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Old 12th Mar 2012, 5:52 pm   #1
Miguel Lopez
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Default Building an oscilloscope at home

Hello everyone

I've been thinking on this since a long time ago. I had used a C1-94 for more than 20 years and now it is requesting "vacations". The C1-94 oscilloscope is intended por educational purposes but it has been very useful to me. For years it has failed from time to time but now I think that the CRT is ending. The beam have been vanishing during the year and nowaday, I have to turn off the room lights to see the trace. I really like this device because it has been my partner for several years of electronics work.

I have thought to build an oscilloscope since some years ago, but I lacked knowledge and parts. I have three 17LO2I CRT's (Soviet made) that would be suitable for that purpose. And now as I'm learning to use valves I think it would be nice to build it with this magical devices, thank to you who have help me to get the valves that I need, I think that I will be capable of doing this project. If the CRT's work OK, then I will see how to make the other circuitry.

The first step would be to test the CRT's. I think that I have to build the schematic shown in one of the pictures bellow. That is the schematic of the C1-83 Soviet oscilloscope which uses this type of CRT. I have some questions regarding this, and I hope you help me to undestand.

The datasheet of the CRT is also shown bellow, but unfortunately it is in Russian, so I don't know if it would be useful.

In my opinion I have to make them (the CRT's) show the trace in the screen as a dot, and then I should be capable of moving this dot across the screen by applying voltage to the deflecting plates. Am I OK?

My main question is (for now):
What are the purpose of the potentiometers circled in red? What do they adjust?

Cheers

Miguel

PS. If achievable, I will only start this project when I finish the one that I'm currently working on. I'm just learning about this.
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Old 12th Mar 2012, 6:05 pm   #2
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Default Re: Building an oscilloscope at home

The pots look like focus and two beam shaping ones.
 
Old 12th Mar 2012, 6:08 pm   #3
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Default Re: Building an oscilloscope at home

Could you explain more please?

Thanks for your quick reply
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Old 13th Mar 2012, 1:52 pm   #4
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Default Re: Building an oscilloscope at home

Hi Miguel,

Making a simple scope is pretty easy...However the first thing I would make would be a simple generator for making the EHT. Making EHT from the mains is VERY VERY dangerous. One slip and you're dead. A simple circuit like the one attached is easy to make up. You will need some high voltage diodes and capacitors but these can usually be found in old TVs...

I've used a circuit like the one here to test lots of old 'scope and TV tubes and it has only given me a shock a few times which made me jump....if it had been mains derived EHT I would have only had one shock!

That tube looks quite complicated but you should still be able to get a spot somehow....

Good luck!

Dom

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Old 13th Mar 2012, 2:39 pm   #5
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Default Re: Building an oscilloscope at home

Thank you dominic.

I have the schematic of the EHT used in this type of tube; it is very similar to the one you showed. I have some diodes of that type and I'm searching for the HV caps in my stuff of Soviet PCB's . The transformer is driven by two transistors in push-pull at 20Vcc, 9kHz. I have some ferrite cores from Krim-218 TV sets. It's easy to built. See the attachment.

Yes, this tube is very complicated. It was used in dual trace oscilloscopes. That is another question I need to ask.

The blanking pins were used to blank the beam while swaping from one signal to another in dual signal operation. I do not intend to make it work in dual trace, so the question is:
What should I do with the blanking signals? Ground them perhaps? Leave them open?

PS: I'm also worry about the EHT danger so, I will be very carefull.
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Old 13th Mar 2012, 4:12 pm   #6
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Default Re: Building an oscilloscope at home

You could use the blanking signal to blank out the trace while it is flying back. A capacitor from the relevant X-plate and a bleed resistor should work.

D
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Old 13th Mar 2012, 5:09 pm   #7
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Default Re: Building an oscilloscope at home

Could you be more explanatory please?. Remember I'm a newbie working with vacuum tubes, including CRT. .

Quote:
Originally Posted by Dominic
A capacitor from the relevant X-plate and a bleed resistor should work.
Which would be the relevant X-plate?
What value for the cap?

Thanks in advance.
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Old 14th Mar 2012, 12:24 pm   #8
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Default Re: Building an oscilloscope at home

Hi Miguel,

Something like the circuit attached. You are taking the sawtooth waveform and turning the quick flybacks into negative pulses to blank the screen while it is retracing from one side to the other.

Which plate just depends on which way round you wire them...try both!

You might need some other resistors to set the normal voltage of the blanking pin, I can't read Russian so not sure what the voltages should be!

Dom
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Old 14th Mar 2012, 3:20 pm   #9
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Default Re: Building an oscilloscope at home

I got it.

I can not read Russian completely but I can replace cyrilic with latin letter and it helps, that's what I always do. I'll try to find what voltage level does it need in the blanking pin.
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Old 20th Mar 2012, 2:39 pm   #10
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Default Re: Building an oscilloscope at home

I found this circuit that could be suitable for what I want to do.

http://www.electronixandmore.com/project/circuit7.gif

It was taken from this site:

http://www.electronixandmore.com/project/7.html


Opinions?
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Old 20th Mar 2012, 4:07 pm   #11
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Default Re: Building an oscilloscope at home

I found that the Cossor 3332 oscilloscope used only a couple of valves for the amplifiers plus the rectifier.

Has anybody the schematic?
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Old 20th Mar 2012, 6:17 pm   #12
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Default Re: Building an oscilloscope at home

Hi Miguel, you would probably find that the cossor circuit was quite low Y sensitivity, There were some Practical television circuits that were similar. Minimum valves, minimum sensitivity.
The Philips book "An introduction to the cathode ray oscilloscope " by Hartley Carter has several good circuits, plus lots of operation and constructional details and uses modern (E series) valves. It is part of the Philips Technical library series.
There was also a good circuit in Wireless World in the late 80's that used a simple tube and transistors/ Op Amps.

