:26 pm   #223
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Default Re: Building an oscilloscope at home

I've noticed that the earthing for measuring TTL signals can be a bit odd. If your scope is earthed then bonding to the circuit under test at its supply pin can make a fair difference to overshoots?

Looking very good!
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Old 3rd Jun 2013, 4:16 pm   #224
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Looks like a fairly average TTL signal to me, remember that 1 (logic high) is anything over 2.8V. You are probably looking at 'real' TTL where it goes high quickly then the pull ups bring it up a bit more. In other words your 'scope is showing the true waveform (well done!). What are the scales (volts vertical and time horizontal)?
 
Old 3rd Jun 2013, 4:23 pm   #225
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Default Re: Building an oscilloscope at home

I think I did not explained myself well. Sorry.

When I said TTL signal, I meant a pulses signal from 0V (low) to 5V (high). On the C1-124 the signal looks perfect with both levels flat (no peaks).

The scales are not adjusted just some fixed components to make tests. I have to say that I have not measure that, in fact. The signal goes from zero to 5V, Frecuency should be around 1kHz, but not sure.
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Old 3rd Jun 2013, 4:45 pm   #226
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Default Re: Building an oscilloscope at home

Hi Miguel,

Well done in finishing the project and making such a good job of it. Just a word of caution about the EHT supplies; those are very big transformers and a low impedance supply like that could easily deliver a Lethal shock so please exercise extreme caution when working with the live EHT supply.
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Old 4th Jun 2013, 4:58 pm   #227
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Default Re: Building an oscilloscope at home

I solved the problem with the peaks. Silly me.

The S51 Y-amplifier has two coils (inductors) in series with the anode resistors, in the diferential amplifier. I connected the deflecting plates between the resistor and the coil (erroneously), and there were the peaks. Now I have corrected that, connecting the plates to the anodes of the diferential amplifier and the square signal looks great.

Yesterday I also finnally supressed the flyback trace by adding an RC network plus a diode, in the grid of the CRT, taking the blanking signal from the screen of the 6BW7 in the time base. It worked fine. I will show oscillograms soon. I'm also drawing all the schematic in an old software (MS-DOS based), to make some kind of manual when finished.

Now I will begin to adjust ranges.

Quote:
Originally Posted by n_r_muir
those are very big transformers and a low impedance supply like that could easily deliver a Lethal shock so please exercise extreme caution when working with the live EHT supply.
Thank you very much for your advice and your concern. I have to say that that has been my concern since I started this project. I always try to be cautious when working on this scope, but I understand that anybody can make a mistake.

I always discharge al capacitor (via a low impedance bleeder resistor) before start working on any part of the circuit. And even if the transformers are big, their output is not low impedance, cause I include 3Mohm resistor to increase impedance in order to make them safer. Anyway, the risk is there and I try to keep my finger out. Thank you very much for your concern
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Old 17th Jun 2013, 2:20 pm   #228
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Default Re: Building an oscilloscope at home

I have been out for a week but I'm here again.

Here some waveforms shown in the screen of my homemade scope. All waveforms match with the ones shown on the C1-124 scope.

Now I will begin to adjust ranges of amplitude and time.
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Old 17th Jun 2013, 3:52 pm   #229
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Default Re: Building an oscilloscope at home

You need to replace your photograph with one showing a very big smile!

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Old 17th Jun 2013, 5:14 pm   #230
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Default Re: Building an oscilloscope at home

Something like this?
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Old 17th Jun 2013, 10:10 pm   #231
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Default Re: Building an oscilloscope at home

LOL - Miguel, you are a remarkable man!
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Old 6th Aug 2013, 6:40 pm   #232
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Default Re: Building an oscilloscope at home

Some pictures to give you an idea about what I've been doing by these days.

Best regards.
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Old 14th Aug 2013, 2:43 pm   #233
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Default Re: Building an oscilloscope at home

By these days I've been adjusting the trace by the controls that this CRT has.

