[time-nuts] SRS FS710 spurs at 50Hz and harmonics?

Anders Wallin anders.e.e.wallin at gmail.com
Fri Oct 2 14:29:14 EDT 2015


Hi all, thanks for all the suggestions.
I am starting to think this is a feature not a bug.
I tinkered with the FS710 a bit today. It shows about 600mVpp 100Hz
sawtooth on the +/-13.9VDC input to the voltage regulators.
I changed the big electrolytic caps and the smaller tantalum caps around
the 7805/7905 regulators - but no change to the sawtooth.
With a 1M 10x probe and a cheap scope there is no visible 100Hz ripple on
the +/-5V regulator outputs.
some new pictures in my blog:
http://www.anderswallin.net/2015/09/srs-fs710-noise-measurement/

Maybe my earlier result with strong spurs was a result of a ground loop
between REF-distribution-amp, 3120A, and this F710-distribution amp. They
were probably plugged in to different AC outlets...

Anders


On Thu, Oct 1, 2015 at 1:52 PM, Bob Camp <kb8tq at n1k.org> wrote:

> Hi
>
> It can also be a blown diode in the bridge. When that happens the ripple
> goes
> way up. A quick check with a scope should tell you what’s happened. 50 Hz
> triangle
> wave = blown diode. 100 Hz triangle wave = blown cap.
>
> Bob
>
> > On Oct 1, 2015, at 4:04 AM, Esa Heikkinen <tn1ajb at nic.fi> wrote:
> >
> > Anders Wallin kirjoitti:
> >
> >> I seem to get very strong spurs at 50Hz and harmonics with an old
> >> second-hand SRS FS710:
> >> http://www.anderswallin.net/2015/09/srs-fs710-noise-measurement/
> >> feature or bug? Anyone looked at the powersupply and figured out what
> parts
> >> to change?
> >
> > If you live in a country with 50 Hz mains network then this sounds like
> a dead capacitor in the power supply. Usually this means that it has old
> fashioned linear power supply (with classic iron transformer). In that
> case, replace the secondary capacitors after the rectifier bridge(s). Check
> voltages with oscilloscope if you wanna see which ones, but it might be
> good idea to replace them all (there should't be many of them).
> >
> > Should be very easy to fix.
> >
> > --
> > 73s!
> > Esa
> > OH4KJU
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