Crypton oscilloscope circuit operation explanation.

[ScottyH]

Forgive me if this is not the right place to ask, but I have a circuit from a 50's automotive oscilloscope I am having trouble following.

Would it be OK to post a sketch here to see if someone can explain what should be going on?

Thanks,

Scott.

[trh01uk]

Its always OK to ask for help. Obviously we need to see the schematic in question, and there are lots of people on here happy to give it a go!

Richard

[Radio Wrangler]

There have recently been threads on a couple of Sun analysers form the 60's, and a bit further back a Crypton one. You may find something interesting with a bit of searching. The electronics of the instrument is OK on this forum, but please avoid engine and vehicle discussions. There are plenty of fora for those and we're trying to avoid everything merging and making information impossible to find.

The family business used to have an old round-screen Crypton machine.

David

[ScottyH]

Thank you. Bear with me, the only schematic is the one I have drawn out as i've found it. (There seems to be nothing anywhere for these machines...grr)

The device is a Crypton circa 1950's, and I have been chasing various ends of this and found the following:

The big capacitors (0.15uF 3.5KV) are Ok.

The 4 20uF electros on what I think is a voltage doubler circuit are good.

There are two EY86 tubes connected cathode to anode feeding the CRT (Brimar D21-102) A4 EHT "button")

It is this circuit I have no experience with. I can't figure out how it generates the voltage for the CRT A4.

Have to change devices to post the circuit so far.

Stand by.

[ScottyH]

The circuit so far. Both EY86 tubes have heater voltage and glow happily. I just can’t see how this is meant to generate several KV for the CRT anode.

[trh01uk]

At the moment your circuit is too meagre to make much sense to me either. As it stands its not "generating" anything, as there are no voltage inputs shown. Is the instrument mains powered?

Is it actually working - or do you have a pile of bits? If the latter, then what remains may well make no sense!

And, by the way, it looks like you have a typo on the schematic: short across the 150k resistor between the 2 x 0.15uF caps?

Richard

[Radio Wrangler]

I can't see where this circuit gets powered from. There seems no reason for having two EY86 in series one of them is good for 22kV. So I suspect they are part of a doubler circuit where bits haven't been drawn.

At a guess I'd expect the cathode of the leftmost EY86 to be grounded somehow, and a high voltage transformer winding to be connected via a high voltage capacitor to the node labelled "PURPLE" between the two diodes.

This will then make the usual few kV negative supply at the two capacitors feeding the resistive divider string which make the grid and cathode potentials then eventually focus potential for the CRT.... the shorted resistor also looks odd.

The doubler means that the transformer secondary voltage doesn't need to be the full HT, which makes the transformer a bit easier.

The pot etc 'horizontal hold' just uses the high voltage and a resistor to charge a capacitor (almost) linearly for the timebase.

BEWARE mains derived EHT can be deadly. Looking back, I'm not sure how I survived building oscilloscopes as a schoolkid.

David

[ScottyH]

Sorry about the short between those caps, yes a typo..

I have drawn what is physically there, the unit is complete, and looks to have not been disturbed.
There are two HV anode caps for the EY86's so I don't think there has been an incorect substitution.

It is mains powered, the rest of the circuit consists of 2 12AT7 tubes (amplifier board) and an EN91 board (Timebase?)

I guess I'll have to draw out the rest of it, but there are no connections that I can see I've missed to the two EY86 tubes.

I have measured 6.3V on the heater pins of these tubes, but did get a small, almost HF arc when the probe came close...?

Here is the circuit for the doubler. I've not measured the outputs yet, and have not checked the diodes (I can't see what they are, they possibly have the Texas logo in red and maybe BY) Both check out in situ.

I will keep drawing it out, maybe it will all fall into place.

[trh01uk]

That circuit isn't a doubler as such. Its just two half wave rectifiers producing +ve and -ve voltages w.r.t. earth. Judging by the voltage rating of the electrolytics, possibly 300V given the 12AT7 etc valves.

Sounds like its partly working if you are going to measure some voltages.

There must be something else producing the EHT. Presumably some other winding on the transformer you haven't yet spotted.


Richard

[Malcolm G6ANZ]

Attached is the circuit for the EHT section for my analyser. It dosn't generate the full EHT for the final anode but supplies a -ve 1kV and a +ve 2kV. This is enough to produce a spot. Other high voltages come from separate windings on the transformer
There are threads on here about trying to get mine to work, at present it's having a rest!
https://www.vintage-radio.net/forum/...d.php?t=171609

Malcolm

[ScottyH]

Thank you all for the input so far. I will continue to trace out the remainder, and see if there is a (big) piece of the puzzle I have missed.

Scott.

[Radio Wrangler]

With electrostatic deflection CRTs (usual sort for oscilloscopes) there is a potential difference of all the electrodes right down the tube form the cathode to the screen at the end (excluding grid 1 which is set more negative than the cathode.)

This isn't exactly a smooth progression but follows how they want to accelerate the electrons and focus them.

But somewhere along this progression come the deflection plates and there therefore is a comfortable average voltage that they should sit at, else the picture and the focus suffers a bit.

Now let's flip that business on its head and say that we have valve amplifiers - maybe - or else just a timebase, and the Y signal is an attenuated version of the spark plug or coil voltage. These are going to be not very far from ground.

So early oscilloscopes had the final anose and screen at ground, and everything was powered by around 2kV, but negative with respect to ground. THe electrons didn't knoe this, they just saw a train of anodes stretching away into the distance to 2kV more positive than the cathode they'd just vacated.

