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Old 1st Aug 2012, 10:25 pm   #81
dragonser
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Default Re: Building an oscilloscope at home

Hi, that is a very nice offer of yours, Ed.
I will have to dig it out as it hasn't been looked at for a while.....
certainly it is all valve, apart from the stick ? rectifiers on the power supply unit, which is a separate box !
regards Peter
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Old 1st Aug 2012, 11:40 pm   #82
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Default Re: Building an oscilloscope at home

Peter,

As Ed says, increasing the EHT increases the speed the electrons are accelerated to. They make brighter light as they hit the phosphor, but they take less time to pass through the deflection plates. This gives the sideways force from the plates less time to deflect the beam, so your plates become less sensitive. The good side of this is that it increases the bandwidth of the tube.

The electron optics in the VCR97 are designed for about 2kV total accelerating voltage. Going away from this will spoil the focus.

Modern scope tubes still use a couple of kV for the basic CRT, but then after deflection, the beam is passed through a domed mesh into an area of very high accelerating voltage, maybe 10-20 kV to smash them into the phosphor with a lot of energy. THe mash screens the strong field from reaching the deflection area... and the dome can be made to work in conjunction with a graded field as a lens, magnifying the deflection.

Have a root round for 'helical PDA' and 'Mesh PDA' if you want to go deeper.

The VCR 97 can give you several MHz of bandwidth, but it does need valve amplifiers to swing the necessary voltage.

Cheers
David
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Old 2nd Aug 2012, 4:03 pm   #83
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Default Re: Building an oscilloscope at home

Quote:
Originally Posted by ajs_derby
There is an excellent article on electromagnetics here: http://ludens.cl/Electron/Magnet.html
Excellent link ajs_derby, thank you very much!!!!
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Old 2nd Aug 2012, 5:47 pm   #84
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Default Re: Building an oscilloscope at home

Hi, thanks for all the info.
I realised that the crt in the home made scope isn't a vcr97, but is a smaller crt.
( sorry my memory isn't as good as it should be ) when I get a chance I will take a couple of photos of the home made scope in case anyone is interested.
regards Peter
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Old 2nd Aug 2012, 7:05 pm   #85
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Default Re: Building an oscilloscope at home

Please, do it. If you've got the schematic, please share it too.
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Old 4th Aug 2012, 4:49 pm   #86
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Default Re: Building an oscilloscope at home

I to would be interested in seeing a picture of your homemade 'scope, just to see what everyone else's efforts look like, also I would be interested in how yours turn out Miguel, will you be using a scavenged case for this project aswell?
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Old 4th Aug 2012, 6:24 pm   #87
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Default Re: Building an oscilloscope at home

Hi Miguel,

I'd still recommend something like the link I put at the beginning of this thread. It will give you more than enough EHT to run a VCR97 or similar. I've also made similar things from old TV LOPTs but they tend to be made to make higher EHTs than you'd want for a scope.

For transformers look at for dead computer PSUs. I'm not sure how many computers are around in Cuba but last time I visited there were a few in hotels. Also a good place to ask is the IT department of universities/colleges. Computer PSUs are notoriously unreliable so may be worth asking round as they are a very good source of parts!

D
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Old 6th Aug 2012, 2:53 pm   #88
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Default Re: Building an oscilloscope at home

Quote:
Originally Posted by Muggy
I to would be interested in seeing a picture of your homemade 'scope
Be sure you will, when I build it.

Quote:
Originally Posted by Muggy
will you be using a scavenged case for this project aswell?
Yes, I will use a case from a Soviet C1-99 scope.

Quote:
Originally Posted by Dominic
I'm not sure how many computers are around in Cuba
Not too many. Really scarce. A PC in Cuba nowadays is like a color TV in UK during the 50s. Parts for PC PSUs are wanted like gold.
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Old 6th Aug 2012, 4:31 pm   #89
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Default Re: Building an oscilloscope at home

Miguel, I have a small 80 page book on building an oscilloscope using valves, i'm going to scan it later today/tommorow, i'm sure you could adapt it to suit your CRT and the valves you have available, would you like me to post the PDF for you when I can?
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Old 6th Aug 2012, 5:05 pm   #90
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Default Re: Building an oscilloscope at home

Of course!!!!

That would be nice. I'm always looking for more info to learn before I start to "cut metal" and solder wires.

