Re: Oscilloscope arrived - black market sucks

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Jan Wagner <jwagner-at-cc.hut.fi> 


On Sun, 4 Apr 2004, Tesla list wrote:
 > Original poster: Kreso Bukvic <kreso.bukvic-at-kc.htnet.hr>
 >
 > After playing with my new scope for almost whole night it look like it is
 > not good!
 >
 > What should i check before i beat the crap from the black market seller?
 > :-))
 >
 > What are the basic check steps to see if it is good?
 >
 > I tried conecting 9 VAC transformer at input, scope measures 15 V with odd
 > showing,

Sounds like it is working then! A 9Vac un-loaded xfmr gives out just about
anything but 9Vac.

Usually the oscilloscopes have a small pin somewhere on the front panel,
which gives out a calibration signal. 0.5Vpp 1kHz, or something like that.
You can test and calibrate your oscilloscope probes by hooking them up to
that pin.

 > ok i tried adding one diode no improwment 2 dioes nothing, greatz
 > then i get only a line it cannot be a line only with a greatz - right? It
 > cannot be full DC with only 4 diodes?

You should get full DC with 4 diodes, but you have to add a capacitor
behind the diodes. 1uF, for example.
The oscilloscope has only a few pF of internal capacitance, diodes have
100's of pF of junction capacitance, so what you might have measured is a
capacitive voltage divider => no DC output.

 > Or when i touch the table the waveform
 > gose here and there it moves by it self, scope is not fizically connected to
 > table it does  alot of crap etc and etc.

Sounds fine too, sensitive input, 50Hz mains coupling in. It's present
everywhere. Try holding a mains cable (isolated, of course) in one hand
and touch the oscilloscope probe, you'll see a huge 50Hz signal...

cheers,
  - Jan

--
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  Helsinki University of Technology
  Dept. of Electrical and Communications Engineering
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