Question on PPI or Radar displays

[TonyDuell]

I don't know if it's of any help, but I have just remembered that the manual for the TDMS5 (a telegraph distortion tester) is on Sam Hallas's web site. This instrument displays a circular trace on an electrostatically deflected CRT. While the circuit diagram is not too clear (nor is the original that I have and scanned..) you can probably make it out. The circuit description is pretty good I think. From what I remember the timebase uses 3 double triode valves

Interestingly the Germans in WWII used a circular scan for their Wertzberg radar as a bent "A scope" type, three times as long on the same sized tube. So near to a PPI.

[AdrianH]

Originally Posted by TonyDuell

I don't know if it's of any help, but I have just remembered that the manual for the TDMS5 (a telegraph distortion tester) is on Sam Hallas's web site. This instrument displays a circular trace on an electrostatically deflected CRT. While the circuit diagram is not too clear (nor is the original that I have and scanned..) you can probably make it out. The circuit description is pretty good I think. From what I remember the timebase uses 3 double triode valves

I have downloaded and will have a read, for now I will try to keep myself to something more simple such as a linear type 'A-scope', rotating a uWave dish or audio system I ound find difficult.

I need to play with phase shift osc to see if I can get 90 degrees, but there is more to do before I get to that stage.

It is good thinking about it though.

Adrian

[AdrianH]

To much time on my hands I think. Just a lash up in a box, DG7-5 CRT and the circuit I had before driving the sawtooth waveform, no Y amplifiers yet so nothing to make it go up/down.

I asked a question before in another thread about getting 800 to 900 volts from 250-0-250, using a voltage triplert. In the mean time I was wondering why some old scopes had negative 650 Volts to plus 350 Volts now I understand why, with the standard HT voltage supplying the valves, adding a negative voltage doubler is simpler then a tripler so the CRT can still have its required voltage going from plus 300 to -600 Volts, but a few less parts.

Things only start to click when I build gear.

Adrian

[barrymagrec]

One of the reasons that old scopes such as Cossor have a negative tube supply as well as a higher positive supply is in order to sit the deflection plates at a suitable voltage to be driven by the deflection circuit.

[AdrianH]

Hello Barry;

That must be why I found I had to strap the X deflection plates to g3 to both keep them free from pickup and keep the display visible, if I took them negative or down to cathode I lost the line.

Adrian

[AdrianH]

I added the Y amplifier stages today. Used an EF91 Pentode as am amp and driver to a ECC85 as a phase splitter to drive the deflection plates.

I can get a 40mm vertical deflection from a 30mV peak to peak signal at audio frequencies so I hope that will be enough gain to be driven from a detector stage.

The time base is not variable and there is no lock/sync circuit in use, I could also do with blanking the flyback pulse, things I need to understand and see how I could achieve this with this Heath Robinson affair.

The transformers for my amp are due to arrive tomorrow so I may be taking a break from this for a while. Never sure if should still be here or in home made as it started with me asking questions on Radar PPI displays?

Pics are the general layout then with an input of 31K and an input of 123K sine-waves.

Line up now is 6D4, EC90 EF91 and two ECC85's plus the DG7-5 CRT.

It may have been easier to get and old valve scope, but the prices seen on Ebay are a bit ridiculous and this may be more fun

Adrian

Picture two, topologically a circle, it needs a shade of refining (the easy bit). I like topology, once you have sorted it out all that is left is bending.

[AdrianH]

There is an issue, in both pictures and it is in the time of the flyback. In post #20 I showed the pictures of the saw-tooth waveform from the 6D4 Thyratron where the flyback would be done in 2uS.

This is driving a EC90 cathode follower so that I did not load the saw-tooth output. This via a potentiometer drives the grid of the ECC85 acting as both amp and phase shifter. But and this is a big BUT I am loosing definition of the waveform. There must be a better way of doing it but I had not figured it out, so this 2uS is now turning to 7 uS flyback time with rounding at the bottom and that is why you are seeing this shape. At least I would think so anyway. It will never be a circle as I am not driving the X axis with a sine input.

