Amplifer Design

The credit belongs to the person who scanned of the manual. The schematic is on one of those super-wide foldout pages, which have to be scanned in sections and re-combined. The pdf's quality is as good or better than my screen capture.

The differential current into the output stage summing junction is -3.6mA, so with the 19k6 feedback resistor, the nominal output voltage on each side is +70 volts. As one side swings up by +50V to +120 volts, the other side swings down by -50V to +20 volts. The deflection electrodes see a nominal 0 volts, and swing from +100 volts to -100 volts, more or less, set by the gain trimpot.

OK, many thanks for that. Whilst I'm absorbing your analysis, I've discovered a problem with Q8. The base-emitter and base-collector voltages are both around -0.68V and static. OTOH, Q8's compliment Q13 has base-emitter voltage of 0.5 - 0.7V cycling steadily in line with the timebase as it should and a healthy base-collector voltage of ~11V. Clearly Q8 can't operate with those voltages and it looks like it's got a significant C-E internal short. Can't say for certain without hooking it out of circuit, but there's not much else in that area it could be!

That's a net peak-peak deflection-voltage swing (difference) of 200 volts, or nearly 300 Vpp at the amplifier's extreme limits. The 7mA current output capability of the amplifier means, if the plate capacitive load, plus the two BJT's Cob, is say 30pF, that the maximum output slew rate is S=i/C = 230V/us. Therefore it can slew the p-p voltage of 200V in under 1 us. That's a pretty respectable performance for a simple, low-cost circuit in 1977.

Indeed. In fact I think IIRC I read somewhere that CRTs beat liquid crystal hands down in this regard - although that's more to do with the superb agility of an electron beam I would imagine.

The video output-stage's modest 1-watt class-A power dissipation is nominally divided between the two BJT's, but a fault situation could force most of it to be in one part. That's too much power for a modern TO-92 or SOT-23 type, but the metal-can parts in the old days had no problem. Furthermore, a clip-on heat sink could easily be added. HP affixed rather large ones to the TO-39 parts. HP only gives HP numbers, but it's safe to say the manufacturer's original is no longer available. Now I struggle with high-voltage SOT-223 or TO-126 parts for my HV amplifiers, if I can find them.

Indeed they did. Here's the board in question and you can't miss the four final transistors with their rather generously-proportioned heat sinks:

This is the kind of layout I like: no smds and plenty of space between the components to probe connections. I can't do that with anything built after SMDs came in, I lack the fine motor skills required.

They have an odd practice of quoting either one or the other. So the first four TO-18 tin can transistors are shown as 2N3251 devices, but the next stage uses "4-404 826" BJTs and goodness knows what they are. I'm not even going to bother to stick that into Google because I know it won't throw up anything helpful at all. But on the plus side, I know enough about the functioning of the circuit now to be able to drop a couple of closely-matched jellybeans in there in place of Q8 and Q13 and in this x-amp at any rate, given its modest task, will quickly be back in business again. All this old vintage stuff remains so readily serviceable. :-)

I really, really admire your resourcefulness, Jan. You clearly have an excellent grasp of your subject. However, why make life hard for yourself? I mean a 10Mhz scope and mixing down??? If you're working with GHz stuff then surely you can afford something a lot better than that! I had a Belgian friend once many years ago and he frequently said the Dutch were "careful with their money" (shall I say it tactfully) and here you are now proving it! :-D

On a sunny day (Mon, 14 Oct 2019 18:31:55 -0000 (UTC)) it happened Cursitor Doom wrote in :

Na... farmers are revolving here against being forced by the government to cut there herd to save on CO2.. They occupied gov buildings and got what they wanted...

I was not referring to mixing down to the 10 MHz. I was referring to mixing down 2.4 GHz to 1.5 GHz or so for use with an RTL_SDR USB stick as spectrum analyzer, that I did. OTOH if you scope the 470 kHz or 9 MHz IF of a shortwave radio with a 10 MHz scope then you can see a lot about the modulation. It is the experimenter, not the instrument. I do remember .. well getting big boat_anchors in your shop may impress potential customers :-)

You never say what you want to do with your 'instrument' if you are a collector OK...

