Quickie Oscilloscope Circuit Technique Query!

[Chris55000]

. . .Just a quickie one on Oscilloscope Circuit Techniques I haven't seen described anywhere to my knowledge!

. . .Can anyone suggest the advantage of using a dual–gang X or Y Shift Potentiometer with the ends of the tracks cross–connected and the wipers connected either directly or by series resistors? It crops up in many of the manuals for the older Solid–State British designed Oscilloscopes!

. . .If anyone can point me to a descriptive reference to this method of wiring Shift Controls I'd be very grateful – the Home Built Oscilloscope I'm currently working on uses this technique for the "Y Shift" but not for the "X Shift" control!

Chris Williams

[Craig Sawyers]

Because the deflection plates are driven differentially, so you need a differential x and y shift control, in both directions. For that you need a cross coupled shift design.

See Tektronix golden era Horizontal Amplifier Circuits and Vertical Amplifier Circuits from

https://w140.com/tekwiki/wiki/Concepts_Series

Craig

[Chrispy57]

Thanks for that reference Craig, but having had a quick look I'm a bit mystified, perhaps you can help? I started by looking at the Horizontal Amplifier option;-

https://w140.com/tekwiki/wiki/Concepts_Series

https://w140.com/tekwiki/images/d/df/062-1144-00.pdf

PDF page 22/108

P18 Fig. 2.1, R11 - one half of a dual ganged 20k potentiometer labelled Horizontal Positioning, has both ends of the track connected to ground ie the potentiometer is shorted out. The other ganged half of the dual potentiometer, R3 20k, has -150V and ground on either end, which makes sense.

Cheers
Chris

[Neutrino]

Originally Posted by Chrispy57

P18 Fig. 2.1, R11 - one half of a dual ganged 20k potentiometer labelled Horizontal Positioning, has both ends of the track connected to ground ie the potentiometer is shorted out.

This might be a mistake. PDF page 28 (P24 Fig 2.4) show the ends of the track connected to PLUS and MINUS. This potentiometer is only used in EXTERNAL mode when the other gang of the potentiometer is not used.

PDF page 64 (P60 Fig 3.12) shows the ends of the HORIZ CTR potentiometer tracks cross connected.

David

[G6Tanuki]

Originally Posted by Craig Sawyers

Because the deflection plates are driven differentially, so you need a differential x and y shift control, in both directions. For that you need a cross coupled shift design.

See Tektronix golden era Horizontal Amplifier Circuits and Vertical Amplifier Circuits from

https://w140.com/tekwiki/wiki/Concepts_Series

Craig

Yes you need a differential offset voltage, as you turn the shift control away from the centre position you need the voltage on one deflector plate to go up, and the voltage on the other plate to go down by the same amount. Hence the dual cross-wired control.