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Amplifer Design

Greetings, gentlemen,

This is the schematic of the CRO x-amplifier I've been investigating lately. It's supposed to provide sufficient potential difference to the x plates in the tube so as to sweep the electron beam left to right in accordance with the timebase setting:

formatting link

At first I thought this was operating as some kind of complimentary pair chain of stages, but it's only quasi-complimentary and I can't fully understand how it works. The signal path is shown in slightly heavier lines. It starts off conventionally enough, with a sawtooth sweep waveform of approx half a volt applied to the base of Q10, but then why does it then passed to the *emitter* of Q15? This is the part I'm struggling to understand most.

What is the name for this type of configuration and why is the input signal to Q15 being applied to its emitter rather than its base? Thanks.

(if it were *truly* complimentary/push-pull then I'd have thought there should be a *minus* 158V supply for the final stage of the lower chain, but it states +158V which makes no sense to me).

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Reply to
Cursitor Doom
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Q10 and Q15 are a diff-amp or long-tailed pair. Q11 is the tail current source.

piglet

Reply to
Piglet

Why do you post that on a Russian company's servers, rather than dropbox?

Aren't you allowed to use/access dropbox or another western company's servers?

Reply to
Tom Gardner

Also, is your name Paul B. ?

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Yup, as in Burridge.

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Reply to
Cursitor Doom

Well I did suspect that initially, Erich, but the input signal is shown only applied to the base of *one* of those transistors!

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Reply to
Cursitor Doom

I couldn't see that at useful resolution.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
jlarkin

Hi Paul, nice to meet you!

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Gleichfalls, Win. :)

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Reply to
Cursitor Doom

I know; it's a PITA but there's nothing I can do as I pulled it off a website myself. I've posted 2 versions, so you might see it better with the enhanced one where I've tweaked things up a bit.

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Reply to
Cursitor Doom

FWIW, on the image and then choosing "View Image in New Tab" from the popup menu, made the schematic readable for me. YMMV.

Thank you, 73,

--
Don Kuenz KB7RPU 
There was a young lady named Bright Whose speed was far faster than light; 
She set out one day In a relative way And returned on the previous night.
Reply to
Don Kuenz

What scope model did it come from?

--
 Thanks, 
    - Win
Reply to
Winfield Hill

To be strictly accurate, it's actually an HP865A RF spectrum analyser. Not that it makes any difference to the question in this instance. In fact the y-amp uses *exactly* the same board as the x; something I'm hoping will assist me in identifying the faulty component. So rarely does something like that arise which can actually be invaluable in fault tracing!

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Reply to
Cursitor Doom

Can you recheck that HP model number?

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Sorry, Win, it's actually 8565A.

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Reply to
Cursitor Doom

  • Q10 / Q15 is a differential pair; Q11 is the current source.
Reply to
Robert Baer

That's how a diff pair is supposed to work, see e.g. Win's excellent book.

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-TV
Reply to
Tauno Voipio

I shall! Thanks, all.

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Reply to
Cursitor Doom

There's a 236MB file of the HP 8565A Operating and Service Manual, dated 1977, 424 pages, on the web. Unfortunately, it's locked, so I could not extract the pdf schematic page. But I expanded it, and made an image of the HV output amp.

formatting link

The 1st diff-amp stage has 562R emitter and 2k15 collector resistors for G=3.8. The 2nd stage has 750 + 500-ohm trim emitter resistors and a current output. The HV output stage has a 19k6 feedback resistor, for G = 15 to 26, 19 nominal. A 1pF feedback cap would set the video bandwidth at 8MHz. I imagine Zout is less than 1k. Push-pull PNP & NPN BJTs, running at 7mA, and dissipating 1 watt, go from 2V to 140V. There are two complementary amps, for two deflection plates.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

And a splendid image it is too. You're obviously way better than me at enhancement.

It seems odd to me that there's no minus 158V supply for the final voltage amp. It looks like they're relying solely on attracting the electron beam from side to side with positive voltages swapping back and forth between plates rather than a combination of attraction and repulsion as I would have intuitively expected. :-/

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This message may be freely reproduced without limit or charge only via  
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Reply to
Cursitor Doom

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