I have notice a problem in the DC mode. I set zero input and it shows a flat line as expected. Then I apply a sine wave and this sine wave is not oscillating around zero, but mostly negative, with some parts above the zero line.

I guess this is due to diferences in the gain of the two triodes used in the diferential output amplifier. What do you think?

In my opinion this can be solved by changing the twin triode which conform the diferential amplifier or simply by using only one output of the diferential amplifier to one of the CRT plates, while the other is connected to a fixed resistive divider. The second option will make me lose gain, but there is enough gain to sacrifice.

There is no problem on AC mode
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Old 15th Aug 2013, 10:51 am   #234
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Default Re: Building an oscilloscope at home

I've had a flick back through but I can't work out what circuits you're actually using in the end. If there is any capacitors in the path in DC mode then these may be getting charged. Post up your circuits and we can have a look..

D
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Old 16th Aug 2013, 9:38 am   #235
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Default Re: Building an oscilloscope at home

A Y-Amp is in post 131.

Maybe a current source in the tail is needed ?

dc
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Old 16th Aug 2013, 9:50 am   #236
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Default Re: Building an oscilloscope at home

Oscilloscopes are often driven off-screen by part of a waveform. Y amplifiers need to be able to clip cleanly and symmetrically, and they must come out of clipping without having created any lasting DC changes because of clipping.

Current sources in long tailed pairs are important. Some scopes used high value resistors from rather high voltage rails as an approximation.

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Old 16th Aug 2013, 11:18 am   #237
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Default Re: Building an oscilloscope at home

I'm not sure that that is the final circuit nor that it includes the details of the coupling to the CRT. I can't see anything in that circuit that would cause the symptoms that Miguel is seeing.

Miguel, is there a capacitor at the output of your signal source - that might be getting charged from the input valve if its leaky. Try placing a low-ish resistance across the scope input cancel this - if the symptom goes away then it is that your input circuit is not at zero volts....
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Old 16th Aug 2013, 1:12 pm   #238
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Default Re: Building an oscilloscope at home

The Y amplifier that I'm using is that of the Telequipment S51 (see attachment). In the input there is only a pure resistive divider referenced to HT-

I did more tests and I found that on AC (with a 0,1uF in series) mode it does it too. I'm puzzled with this thing.

On the other hand, not everything is problematic. I had sweeps of 0,5uS/div by these days. Wait for the pictures.
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Old 16th Aug 2013, 2:55 pm   #239
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Default Re: Building an oscilloscope at home

My guess would be that its due to non-linearity of the OP long-tailed-pair. Of course the the IP is a long-tailed-pair too but its not swinging nearly so much voltage.

I only have very basic valve knowledge but wouldn't the OP be better with some degeneration in the cathodes ? I bet a nice transistor current source would help too

dc
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Old 16th Aug 2013, 3:00 pm   #240
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Default Re: Building an oscilloscope at home

I was testing how fast the scope can sweep. My homemade signal generator is only capable to provide a 50kHz (in fact maybe 53 or 54 kHz) signal so that was the fastest signal I could apply. Anyway, I'm very happy with the results.

As the range sellector is not fitted yet, I use a 5M potentiometer as in the Serviscope Minor and I tried to get the same picture as in the C1-124.

With the C1-124 at 2uS/div, the 50kHz signal appears as a complete cycle in the screen. I was able to get that in my scope too.

Then I decided to go faster and I set the C1-124 at 0,5uS/div. The 50kHz signal appears just as a portion of a cycle. And then I was able to get the same pattern in my scope. This means that if I would had applied a 1MHz signal to my scope. This signal would had appeared showing a cycle in two divisions. That's great for me!!!

I have gotten this sweep using two 47pF capacitors in series (23,5pF). I will try with a 10pF capacitor to see if I can get a faster sweep, cause there is a point at which it simply stop sweeping, at least while moving the potentiometer.
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