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Old 25th Nov 2020, 3:01 pm   #1
vampyretim
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Default Farnell LF1 oscillator

Hi,

I've just recapped this old sig gen but am getting (and was prior to recapping) an out of phase second oscillation on my scope which is fainter than the main one. The oscillator seems to be a bit unstable on lower frequencies and on the lower p to p settings, ie; I get distorted waves that bounce about a bit on the x1mv and x10mv (x10 and x100).

I've been reading up on the wien bridge circuit and was wondering if there might be a problem in the feedback loop hence why I have an out of phase trace. I've never attempted to repair a sig gen before and was looking for some guidance, I have the service manual for it but don't know how to properly shield the oscillator circuit whilst also being able to tweak it.

Any help greatly appreciated.
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Old 25th Nov 2020, 4:51 pm   #2
dave cox
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Default Re: Farnell LF1 oscillator

Your scope may be struggling to trigger properly on the low output ranges. Try connecting the 'sync' output to the scope 'trigger in' if you haven't already.

Wien bridge circuits do take quite a while to stabilize, its not unusual to take a few seconds when you change frequency for the amplitude to settle. The 'gain control' feedback loop needs to have a long time constant to keep the distortion low so the settling will look 'under damped'*.

dc

* 'under-damped', 'critically damped' and 'over-damped' refer to control theory - critical meaning there is (only just) no overshoot! If it take longer then that then it is over (damped) and if it bounces around then its under (damped).
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Old 25th Nov 2020, 7:34 pm   #3
frsimen
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Default Re: Farnell LF1 oscillator

The LF1 has a spring contact that earths some of the case. If the contact is poor, the oscillator output around 50Hz and 100Hz will be very unstable and can fool you into thinking you have a problem somewhere in the oscillator circuitry. Check that earthing spring is making good contact in case that is your problem.

Paula
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Old 30th Nov 2020, 1:34 pm   #4
vampyretim
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Default Re: Farnell LF1 oscillator

Thanks, I will do as you both suggest.
Thanks again.
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