Ed
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Old 20th Mar 2012, 6:51 pm   #13
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Default Re: Building an oscilloscope at home

Thanks Ed, saddly that technical literature is beyond my reach (by all reasons)

The purpose of take a look to the Cossor 3332 schematic is to get some ideas for the final design. In the end, I will probably need to adapt the design to the valves that I have. I think that extra Y amplification could be done with an additional triode or pentode. Is that possible?

I found this picture that looks suitable for an oscillator to obtain EHT for the CRT.

http://savepic.ru/431667.gif

I would like to replace the 6P44S with a less powerful tetrode or pentode, such as the EL86. Is this possible with small changes in this circuit?

This is the circuit used in the Soviet B/W TVs. Along with the 6D20 half wave rectifier, it provided 18 to 25 kV for the CRT. I only need 4kV and -1,5kV. I do not intend to obtain the EHT with a valve rectifier. I only intend to make it oscillate and then multiply it to obtain the EHT using silicon diodes and capacitors. Of course, I will not rectify the mains for the supply voltage, instead I will use a transformer.

Any opinion will help.
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Old 20th Mar 2012, 7:47 pm   #14
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Default Re: Building an oscilloscope at home

Hi Miguel, the easiest way to get + &- EHT is to use the Ht transformer (AC side) and then use voltage multipliers to develp the voltage required.
PM me your e-mail address and I'll try and get you a copy of some circuits.

Ed
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Old 20th Mar 2012, 8:01 pm   #15
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Default Re: Building an oscilloscope at home

Yes, but as it works at 60Hz, I think that the required capacitors have to be huge in capacitance, and then in size and weight.

How many current is needed to bias the CRT? Could I use 0,1uF caps for this purpose?
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Old 20th Mar 2012, 9:26 pm   #16
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Default Re: Building an oscilloscope at home

It is in the order of microamps, 1uf would be adequate. As the circuit for the 'scope you suggest has unblanced deflector plate drive which will give focussing issues at the end of the deflections, build it with a big chassis so you can add valves later. It is always best to build someting that works but isn't perfect than a perfet one that doesn't!
 
Old 20th Mar 2012, 10:04 pm   #17
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Default Re: Building an oscilloscope at home

The only 1000V caps that I have, are 0,1uF, so, not suitable for 60Hz. The 1uF that I have are only up to 400V so I would need more than 10 to reach 4kV. That's why I thought to increase frequency.

Quote:
Originally Posted by merlinmaxwell
build it with a big chassis so you can add valves later
Yes, it will have space and holes for further valves. I did it in the amplifier, so I've got some experience in this kind of "precaution".
And yes again, the chassis will be BIG. It's Soviet!!!!

Quote:
Originally Posted by merlinmaxwell
It is always best to build someting that works but isn't perfect than a perfet one that doesn't!
So true. +1
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Old 20th Mar 2012, 10:52 pm   #18
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Default Re: Building an oscilloscope at home

Hi Miguel, many of the "war surplus2 based scopes used this method to obtain high voltages. Caps were in the range 0.1uF to 1uF.

Ed
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Old 20th Mar 2012, 10:56 pm   #19
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Default Re: Building an oscilloscope at home

I lose nothing trying. That's the simplest way. I will try soon.

Thanks Ed
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Old 21st Mar 2012, 12:53 pm   #20
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Default Re: Building an oscilloscope at home

Building a simple oscilloscope. If you are looking for ideas, may I suggest you look at the manuals for some of the commercial valve scopes. Cossor 1039 (Mark 1 or 2), MetroVick CT52 or CT84, EMI WM1. With 3 or 4 valves only.
More extensive with 6 valves or so are Telequipment S51, S31 or S32, Hartley 13A, Cossor 339 and 1035 Mark 1 or 2 (Mark 3 is much more elaborate and uses modern valves.) All these manuals are widely available, start your search with Jon's Valvepage.
Similar are the three designs in the Philips book: "Introduction to the Cathode Ray Oscilloscope", 1960 by HarleyCarter, quoted by Ed Dinning. I can scan if you are stuck, though the book seems to be readily available second hand through ABE or Amazon, and you may find helpful to read.
As this may be of interest to others on the Forum, I have details of Wireless World scope designs. Of May 1948, converting a wartime radar unit, by Vaughan; with additional Wobbulator of Dec 1948 by S A Knight; a more elaborate Television one by W Tusting in June, July 1952. Also the Jason OG10 of Radio constructor August 1959 and the very similar Heathkit OS1. The Scroggie 2 valve design of W.W. March 1950, also appears in his Radio Laboratory Handbook, most editions up to 6th 1971, not final 9th 1980. These all fairly traditional to late 1950's, and easy to build and adapt.
A modern valve design in Wireless World August 1963 into 1964, using 10 modern valves with an oscillator for the EHT supply.
Others include a minature one using 5 EF91/6AM6 from Short Wave Magazine Feb & Mar 1955, which I built and found very satisfactory. I am sure that there will be a Russian equivalent to this very common valve.
Transistorised scopes often much more complex, the Wireless World 50MHz design by C M Little of May to Aug 1975 using an oscillator for the EHT supply, and various designs from Practical Wireless and Elecktor. I have all this information and can scan if required.
Use of an Oscillator for the EHT supply was introduced by Tektronix in late 1950's, and rapidly followed by other manufacturers as it was much lighter, uses smaller capacitors, so cheaper, and was much safer. Look up Cockroft Walton Multipliers or Westinghouse Brake and Signal UK for notes upon multiplying H V rectifier arrangements. Billn m0wpn
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