2 inch oscilloscope

[Hermit6345]

As I was bequeathed a couple of 2 inch CR tubes, 2AP1, 2BP1 etc I decided to see if I could make a simple scope using one or other of them using only parts from my junkbox. The only transformer I had was a Parmeko Neptune series oil filled device which has enough windings to supply all the various voltages. I took a chance on stray magnetic fields from the transformer and hoped that its casing would contain most. However, I get a very slight ac ripple on the x display, probably because the y plates are parallel to the top of the transformer. I don't have a mumetal screen for the tube, I can, I think, get mumetal sheets. Is it necessary to wrap these around the tube with overlapping ends to form a "shorted turn" or could I just mount a plate of mumetal between tube and transformer? Apart from the tube, the whole thing is transistorised as I wanted to keep it compact. Any ideas?



Ian

[beamcurrent]

Hi Ian

The positioning of the transformer will have an effect on the degree of hum induced. The best spot for least effect is behind the tube base on the tube axis. It is also worth experimenting by turning the transformer orientation for minimum hum.

regards Brian

[ParcGwyn]

Hi Ian,

Glad to see you have your scope working, you say you want to keep the design compact. Have you thought of building a high frequency inverter and winding your own transformer. I built a very simple oscilloscope based on a Mullard design running from a 12V supply and fitted the transformer behind the tube, as Brian says, with no magnetic shielding and do not have any ripple on the display. See: http://www.parcgwyn.talktalk.net/mullardpupil.htm for photographs and transformer winding details.


regards

Dave

[Ed_Dinning]

Hi Ian, if you need a more compact scope there are a few 1CP1 tubes about (only 1" dia)

Ed

[Hermit6345]

Thanks for all the advice, gratefully received. Now, since the scope had to be built from the junk box, I had to use the largest diecast box that I had. That meant the transformer had to be mounted under the tube and given it's construction and the way it's mounted, I don't have a hope of moving it around unfortunately. That would involve s complete rebuild. The transistorised PSU idea from Dave looks interesting, I need about 370 vdc for the Astig voltage to my tube and I have zenered that down to 150vdc for the HT to the long tailed pairs which do the x and y scanning and I need +/-5 volts to power the TL082 op amps used to drive those and of course 5 volts for the timebase generator. All that could be powered from the 12 volt rail with a little-ve five volt generator chip. I am currently nder running the tube heaters as 5 volts might give the tube a longer life. Blimey Ed a 1 inch tube! I reckon that my 2 inch things are small! I have a couple of three inch tubes here and one of my 2BP1s actually has a proper socket on it. The induced ripple is very small but irritating because in the y axis the trace is razor sharp whereas in the x axis the line is very slightly thickend. I learned a lot about timebases whilst knocking up the scope and finally decided on one that uses a 555 to generate the timebase in 6 ranges. I breadboarded three different circuits all told. I did have some success with one of them by feeding in a pulse on the Z axis right at flyback time and that did hide most of the faint flyback visible on the trace. Now its time to get the breadboard out again by the sounds of it. Thanks all.


Ian

[Hermit6345]

Scope now finished, interesting exercise. Never intended for serous use but just to keep me busy!

Next is to make an inverter HT unit as Dave suggested.


Ian

[dominicbeesley]

Hi Ian,

That's really neat and very pretty!

Playing with little CRTs is great fun! Though I get too distracted by the next idea to make anything nice like your scope!

I've mucked about with a few smaller CRTs and they are fairly easy to work with and can produce really fine results:

http://vintage-radio.net/forum/showt...&highlight=EHT
https://www.vintage-radio.net/forum/...ad.php?t=24304
And lots of different CRTs here:
http://www.taswegian.com/NBTV/forum/...ht=eht&start=0 (you'll need to log in to see pictures though)

There's a dead simple EHT supply I made here (good for a few thousand volts - get rid of some of the ladder for less volts):
http://www.taswegian.com/NBTV/forum/...744&highlight=

Or a more complicated one I started but still haven't finished here:
http://vintage-radio.net/forum/showt...ht=NBTV&page=4

Cheers

Dom

[Hermit6345]

Hi Dom, thanks for your comments and all of those links. I removed an EE core from a computer psu and soaked it in varnish remover, horrible stuff and that stank as well! I now have some other ideas thanks to your links. I have fitted a bnc socket on the back and built a simple Z-axis amp so with suitable timebases might see how it gets on with some video!

Ian

[ppppenguin]

Seems like a lot of work for your EHT transformer. The EHT transformer from an old CRT monitor fed with a very modest amount of square wave will generate loads of EHT. The problem may be keeping it down to a sensible value if you only want the odd kV or 2.

Another problem with this method is if you want -ve EHT as often done in scopes. Can't see a solution to that one since you don't want the LT side all floating at EHT potential.

[dominicbeesley]

Work - not likely! Took me longer to drill the holes in the box than to make the circuit! I "cracked" a pile of PSU transformers a while ago by baking them in the oven and pulling - about 1/3 of them broke but the rest have been very useful for all manner of larking about.

