[GreenKeys] CRT Question

[Jim Haynes]

On Fri, 21 Aug 2015, Lester Veenstra wrote:

> In the Altronics Howard Model L they took the channel filter (Mark and space
> filters) and passed them to a dual channel amplifier, driving the
> oscilloscope plates, However, across the plates were an additional high q
> tank (the usual 88 mhy torroid based). Since this was a very high impedance
> point (the scope defection plates), the high Q allowed a very narrow
> bandwidth. Therefore, instead of the usual sloppy bananas tuning display, he
> was able to present a clean crossed line display that allowed unambiguous
> tuning.
>
This kind of thing can be overdone.  With simple tuned circuits you get
crossed bananas (or an airplane) and you can tell which direction you need
to tune if you are off frequency a little.  The Electrocom 440 model
uses signal strength in the channel filters to modulate the amplitude
of the signal sent to the scope.  The result is a perfectly sharp cross -
but if you are off frequency the arms of the cross get shorter but you
don't know which way to turn the tuning to correct the problem.