I will try to draw out the circuit and let someone suggest better ways of driving the X axis.

Adrian

[Ed_Dinning]

Hi Adrian, plenty of design details for valved scope amps and time bases in Practical Wireless service manual as well as a host of the Babani small paperbacks that were popular in the 50's and 60's most for not much on ebay as well as always represented at Golbourne.

Ed

[AdrianH]

Hello Ed;

I have been looking at the old end of the oscilloscope diagrams circa 40's era such as Philips TA155, Heathkit IO-12 and Philips GM5659 to see how things have gone together. The line up for the CRT came from the latter.

They all use valves that I do not have so a case of what I have can I make it do, is the sort of exercise I am trying to achieve, for the most part I get on OK (for me), but it is the flyback shape of the X axis that is my concern.
I have attached 4 pictures. All pictures on the HP 182C scope are 10uS per div horizontal: -

The first is the waveform on the grid of the ECC85 after Capacitor (C). It shows nice sharp corners to the waveform especially on the steep discharge section.

The second picture is on the Anode of the first section, so it is inverted, the start of the discharge is sharp but then starts to round off meaning it now hits peak volts at 10uS after start of discharge. This is a 150 Volt peak to peak waveform centred around 150 Volts so around +/- 75 Volts swing.

The third picture is the second Anode 180 degrees out of phase so back as the grid signal? Again the same voltage swing +/- 75 Volts around 150 Volts.

The last picture is my hand drawn circuit just for the X axis. I have a B+ of 325 Volts, use a couple of Zener's to provide 260 Volts stabilized for the 6D4 and for the EC90 cathode follower.

The caps (C) are 4.7 nF (4n7), and I have tried various values to ensure I am driving the grid OK from 100pF to 0.1uF, I basically lost some gain with the 100pF as the reactance started to make a difference.

The cathode of the ECC85 sits at 3.4 Volts and I have checked that I am not driving the grid positive.

The normal anode current with no signal will be around 1.16 mA with the anodes at 150 Volts so will drop to 0.66 mA on peak voltage, may this not be enough to drive the internal capacitance?

I look at the spec sheets for the ECC85 see it is for an RF amplifier, see quite low C's and just think if I set the valve at such a point it should be able to do it, but think I am missing something basic.

Adrian

Just noticed I have drawn the X position pot wrong on the RHS the 4m7 resistor goes to the wiper.

[Ed_Dinning]

Hi Adrian, nothing obvious stands out (others may see something).

I was thinking of the booklets for home construction and ex gov conversions. They often explained the defects (slow flyback etc) rather well and often showed solutions.
Usually used octals, of which I have plenty if you need them.

Timebases by Puckle and the cathode ray tube bu Parr & Davie are good works of reference

Ed

[AdrianH]

Hello Ed.

The slow flyback has me baffled as it looks perfect on the grid of the ECC85, I have 4 of them and all the same so can not put it down to an individual poor valve, they may all be a bit low, but are operating close enough to the graph spec to makes no difference.

The type S51 scope uses ECC88's as a phase shifters driving the deflection plates, for some reason I had poor results on the second anode if I used similar so switched to the see-saw phase inverter on the valve museum pages, I got the second anode swing but nothing seems to deal with the slower flyback response.

I temporary wired up a sync input from and anode of my Y amp to the grid of the 6D4 at point (A). This enabled me to lock the saw-tooth to incoming waveforms at various frequencies, just to prove to myself it can. Now back as original.

And at the moment I am shoehorning another EF91 on the board so that I can amplify the discharge pulse going though the cathode resistors on the 6D4, there is a very fast 2 Volt pulse across the 47 Ohm resistor, so hoping I can use this when amplified and inverted through the EF91 to provide some flyback blanking for the CRT and to be able to drive something to 'Ping the air'.

It is all interesting, not everything works as expected and I do not sit down and draw out loads of circuits before hand, just look at the valves I have accumulated and dive in.

I will have a look for the articles you mention on line as it may shed some light on the subject for me.

