Re: 7904A readout bounce


NigelP
 

Seems odd, but let's be quite specific about what I see; the readout jumps up and then down exactly at the start of each alternate sweep; difficult to imagine that this is a thermal problem... it's almost as if a switch is being thrown one way and the other at the start of each sweep (and indeed somewhere in the sweep circuitry that is probably exactly what is happening.... by design)? Also, the effect is quite flip-flop, so not increasing with sweep position but actually at the start of each sweep; one might argue that it's coincident with each sweep gate.

What I have not yet checked is whether fully overlaying two or four alternate traces on the screen minimises the effect; in other words whether it is the shift in vertical displacement of the normal two or four scan lines that is attributing to the readout display shift.

Anyway the bottom line is that the thermal compensation adjustment process is not actually solving the issue. I can see that if I adjust (eg R131 @ 50mS sweep) to one extreme or the other I can observe readout jitter going in or out but it doesn't seem to minimise the bounce.

Nigel

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