Building My Own Oscilloscope

[Miguel López]

I did it with a high frequency oscillator at 20kHz, but I had a noise in the trace. See post#34

[boiss]

Your displayed waveforms have quite a few components present. What does the waveform represent, was the input S/C ? I think that you can you can use your pc as a scope to fault find at least to 20KHz (free software available on the net). I presume that the ferrite cored transformer is the EHT source. This must be screened with magnetic material.
It is necessary to find out where the noise!!!!! is getting into the deflection cct.
can you swap X and Y amps? is the result the same?
Great project !
Mike

[Skywave]

I built a 'scope many years ago using an old 3BP1 CRT. . . . The CRT supplies were mains derived (UK 50 Hz) and I don't recall any ripple effects on the CRT display.
Lawrence.

That brings back memories: yes, so did I: sounds like a very similar cct. to what I used. Mine was based on a design from a Babini book: was yours?
I well remember the PSU 'arrangements' though - again, similar to yours. Thinking about it now, in retrospect, it's remarkable that it didn't kill me! But when you're young and bursting with unbridled enthusiasm . . .

Al.

[Skywave]

I did it with a high frequency oscillator at 20kHz, but I had a noise in the trace. See post#34

Have a close look at the earthing arrangements. You might have a return EHT current passing through a conductor (which could be the chassis itself) and modulating the scanning voltages or getting into the 'front end' of the vertical amplifier.
It looks like you have a mu-metal screen for the tube. Is it earthed? If it is, make sure it is earthed at one point only and choose that point carefully.
I've also looked at the under-chassis wiring. The EHT circuit, X and Y circuits could probably benefit from some screening: I see a lot of unwanted capacitive and inductive couplings there, on account of the layout. It would probably be beneficial to put the EHT transformer into a metal box too.

Al.

[pwdrive]

I built a 'scope many years ago using an old 3BP1 CRT. . . . The CRT supplies were mains derived (UK 50 Hz) and I don't recall any ripple effects on the CRT display.
Lawrence.

That brings back memories: yes, so did I: sounds like a very similar cct. to what I used. Mine was based on a design from a Babini book: was yours?
I well remember the PSU 'arrangements' though - again, similar to yours. Thinking about it now, in retrospect, it's remarkable that it didn't kill me! But when you're young and bursting with unbridled enthusiasm . . .

Al.

Can't remember Al, I seem to remember a couple of EF50's in it somewhere, I think the EHT was around 750 V straight from a doubler off a 350 V HT winding.

Lawrence.

[AlanBeckett]

In my old Marconi TF1331 the Tube has a Screen and the EHT Generator is enclosed in a metsl box.

Alan

   

[pwdrive]

I want to see the unfocused blob on the display.....?

Lawrence.

[Miguel López]

Thank you all for your replies.

I decided to use a bridge rectifier to get the EHT that I need, directly from a 60Hz AC winding.

I will use a bridge rectifier with 2,3uF (4.7 + 4.7 in series) cap for the -1,5kV voltage. It will be obtained from a center tapped winding. I will use the center tap to assure that both capcitors will be at the same potential. This will give around 800V, and added in series with the -600V that I already have will give me -1,4kV (+/-) which is enough. In the simulation I got a ripple of less than 200mV with that values.

For the +4kV, I also intend to use a bridge rectifier from another center tapped winding. Same purpose for the center tap. This time I will use around 5 or 6nF to filter the EHT. With a 30uA current (which is the current for the CRT according to the datasheet), the ripple in this PSU will be around 300mV according to simulation.

I guess this levels of ripple will be negligibles. What do you think of this arrangement and ripple level?

I will use a separate transformer for the EHT's and one for the rest of the voltages needed. Both will be enclosured on a steel sheet box, as well as the EHT circuitry. I will take care on isolation.

[pwdrive]

Hello Miguel, if you can achieve those ripple figures then I would see no problems at all.

Lawrence.

[Miguel López]

Let's hope simulation be near reality. Thanks.