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Old 22nd Oct 2012, 7:47 pm   #121
Miguel Lopez
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Default Re: Building an oscilloscope at home

Here a picture of the 4000V multiplier and the arrangement that I have prepared to test the CRT. It looks a little "spartan" but I promise to improve it when finished.
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Old 24th Oct 2012, 8:43 pm   #122
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Default Re: Building an oscilloscope at home

Finally I did it!!! Well at least the CRT showed the beam.

I was able to vary the brightness, focus and astigmatism. I also moved the beam in the X axis varying the voltage in the corresponding plates. But there are a couple of things that I do not understand.

1.) It is not a single dot. I was expecting a bright dot and instead I've got a bright region. Can anybody tell me why? I couldn't reduce it moving the potentiometers mentioned before.

2.) There is a coil inside the metallic cover for the CRT (not showed in the picture) which I don't know what is ir for. It is placed in the section in which the CRT begin to get wider (+/-). Any suggestion about its purpose.

Anyway, I'm happy with the result. The CRT is working, now I have to improve it
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Old 25th Oct 2012, 11:43 am   #123
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Default Re: Building an oscilloscope at home

That could be mains hum on the Y amplifier.
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Old 25th Oct 2012, 1:16 pm   #124
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Default Re: Building an oscilloscope at home

Nop. There is no Y amplifier yet. Plates terminals are short-circuited and grounded.
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Old 25th Oct 2012, 2:37 pm   #125
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Default Re: Building an oscilloscope at home

In the link bellow you can see how the bright pattern varies while adjusting the potentiometers. It is my voice in the forefround saying: "Brightness", "Focus", "Astigmatism". Please, forgive the background voice, it was the TV.

I consulted with a friend who speaks Russian and he told me that in the C1-93, the coil in cuestion is called, "deflecting correction coil" with no further detail.

http://www.ziddu.com/download/20709164/Ajustes.avi.html
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Old 25th Oct 2012, 7:50 pm   #126
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Default Re: Building an oscilloscope at home

Hi Miguel, coil inside the cover is a "twist coil", energised with low voltage DC this will allow the trace to align with the screen graticule.

You may need to experiment with the voltage swing you get from the focus control. If the bright region can be made smaller or larger, set contol to smallest and look at the voltage on the focus electrode.
It then needs to be made to go more in the "smallest" direction.

Astig setting should not be a major problem.

Be carefull when you have a focussed spot, as if well focussed and bright it will damage the screen, unless it is a moving spot( as when the tube is being scanned).

Ed
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Old 25th Oct 2012, 10:15 pm   #127
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Spartan? No, just good honest workman like construction, and very neat too!
 
Old 26th Oct 2012, 4:34 pm   #128
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Default Re: Building an oscilloscope at home

I found the problem. It was an interference from the mains cable of a small fan that I use to refresh. I noticed it because when I applied a 60Hz voltage to the horizontal plates, it showed an ellipse, then I thought that the interference should be at the same frequency, and then I disconnect everything in the bench.

When I disconnected the fan, the trace became a straight line. See pictures for a graphic explanation of what I did. I'm very happy with this result.
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Old 26th Oct 2012, 5:23 pm   #129
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Default Re: Building an oscilloscope at home

It was indeed mains causing a vertical trace.
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Old 27th Oct 2012, 10:16 am   #130
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Default Re: Building an oscilloscope at home

I'm very impressed, Miguel, well done.
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Old 3rd Nov 2012, 4:00 pm   #131
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Default Re: Building an oscilloscope at home

I'm going to try this circuit this weekend. Wish me some luck . It is for the "Y" amplifier.
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Old 3rd Nov 2012, 5:12 pm   #132
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It is always worth having the mean (average, centre) voltages on the plates variable so you can minimise 'spot smearing' or astigmatism. If the plates are not near the 'voltage' of the beam they will distort it. It will be different for X and Y as they are on different parts of the electron beam. (usual disclaimer, if I am wrong please correct me) Get it going first, these small adjustments can be added later.

I do admire your tenacity in this project. This must be one of the longest and most interesting threads on the forum.
 
Old 5th Nov 2012, 2:28 pm   #133
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Default Re: Building an oscilloscope at home

I mounted the circuit for the Y amplifier. It seems to work fine. I couldn't test the output (lack of time) but the input and first amplifying stage work OK.

Quote:
Originally Posted by merlinmaxwell
If the plates are not near the 'voltage' of the beam they will distort it.
Which would that voltage be?

I included a potentiometer (in the second triode anode) to "move" the level at what the "0 plates voltage" will be, referenced to HT-
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Old 5th Nov 2012, 3:25 pm   #134
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Quote:
Which would that voltage be?
Somewhere between the electrodes either side of the plates. Can't be anymore specific, depends on the tube.
 
Old 5th Nov 2012, 3:52 pm   #135
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Default Re: Building an oscilloscope at home

I will try to see what voltage is that from the schematic (got a friend who speaks Russian).

The transistors that the C1-93 and C1-83 scopes use for this CRT are the 2T602 which has a maximum voltage across collector-emitter of 70V.

Anyway, all voltages in the CRT are negative (except +4000V).

Now a couple of pictures of what I did on weekend.
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Old 5th Nov 2012, 9:29 pm   #136
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Default Re: Building an oscilloscope at home

They will have juggled the CRT design so that the optimum average potential for the Y plates suits an amplifier based on those transistors which will put the Y plates about 25v above ground. This is a good wet-finger estimate. It does mean that the CRT is more difficult to drive with a valve amplifier.

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Old 6th Nov 2012, 3:01 pm   #137
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Default Re: Building an oscilloscope at home

If it distorts the signal using the usual voltage at which valves are operating, I could try to reduce this voltage to around 25V using resistive dividers across ECC81 anodes and HT-.

Would that work?
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Old 6th Nov 2012, 9:24 pm   #138
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Default Re: Building an oscilloscope at home

I think it would. I know you're primarily interested in doing it with valves as a personal exploration, but I thought I'd better mention that your CRT is pretty certainly designed to work best with transistors, or with a very odd valve amp.

You could always add a -200V supply and have the final stage cathodes and grids at about - 75 volts.

Oh, there are articles sprinkled through all those HP journals on the web about the CRT designs in their oscilloscopes, right from the early types.

Cheers
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Old 8th Nov 2012, 3:21 pm   #139
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Default Re: Building an oscilloscope at home

I did the input-output test, Sensitivity was not very good, but I didn't expect it to be. That's something I can improve later. The most important thing was linearity, which was perfect. Gain was around ten.

Quote:
Originally Posted by Radio Wrangler
I thought I'd better mention that your CRT is pretty certainly designed to work best with transistors
I'm sure it is. Looking for info in the manual of C1-93 (in Russian) I found that the output stage for the Y amplifier is supplied with 80V DC, and knowing that 2T602 max VCE is 70V, the value given by you of around 25V should be correct. I will try with the resistive divider once mounted.

The pictures that I showed earlier with the traces in the screen were using a floating voltage, that is, not connected to any part of the circuit except the deflecting plates. How this could affect distortion?
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Old 19th Nov 2012, 4:16 pm   #140
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Default Re: Building an oscilloscope at home

In the Serviscope Minor manual you can read:

V3 acts as a Schmitt trigger circuit. In the abscense of an input signal, it will oscillate at a frequency determined approximately by R22, C5.

I built it and it did not oscillate. Looking once and again, I don't see this circuit as an oscillator but as a simple Schmitt trigger voltage comparator. I need some help here please to understand what's happening. Thanks

Here is the circuit that I built.
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