[sdiy] Old 20Mhz scope high on LSD!!

[Roy J. Tellason]

On Monday 27 December 2004 01:32 pm, TIm Daugard wrote:

> I too am interested in comments. Aren't the final plate drives relatively
> high voltage? Or are they on the order of 50V and under. Every schematic
> I've see has the intensity and focus in the high voltage plate line. I
> would love to improve focus and add Z axis modulation.

There were a number of old (tube) scopes that I've messed with over the years,  
and they all had the same basic arrangement -- the "high voltage" was a 
_negative_ supply,  applied to the cathode of the CRT,  and there was a whole 
string of stuff,  resistors and controls,  which typically included the 
brightness (intensity) and focus controls.  This was done so that the 
amplifiers that were driving the deflection plates wouldn't have to be too 
far away from ground potential.

If you're trying to refurb some old scope like that it's not uncommon for 
something in that string of stuff,  typically high-value resistors,  to 
change in value and mess everything else up.

The other thing I did with those was to change out all of those wax-coated 
paper caps,  for mylar.    :-)