My suggestion to you is: design one. Start designing a simple scope, look at how others did it, take it from there.

So, as to GHz:

Build something.

Ludicrous.

I couldn't possibly bring myself to use such a device. Those things are an abomination in the sight of God. I only ever use massive, discrete items of test equipment that are terribly inefficient and require a *lot* of bench space to set up and operate. I wish I could be more like you in this respect, but I can't.

I seem to exhibit a magnetic attraction towards boatanchors and the heavier and bulkier they are the stronger the attraction. A bit like gravity.

Metrology has always been a major interest of mine, ergo anything that measures something else is ripe for acquisition. This fascination goes back a long, long, long, long way and probably stems from late 50s/early

That's an excellent suggestion for an intermediate level electronics student, Jan. There are so many key aspects of the subject that one can learn from building such an instrument. In my case, however, that would be absolutely inexcusable behaviour as I already have 13 commercial scopes and I need to get rid of at least ten of them. Any other suggestions?

Why change both if only one has failed??

The two BJTs are npn and pnp types and cannot easily be closely matched, nor do they need to be.** Plop some parts in there and let us know what happens.

It wants a sign in or something so I will never see it unless you get some hosting. I was going to but I figured I don't really want to show people s*it anyway.

It is not complimentary it is differential. Don't conflate the two and maybe you'll get it.

On a sunny day (Tue, 15 Oct 2019 17:21:46 -0000 (UTC)) it happened Cursitor Doom wrote in :

Very powerful, today they are blocking main highways by driving trackers (thousands) side by side on all lanes...

The army has been called in in The Hague to keep them from getting to the city center.

Let us be real, the anthropogenic climate hoax forces people to do huge investments and damages business. I am with the farmers 100%!

So better invest in higher dikes and more power stations for aircos etc while it is still possible and technology is not lost.. Eventually climate driven mass migration will cause major changes in human population.

Do you have a cellphone? As a kid we tried two empty cans with a stretched wire in between as walky-talky :-)

Seems we are from similar ages yes, but me myself I was happy when transistors became available.. I never had much test equipment, what came was what I build myself.. Later at work we had nearly unlimited equipment (National TV network), hundreds of scopes, Teks, vector-scopes, any test equipment you can imagine.. I had fun in my school days building my own scope and then later designing some myself.

LOL not everybody is the same, I am fighting with software. Real fun, my systems got attacked by one of the most evil ones, the well known Jan Panteltje. I bought a 2400 W water boiler, for coffee that is. Plugging it in did indeed boil 1.5 liter water in about 3 minutes, but it also caused a dip in the mains that in turn caused my main PC reiserfs to see bad sectors, Rescue failed. Disk works OK now but lost lots of data, yes I make backups, but not of everything, so I moved to the laptop now as main PC while thinking what to do with the rest. One can buy a no-break supply, but maybe I will simply buy some more hyper super laptops as those have automatic battery backup... All that to make more coffee real quick, and actually: Coffee or caffeine does NOT keep you awake I found. Or maybe I will just add some Raspberries running from some huge lifepo4 cells connected to my solar panel.

When government is replaced by angry farmers and temperatures reach boiling point due to climate change I will ask Elon for a seat on his next mars mission. It would be just the same situation, hopefully he brings a nuclear reactor along for the water maker.

Indeed so. I'm guessing the hardest part is going to be finding PNPs in my junk box that are fast enough. And I have a *massive* junk box. But for some reason it never seems to be quite massive enough. :(

Sorry to hear that; serious PITA. Try plugging in your kettle to an outlet on the opposite side of your house/apartment. Line impedance is your friend. ;-)

so many failures are due to water haters being unnecessarily powerful.

NT

Why? Does rabies make them bite into the mains cable?

Got it here in English from a trusted news site:

Good for them! Someone needs to take a stand against this Globalist madness. Those bastards will never be happy until we're all eating insects - and that's no exaggeration. They're already trying to make it an acceptable alternative source of protein. But *they* won't be eating any flies, oh no. It'll be the finest steak reserved just for them. Such is the way these self-entitled 'elites' think.

Any old PNP will be fine, tit doesn't have to be fast, the collector is going into a summing junction. Even a 2N3904 / 2N3906 will work fine.