These oscilloscope tubes were a bit dim. They really needed more volts to accelerate the electrons more to make more of a visible splash when the smacked into the screen.

But they wanted to keep the potential on the plates where it was, and pushing the cathode, grid and focus anode more negative would get messy and waste more power in the chain of resistors dividing up the negative EHT.

So the answer was to leave the negative supply and the anode voltages much as they were, and the deflection plates. This left only the possibility of making the screen and that graphite coating on the inside of the more positive. So thats what was done. That graphite coating is usually called the 'Dag' short for Aquadag, the brand name of the colloidal graphile liquid that was painted onto the glass (TV people are mostly unaware of the other meaning of Dag, a shepherds term for muck sodden dangling bits of fleece!)

So Malcolm's CRT has the later sort of EHT arrangement. Your diagram doesn't yet make it clear which yours is. I never went inside dad's old Crypton machine, it just kept working. Picture brightness was OK.

David

[ScottyH]

That would have to be the most readable explanation I’ve come across. Thank you.
Looking at the Brimar data sheet I think this tube is dag-less. I can almost see through the paint coating.
Tonight I’m going to go over the transformer to see if I can find the missing HV windings. Got to be there somewhere.

[ScottyH]

I'm going to try tracing this thing from another direction. There are no other unaccounted windings from the transformer that are obvious.
I've attached a photo of the adjustment/calibration panel, am I right in assuming the HV cal voltages would be present here?

If so I might be able to trace them back to source...(?)

[Radio Wrangler]

Those "KV TEST" buttons relate to checking the calibration of the scale for ignition coil voltages... in other words the attenuation of the voltages from the engine being tested.

The deflection plates in your CRT may need a few hundred volts to deflect the spot across the screen, but the ignition pulses are many thousands of volts. So the engine analysers need no amplification to drive the Y plates, rather they need attenuation, because there is far too much voltage. So the set has two ranges of attenuation and it has twiddlers to adjust the attenuators to get the voltage scales to read accurately.

Don't touch these adjusters, you may need a high voltage calibrator to be able to set them, OR the thing may have an internal calibrator, but you need the manual before you should dare touch this area (or a lot of reverse engineering in lieu of a manual)

So those KV adjusters have nothing to do with the settings of the voltages in the electron gun of the CRT. It's just coincidence that both are in the kilovolts range.

David

[ScottyH]

Gotcha.
Leaving them alone....

[Radio Wrangler]

No dag?

Now there's a puzzle for you. You have an electron gun firing negatively charged electrons at the screen. Then where do they go?

Left to themselves they'd just build up, creating an electric field and repelling any more coming to join them.

That isn't going to work!

The answer is that the current is very low and the tube relies on conductivity of the glass and the electrons meandering back to the final anode region. It can be a bit hit-and miss.

Also electron beams repeating the same baseline will also slowly burn the phosphor.

So the first step after the original crude CRTs was to put an 'Aluminisation' coating on the inside face of the phosphor. This improved thermal dissipation from the phosphor and provided a drain path for used electrons. The coating was a very thin deposition of aluminium. Thin enough that electrons arriving weren't much impeded. You could now run CRTs with faster electrons and get brighter pictures. Faster meant having more accelerating voltage.

That business of having only a couple of kV down the electron gun to the plates still applies, so the electron gun voltages and the plate voltages stay where they were and the extra acceleration that the tougher phosphor can take has to be done with acceleration after the plates, by running the screen itself at a high positive potential. So you wind up with aluminium on the screen and dag inside the glass bulb, connected to a high positive voltage. It's called a PDA, Post Deflection Accelerator.

If you read up on CRTs you'll find lots on electron guns, focusing, deflection, beam currents etc. But if you have a suspicious mind, you'll notice something missing, that very little is said about the electrons once they've smacked into the phosphor. It's still important to complete the electrical circuit. Those electrons in the beam are an electrical current. Few people ask "Current to where?"

It's like building a house, putting in a water supply and ignoring the drains. It's going to get nasty.

David

[ScottyH]

I found this on a site where they re- purpose a Brimar CRT as a clock. My tube has no outer graphite coating, the green spiral clearly visible.

“The more difficult issue was removal of the graphite coating. During manufacture, the front-most 8 cm of the glass of each SE5F was sprayed with a conductive graphite-based paint. Why? To make a high-voltage capacitor with the spiral accelerator anode (the beautiful green stripes) and similar graphite coating on the inside of the glass. By connecting the external coating to ground, the thrifty circuit designer could avoid using a separate (and expensive) high-voltage filter capacitor in the anode power supply!”

Seeing as how there are three HV capacitors in this thing, I guess Crypton didn’t scrimp.

[Radio Wrangler]

Yes, A dag coating INSIDE the tube is at high voltage, it serves as the PDA and the return connection for the screen.

Add a dag coating on the outside of the tube as well as one inside and they make a small capacitor.... enough to work for high frequency EHT generators like TVs and later scopes have.

There is not enough capacitance achievable this way for use with a 50Hz driven EHT from a mains transformer with a high voltage winding.

Mention of green spiral and no outside dag.... does it have dag inside? that would be good. Dag outside wouldn't do anything useful, so Crypton had to fit big capacitors anyway to get enough EHT smoothing.

David

[Malcolm G6ANZ]

Can you get the model number of the tube? With that you can get the data for it, from there you can chase the voltages from the tube base/anode cap back to the supplies.