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Old 29th Mar 2012, 4:00 pm   #41
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Default Re: Building an oscilloscope at home

Quote:
Originally Posted by Miguel Lopez View Post
friend told that it was the "delay line", but he couldn't explain me what the delay line was
That delay line is to delay the Y signal so you can see the event that triggered the 'scope. Its delay is slightly longer than the time the 'scope takes to trigger giving you a chance to see it.
 
Old 29th Mar 2012, 9:45 pm   #42
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Default Re: Building an oscilloscope at home

Hi Miguel,
As MerlinMaxwell said, the delay line is included so you can see the rising edge of a waveform.

In a bit more detail, the trigger circuit picks-off a little of the signal from early in the Y amplifier chain, and this starts a scan of the timebase. The Y signal continues through the Y amp cahin and passes through a delay line... something of the order of 50ns delay. So the timebase is up and running by the time the feature in the signal which triggered it reaches the deflection plates.

This amount of delay will do nothing for you at audio, but it's valuable on RF and fast logic.

Some scopes use that looks like a long length of coax, thought the centre may actually be twisted pair to go with the balanced circuitry. Also the impedance will be higher than the usual 50 Ohms.

HP used a rolled up length if flexible PCB with zig-zag balanced lines printed on it in many of their scopes.

That Russian scope doesn't sound too bad... Mesh PDA, good sensitive tube Y plates, delay line. Certainly not lacking in features.

Cheers
David
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Old 30th Mar 2012, 5:58 pm   #43
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Default Re: Building an oscilloscope at home

In fact, that Soviet CRT (17LO2I) and its fellow brother, the 17LO2J were used in several types of scopes. I use to have in the job a C1-93 which used this CRT. Indeed, very good scope while lasted.

The C1-83 was similar to the C1-93 but without automatic synch (manual only). I had never touched a C1-83.

These both scopes were capable to show signals up to 50MHz.

The C1-99 was a bigger type which also used this CRT. It was capable to measure up to 100MHz.

Problem with this scope was mostly with HV generators. (As some friends explained me) The ferrite cores used to generate HV, use to lose its magnetic properties and stop to induct voltage in the secondary. This never happened with earlier designs. Don't know why such problem. Most C1-99's were unusable being brand new by this reason.

http://republika.pl/rubi/oscyloskopy/c199/1.jpg

http://www.rlocman.ru/i/Image/2010/11/11/s1-83.jpg

http://www.elektroda.pl/rtvforum/fil.../c1-93_443.jpg
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Old 30th Mar 2012, 7:39 pm   #44
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Default Re: Building an oscilloscope at home

Hi Miguel, a common problem with ferrites if they are badly designed and get warm, they exceed their Curie temperature and lose magnetic properties.
This can be made worse by heavy clamping of the core, or running at too high a flux density.

Ed
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Old 30th Mar 2012, 8:57 pm   #45
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Default Re: Building an oscilloscope at home

I understand that, but older scopes did not suffer that problem. I think it was a manufacturing problem of the ferrites in the late 80s and early 90s.
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Old 3rd Apr 2012, 8:38 pm   #46
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Default Re: Building an oscilloscope at home

I was studying the schematic given in this thread, in order to make an hybrid with the Y amplifier of the serviscope (Post#32 by n_r_muir) and the X amplifier and time base generator of the PE65 (Post#27 by KeithsTV (first link)), and I found a little problem (there is always a little problem)

The X amplifier and time base generator is capacitor coupled with the synch signal from the Y amplifier (whatever be).

May I remove the coupling capacitor to feed the synch signal directly?

Please, forgive me if I talking silly things. Till now, I do not understand 100% how the sinchronization works in a scope. I'm learning here.

The other problem is what it's said in the lines I extracted from the PE65 document. I will appretiate any explanation as I don't understand that part well.
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Old 4th Apr 2012, 9:47 pm   #47
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Default Re: Building an oscilloscope at home

Hi Miguel,

The serviscope was a very simple oscilloscope intended for use in schools. The paragraph that you highlighted is basically about using the oscilloscope with the time base switched off; showing that if you connect a 1.5 Volt battery to the Y amplifier input, with DC coupling selected, the spot moves towards the top of the screen and stays there as long as the battery is connected to the input. Remove the battery and the spot goes back to the middle, connect the - ve end of the battery to the input and the spot moves down towards the bottom of the tube and stays there as long as the battery is connected.

Compare this with what happens when you select AC coupled input, this time the spot moves up when you connect the battery and goes back to the middle again. Similarly it moves down briefly when you connect the battery the other way round. This is a simple illustration between the difference between AC and DC coupling of the input.

Triggering: the serviscope has an auto-trigger, there are no user adjustable triggering controls at all. The time base will automatically trigger from the input signal as soon as one is applied to the vertical amplifier input. The double triode is a Schmitt trigger circuit which will produce a nice square pulse from the Vertical amplifier signal and trigger the time base to produce a steady trace on the CRT.

Even when there is no input signal we still want to see the time base line sweeping across the screen, this is achieved by R22 and C5 which basically changes the Schmitt trigger to act like an Astable multivibrator producing pulses to trigger the time base at roughly 10 times per second, at this low trigger rate the trace will appear dim but will brighten up as soon as there is an input signal for the Schmitt trigger to trigger from.