As to having the circuit supply -ve just reverse all the diodes in the ladder?

Have you got a circuit diagram Ian?

Dom

[ppppenguin]

Originally Posted by dominicbeesley

As to having the circuit supply -ve just reverse all the diodes in the ladder?

Can't do that with an EHT tranny from a monitor. They are diode split designs with the diodes well buried in the windings.

[dominicbeesley]

Sorry, I just meant on my cheapo cheerful one - it was originally -ve but I swapped it round when I started playing with magnetically deflected tubes and because I never got my head round NOT touching the cathode wires....bbzzzzzzzt

D

[ParcGwyn]

Nice to see the scope working Ian, good look with the inverter.


Dave

[ppppenguin]

Originally Posted by dominicbeesley

Sorry, I just meant on my cheapo cheerful one - it was originally -ve but I swapped it round when I started playing with magnetically deflected tubes and because I never got my head round NOT touching the cathode wires.

Unfortunately if you use a normal electrostatic scope tube the deflection plates must be at approximately final anode voltage. This means either the deflection circuits operating at high voltage or the cathode being negative. Some early TVs with ES CRTs used capacitative coupling to the deflection plates. Not possible in scopes if you want DC coupling.

BTW, if you're doing clever stuff with PDA CRTs then you need a high +ve voltage for that as well as a high-ish -ve voltage for the cathode.

[Tim]

That's fabulous! I wouldn't even know where to start.

[dominicbeesley]

Originally Posted by ppppenguin

Unfortunately if you use a normal electrostatic scope tube the deflection plates must be at approximately final anode voltage. This means either the deflection circuits operating at high voltage or the cathode being negative. Some early TVs with ES CRTs used capacitative coupling to the deflection plates. Not possible in scopes if you want DC coupling.

Yes, for electrostatic ones I tried both ways but for NBTV the slow 12.5Hz frame rate needed a lot of capacitance. With a -ve cathod it needed fewer big capacitors but still needed an isolated circuit (or a big capacitor) to drive the grid/cathode...

Originally Posted by ppppenguin

BTW, if you're doing clever stuff with PDA CRTs then you need a high +ve voltage for that as well as a high-ish -ve voltage for the cathode.

Aah, what is a PDA crt - I've seen it mentioned before but not found out what it means....

D

[ppppenguin]

Originally Posted by dominicbeesley

Aah, what is a PDA crt - I've seen it mentioned before but not found out what it means....

For once google didn't give a good answer. Entering PDA CRT didn't give good hits. Post Deflection Acceleration. In other words an accelerator electrode after the deflection system. Either the screen itself or a spiral of conductive material on the inside of the cone is held at a high +ve potential. This allows brighter traces without wrecking deflection sensitivity. This article looks sensible: http://www3.ntu.edu.sg/home/askklee/...rinciples.html

[Hermit6345]

Thanks for all the comments and interesting points. My tube operates with about 750 volts negative on the cathode and the heaters are tied to that. I found that using the full 370 volts HT rail on the deflection plates via the ZTX458 deflection drivers was not as good as using 150 volts. I used a 0A2 in the prototype but replaced that with a zener due to lack of "real estate" on the chassis. I have a circuit for EHT derived from 12 volts so will replace the rather large Parmeko oil filled transformer with a much smaller inverter unit in due course
One of my concerns was the capability of the Intensity, Focus and Astig. pots to withstand the high voltages when mounted on the aluminium front panel. My fears were groundless though.

I used DC coupling throughout as I found that capacitive coupling affected the rise times when looking at square waves. I can't claim originality for much of the circuitry, I gleaned most of it from the web, built various circuits on protoboard and then modified on an empirical basis to give best results.

The Z-axis circuitry is a bit feeble and I have found a cicuit that uses a couple of 2N3823 FETs in cascode for a video amplifier and having the devices in my junk box, I will have a look at that.

I have a spare 2BP1 tube with slight burning on the screen if anyone wants a play.



Ian

[Hermit6345]

This is by way of a postscript.

I obtained some very thin mumetal sheet and wrapped it round the tube, secured it with a tie wrap and, voila, the magnetic interference was gone. Subsequently after a lot of www browsing, I managed to obtain the correct( second hand) mounting hardware for a 2BP1/2AP1 which incorporates a magnetic shield, front panel bezel and tinted plastic filter. All made by James Millen & Co, a name that will be familiar to anyone with old ARRL handbooks. I had to rebuild the amp/drivers because the HF response was not suitable for my final addition, a scope clock. I put the poor HF response down to veroboard with it's parallel tracks so I used matrix board this time and used point to point wiring. I have also designed a pcb but have yet to get that made. So I have a fun item in my scope clock, see the pictures below.

It's a bit big for the bedside table though!


Ian

[dominicbeesley]

That really looks the part now!

D