Tek 555 was Re: Need pointers on Oscilloscopes - test-drb@ccmp.vtda.org

15 Jun 2006

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Subject: Re: Tek 555 was Re: Need pointers on Oscilloscopes
   From: ard at p850ug1.demon.co.uk (Tony Duell)
   Date: Thu, 15 Jun 2006 02:03:55 +0100 (BST)
     To: cctalk at classiccmp.org

 On Saturday 10 June 2006 06:03 pm, Tony Duell wrote:
  Somewhere I have the issue of Practical
Television magazine which gives
 construction details for a 1 valve + CRT scope. OK, there are some
 semiconductor rectifiiers in the PSU too. The valve is for the timebase,
 the Y input goes straight to the CRT plates. But it was useable for the
 signals in vavle TVs of the time, and a lot better than nothing.  
 Bet you could do the timebase without a valve,  too -- I was looking seriously 
 at a circuit that used a couple of neon bulbs as a triggered devices in a 
 relaxation oscillator some years back... 
I think the single valve was a thyratron (gas-filled triode) used much as 
you suggest.

 What did they use for a CRT?  I have a 3" one in storage that as I recall 
 wants something like 1000V for an accelerating potential,  and a transformer 
 to develop that isn't exactly trivial. 
I think it used a normal valve HT transformer (which would give, perhaps, 
700V between the ends of the secondary winding), with a voltage doubler 
circuit.

-tony 
Over the years I've bult a few 'scopes for special uses and a 2" crt needs
anywhere from 700-1500 for good focus and brightness (also depends on the 
tube).  The upside is you only need maybe 2mA so a voltage multiplier is 
reasonable.  One I built used a quadrupler from a 200V transformer that 
also ran the 5 tubes used (mostly 12ax7 and similar).  The CRT was 3AP1
(3") that gave ok brightness at 1000V.

At the other end I built a small scope for a HB spectrum analyser using 
a D170 (about 1.3"h x 1.5W) square tube that was really decent looking 
at 800V.  For that I used an old surplus audio transformer as a stepup
using a pair of transistors in the classic self excited power oscillator.
With a voltage multiplier it was easy to get the needed CRT voltage
and because the osc frequency was around 800hz the caps were smaller 
too.  The H and V amps were transistor and bandwidth tested out around
5mhz with no special effort as it was way more than needed.

It's very doable and with modern HV diodes and transistors it's pretty 
straightforward.

Allison