Thanks Muggy
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Old 6th Aug 2012, 8:01 pm   #91
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Default Re: Building an oscilloscope at home

If you want to be a little more elaborate in your oscilloscope, Miguel, it is common to have dual windings and rectifiers for the -1.5 kV (or whatever number it is) supply. One of the dual supplies feeds the cathode, the other feeds the grid.

This way your brightness control can be close to ground potential, and you can use a DC coupled amplifier to blank your trace during flyback and if you want to chop/alternate traces, during the transitions. The DC blanking/brightness arrangement allows you to have blanking which is still good at slow timebase speeds. You can also use a linear DC coupled amplifier as a brightness modulator.

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Old 27th Aug 2012, 6:05 pm   #92
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Default Re: Building an oscilloscope at home

Analizing the Serviscope Minor input stage which I intend to build for my projected scope, I found an interesting detail (at least for me) that I would like to talk about to understand it well.

This stage receives the input signal diferentially, so both the (+) and the (-) terminals are floating.

Has this any advantage vs "referenced to HT-" input?
May I mofify it to make it a "referenced to HT-" input?
What are the purposes of capacitors C3 and C4?

Thanks
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Old 27th Aug 2012, 9:12 pm   #93
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Default Re: Building an oscilloscope at home

The input circuit as shown is a simple differential amplifier, and also includes the 'Y shift' control. They've kept it simple and cheap by allowing the 'LOW' input terminal to hang somewhere between HT+ and HT- via the Y shift network. That probably saves a valve, or at least a negative HT rail.

As far as the input terminals are concerned, it's not a true differential amplifier because the 'LOW' terminal isn't really floating. It's grounded via C3, so it can really only be used for an input which as at a fairly constant DC voltage with respect to HT-. As far as I can see, both C3 and C4 are decoupling capacitors to keep noise out of one side of the differential amplifier formed by V1 and V2. I'm not sure why C4 is connected with one end to 'LOW' rather than to ground.

If the 'LOW' terminal was connected to earth externally, as is quite likely, the HT- rail (labelled 0V in the diagram) must not be earthed inside the scope, otherwise the trace would disappear off the top of the screen.

I hope this helps
Chris
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Old 31st Aug 2012, 9:01 pm   #94
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Default Re: Building an oscilloscope at home

I think that this input stage is somewhat similar to the attached circuit image but taking the output in the plates. In this case both input terminal will be isolated from the chassis. Or should I isolate HT- and fix input- to chassis?
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Old 4th Sep 2012, 5:00 pm   #95
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Default Re: Building an oscilloscope at home

This input stage correspond to Telequipment S51 scope. It seems like a Cascode stage, but taking the output by the plate of the bottom valve instead by the top one. But this output is AC grounded via C3.
I really do not understand this circuit.

What is the purpose of this configuration?

What is the purpose of L1, L2, L3 and L4?

Any help most appretiated.

Thanks
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Old 4th Sep 2012, 5:33 pm   #96
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Default Re: Building an oscilloscope at home

I don't think that is actually a cascode but the top triode is being instead used as a voltage regulator.

The inductors you mention look to be to increase the bandwidth of the stages.
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Old 4th Sep 2012, 6:16 pm   #97
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Default Re: Building an oscilloscope at home

How an inductor increase the bandwidth?
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Old 4th Sep 2012, 7:25 pm   #98
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Default Re: Building an oscilloscope at home

Hi Miguel; the alternative name for these coils is a "peaking choke". As the gain starts to fall due to the R/C circuit this adds an additional "lift" on the response curve. It is quite a common trick on video amplifiers.

Ed
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Old 11th Sep 2012, 5:50 pm   #99
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Default Re: Building an oscilloscope at home

A couple of pictures of the simulation that I have done in the PC.

Blue trace: Sawtooth
Red trace: Beam supression pulse.

Circuit of the PE65 scope.

Would the small sine wave on the beam supression pulse affect the trace?

When I vary the input level with the pot, the sweep increases or decreases. Is that normal? Should it be constant for the selected range?
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Old 11th Sep 2012, 10:19 pm   #100
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Default Re: Building an oscilloscope at home

Where's the sawtooth coming from? Is it supposed to be the X deflection? It's impossible to interpret the simulation scope traces without knowing the full circuit and what the input signal is.
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