I tend to stick with the 7 pin and 6.3 Volt 9 pin valves, Octals I have very few holders or valves other then efficiency diodes from old TV sets. I doubt very much I will become a collector, just a tinkerer and builder when I can.

Cheers

Adrian

[AdrianH]

I finished the extra stage last night so the 2 volt positive pulses are now 150 Volt negative pulses still very fast and can be 'loaded' even with sticking my DVM on at the same time as the scope dropping to around 60 Volt peak. Pictures are of 20uS per division and a single pulse captured at 5uS per division.

I would like to square and lengthen (variable) this pulse if possible, in digital mode I think use of a monostable would do it, but stuck on it using valves, guessing on some CR timing and clever biasing?

Adrian

[Ed_Dinning]

Hi Adrian, mention of TV and monostables makes me think of the Philips Tech library series, possibly on line somewhere, a few dozen books covering all aspects of Valve, radio and TV design. There must be an answer in there to your problem.

Ed

[Radio Wrangler]

Christopher, 'High Vacuum House' has some CRTs up for sale right now. One of them is the oddball radial deflection tube.

David

[AdrianH]

I have not long been back from Christopher's, I picked up a Solartron scope for £5 from his list, he also mentioned the CRT's when I was there so I picked up one of the DG7-5's. Yes lots of odd ball tubes but I would struggle with some of the bases so left the others well alone.
Then to Warrington to collect some more TR1998 military gear from an Ebay purchase, so sat here now with a coffee having a rest.

I was reading the Cossor manual for the 1039M last night and what an interesting way to generate a time base, Now that is a scope I think I would like to own, just because of it's simplicity.

OK Ed on the articles, think I will start searching the Radio and Broadcast History library for articles. With google or duckduckgo I end up on anything but valve/tube tech.

Supping coffee and chill time.

Adrian

[AdrianH]

I am sitting in the corner with the hat on.

The slow flyback is down to capacitance, when I took the leads off going to the CRT the slow-flyback times on the anode of the valve improved a good deal, well to around 7 uS from 12 uS.

So in my head I am seeing a mixture of things I believe to be wrong.

Using twisted cable from the anodes to the CRT, The CRT deflection plate capacitance is listed as 0.8 pF, well using my tester I seemed to have added an extra 22pF with the twisted cable, so will try and get hold of sort short lengths of 450 Ohm ladder feeder to use instead.

My construction using tag board holding the components and then having wires going down to the valve holders has probably added a few more pF then needed.

I am assuming that having 22pF between the anodes is not helping as one anode goes positive the other going negative is completely negating the internal screen in the valve. But does it add to the Cag capacitance with the miller effect I can not understand, as I am not seeing the slow effect on the grid of the ECC85.

Adrian

[Ed_Dinning]

Hi Adrian, yes I thought of you when I saw that special tube.

Scope tube capacitances are usually low and the bases should add little. Twisted leads to plates are not a good idea, I've seen professional scopes that used a wider spaced version if twin feeder as well as low capacitance coax

Ed

[AdrianH]

Hello;

I redid the components around the ECC85 socket, taking what I could direct to gnd from the pins. For a temp co-ax I have used two wires trapped in insulation tape to make my own ribbon cable. 7 uS flyback is the best I can achieve.

Next I added a silicon signal diode and 18pF cap, the diode feeding from the 47ohm resistor with the 2 Volt pulse, to the grid of the pulse amplifier (EF91), the 18pF cap between grid and cathode.

This gave me a longer pulse width of just under the 7uS, this is then applied to the grid of the CRT via resistor and a ceramic variable trimmer. Using the trimmer rather than a small cap as there is around 800 Volts across it, non of my normal caps are up to that working voltage.

Last, although I need a better variable pot I put the sync back on to the 6D4 from the Y amp output and the results is the two pictures shown below: -

This is fed with a sine input at 110KHz, not perfectly linier but should be good enough for my playing, I am happy to get this far.

The Y amp to deflection plates is still twisted cable so must sort that out at some point, will probably get a better freq response when I do.

But definitely enough for tonight, must now tidy.

On the CRT, yes saw it and left as was, its bad enough with what I have got.

Cheers

Adrian