If you want a more complex trigger circuit that will trigger on DC levels as well as AC and where you can select to trigger of either the positive going or negative going edge of the input signal then look at the circuit diagram of the other telequipment oscilloscope that I sent you some time ago.

Good luck with your Home Build Oscilloscope project.


Ross
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Old 15th May 2012, 2:22 pm   #48
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Default Re: Building an oscilloscope at home

I've been searching for the parts needed to begin to do some test about the scope. I found three 6K13P RF pentodes which I intend to use in the Y amplifier. Those pentodes are call "variable mu pentodes".

What does that mean?

How do they control mu?
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Old 15th May 2012, 2:44 pm   #49
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Default Re: Building an oscilloscope at home

That means the more negative the grid voltage goes, the less gain you get. This is used in radios to make an automatic gain control.

However, this will make them a poor choice for an oscilloscope as they are quite non-linear.

D
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Old 15th May 2012, 2:47 pm   #50
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Default Re: Building an oscilloscope at home

Oops!! Then they can not be used. I'll keep searching.

Thanks for your quick reply, Dominic
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Old 15th May 2012, 2:51 pm   #51
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Default Re: Building an oscilloscope at home

Hi Miguel,

Variable-mu valves (or remote cut-off tubes in American terminology) are designed to adjust their gain automatically as the bias (or signal) level changes. This is particularly useful in radio receivers where AGC is applied to their control grids in order to keep the audio output constant as the radio signal varies in strength by adjusting the gain of the RF and IF amplifiers.

This 'trick' is acheived by having a non-linear pitch (or turns space) on the control grid of the valve. Standard or 'straight' valves have a linear pitch whereas variable-mu types have the control grid wound so that the pitch reduces, or the turns are closer together, towards the ends of the control grid support rods. If you study the Ia/Vg1 curves of both types of valve you will notice that the curve is more bent and reaches the Ia = 0 axis at a much greater negative voltage than a similar 'straight' valve. As the 'mu' (or mutual conductance) is proportional to the slope of this curve then you will see how the gain of the valve varies with control grid voltage.

As variable-mu valves have a gain which is dependant on signal level, I would doubt that they are of any use in the Y amplifier of a scope unless the amplifier was enclosed in an overall gain stabilising feedback loop.

I hope that this helps.

Regards,
Adrian

Last edited by RF Burn; 15th May 2012 at 2:52 pm. Reason: Bad grammar!
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Old 15th May 2012, 2:59 pm   #52
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Default Re: Building an oscilloscope at home

Thanks Adrian. I understood now. Definitively I won't use them in the scope. Have to keep searching a "straight" pentode as you said. Maybe I will use this valves in another circuit I've been researching: a constant output amplifier. But that's another project.
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Old 15th May 2012, 5:25 pm   #53
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Default Re: Building an oscilloscope at home

Most of the time a non variable mu valve can be found with the same general characteristics, build the 'scope with what you have and look for the others later. The variable mu valves may be OK in the small signal sections only leaving the output valve to be a normal one. Mind you a non linear 'scope may be quite useful, saving a lot of range switching once you know what the vertical axis means.
 
Old 15th May 2012, 6:05 pm   #54
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Default Re: Building an oscilloscope at home

But the output valve wil be the same. The input stage that I intend to build is a diferential amplifier which uses two pentodes, and from both plates, the crt takes the Y signal.
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Old 15th May 2012, 11:52 pm   #55
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Default Re: Building an oscilloscope at home

Hi Miguel,

If you go looking at manuals for reasonable quality valve era scopes, you'll find that the first stage is a pair of cathode-followers then a voltage amplifier stage, usually a push-pull amplifier, cascoded for speed and linearity.
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Old 16th May 2012, 5:51 pm   #56
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Default Re: Building an oscilloscope at home

Can you suggest me something regarding this? I'm interested.
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Old 16th May 2012, 6:03 pm   #57
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Default Re: Building an oscilloscope at home

The manual for the classic Tektronix 545 can be downloaded here:

http://www.thevalvepage.com/servdata...q/tektronx.htm

The 545 is perhaps THE classic Tek scope.
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Old 16th May 2012, 10:08 pm   #58
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Default Re: Building an oscilloscope at home

I am probably being a right bore here, but why would anyone contemplate building their own scope? If you go to any ham jumble sale you will find they are virtually being given away free as there are very few industrial applications where they are still used , dont get me wrong here for a amateur experimenter they are still a very powerfull tool but just now very cheap to buy

regards

de MM0CQT
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Old 17th May 2012, 7:55 am   #59
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Default Re: Building an oscilloscope at home

I think they might if they live in Cuba and getting out to a ham jumble is a bit tricky - and once they'd done that they'd still have to deal with the export and import legalities to Cuba ...

But then, what do I know?

Miguel, I have huge admiration for you!
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Old 17th May 2012, 9:04 am   #60
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Default Re: Building an oscilloscope at home

Having just acquired a fourth such orphan, I find myself still following this thread as these things have always fascinated me .. it would also be great fun to build a simple 'scope, and I find the construction form scrap congruent with interest in vintage gear. The appeal of valve tech for me is that it is not throw away, and even if non functional can be taken to bits and made into something else. Construction is as essential as restoration to maintain these skills and throws up different challenges - as does enactment archaeology! Strength to your elbow, Miguel.
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