[SOLVED] Problem with oscilloscope

[erchiu]

hi everyone,
I have a problem with my oscilloscope.
it shows on the screen a signal also without a probe.
Is it possible to make some regulations?
thank you,
erchiu

 

[barry]

Re: problem with oscilloscope

Sounds like your scope needs to go to the scope hospital. You shouldn't see ANY signal with no probe connected. What is the amplitude/frequency of the signal? Is it noise or something periodic (maybe line frequency?)

 

[erchiu]

Re: problem with oscilloscope

thank you for the answer.
the signal that i see has an frequency of 100 Hz (very precise), but its waveform and its amplitude changes as a function of the position of the track, as shown by the photos.



when i connect the probe signal is added to this by altering the original.
What do you think is a serious or is this some calibration.
it has worked very good until this morning, when there was a lack of electricity on the grid
at that time the oscilloscope is power on.
thanks for the help you're giving me
erchiu

 

[BradtheRad]

Re: problem with oscilloscope

The waveform looks like rectified AC mains hum.

Your power supply appears to no longer be adequately filtered. Perhaps one or more capacitors blew when grid power was restored?

 

[erchiu]

Re: problem with oscilloscope

i opened the chassis of the oscilloscope,
but i don't see some components damage.
there are many tantalium capacitors. i think it's the input voltage.
these are the photos of the interior

thank you erchiu

 

[Audioguru]

Re: problem with oscilloscope

Be careful inside the 'scope because parts of it have thousands of volts that is deadly.

It has some very old parts like the 741 opamp in an old metal case in the fist photo. In those days my little tantalum capacitors simply disappeared but left their pins behind. Look for large electrolytic capacitors with swollen tops.

My 'scope is older at about 48 years old and it still works fine.

 

[erchiu]

Re: problem with oscilloscope

I looked inside but I have not found electrolytic swollen.
There are many tantalum capacitors scattered throughout the oscilloscope.
the pcb where could be the problem is that the first picture. this is very near to the transformer.
thank you
erchiu

 

[tfinks]

Re: problem with oscilloscope

I looked inside but I have not found electrolytic swollen.
There are many tantalum capacitors scattered throughout the oscilloscope.
the pcb where could be the problem is that the first picture. this is very near to the transformer.
thank you
erchiu

Do you observe the problem with the input shorted, or the probe shorted to the ground connection? Do you see the 100 Hz signal on top of an applied signal (like a square wave from a generator)?

 

[BradtheRad]

Re: problem with oscilloscope

Looking at your first image...
A diode or zener has a charred ring around its middle, as though the inner coating was zapped away. (It is near the label R548)

----------------------------

Your main supply may still be filtered, but it appears your vertical amplifier is getting some amount of rectified AC. Its polarity shows as + or -, depending on whether the beam is above or below the zero line.

The vertical plates should get a DC voltage. (On my CRT scope it is 100VDC.) Your plates are getting some DC, but they are also getting some additional rectified AC component.

Look for the vertical amplifier circuit, and examine components which supply power to that circuit.

You may end up having to clip capacitor leads, so that you can test them out of circuit.

 

[erchiu]

Re: problem with oscilloscope

i observe the problem without probe and the selector on gnd position.
when i connect the probe the signal that i must to measure is added to this waveform.
the frequency there is also in position gnd
as can you see from the photos, there are 2 divisions with 5 ms time base = 100 Hz
as i can find the vertical amplifier circuit?
thank you
erchiu

 

[BradtheRad]

Re: problem with oscilloscope

You'll need a service manual for your scope. Or else look on the internet for schematics (if they are there).

You'll need to examine the schematics, and search for circuitry which mentions:
vertical deflection
vertical plates
vertical amplifier

That should lead you to the components which are suspect. Or to the power supply which is suspect.

If you cannot obtain schematics, then you can try looking on the pcb for a label with the word 'vertical'...
But only if you are very lucky will you find the faulty component.

Or, follow the wires from the vertical position knob, vertical gain, etc.

 

[erchiu]

Re: problem with oscilloscope

ok, i will try to look again the pcb and i will search an schematics of this scope. I will hope to find it.
Thank you
Erchiu

 

[barry]

Re: problem with oscilloscope

I would suggest getting a schematic and then probing the power supply lines with your oscilloscope. Oh, wait... ;-)

As suggested, it might be a bad capacitor (even if it doesn't look swollen). It could also be a bad rectifier diode/bridge. But it's fairly evident that it's a power-supply issue since it's exactly 2x50Hz. It looks like a fairly old scope, and electrolytics do dry out over time, so it could just be coincidence that the failure occurred at the same time as your power outage.

 

[erchiu]

Re: problem with oscilloscope

yes,
it is an old scope, but until yesterday it worked very well.
the frequency of noise is 100 Hz perfectly.
on pcb i could not read the word "vertical", but i found three voltage regulator IC :
two ua7812 and on ua7824

near these components there are several electrolitic capacitors:
three of these are 47uF/250v while other three are 1000uF/35v
can be this the section of supply?
regards
erchiu

 

[barry]

Re: problem with oscilloscope

Those are definitely part of your power circuitry. But your vertical power supply is on the order of thousands of volts. Unless you can borrow another scope, you might just check the output voltages on those regulators with a meter. You can verify that the DC level is correct, and you can then switch your meter to AC and see if there's any ripple on the regulator outputs.

And judging from your pictures, it looks like either that scope has had some work done to it (or else it was assembled by the worst electronics assembler in the world)

 

[BradtheRad]

Re: problem with oscilloscope

Closeup of the diode I referred to earlier. It looks as though an inner wire vaporized, etching away the coating around the middle. (I'm not saying it is necessarily the cause of your problem.)

 

[erchiu]

Re: problem with oscilloscope

on the pcb the are other zener diode as that you mention. i think that are in this mode.
i done the measurement after the regulators voltage and i read:
24.5 after ua7824
12.0 after (one of ua7812)
and -15.0 after the other ua7812, but in this case the measurement from common and gnd)
the capacitor 47uf/250v can be supply of the vertical amplifier
thank you
erchiu

 

[E-design]

Re: problem with oscilloscope

If you have a good meter like a Fluke (not all meters block DC in AC mode), put it in AC mode and measure across the electrolytic capacitors in the PSU. The meter will give an indication of the ripple voltage present on the capacitor being measured. If it is unusually high compared to other capacitors, it is a good indication that the capacitor is suspect.

 

[erchiu]

hi everyone,
i checked one of the voltage regulators on my workbench and i found it faulty.
in output i found same voltage of the input.
so i replaced it with an new component and the oscilloscope is come back as before.
now on gnd position the trace is perfectly horizontal and with an signal on the probe, it is shown perfectly.

Thanks to all for the help you have given me
best regards
erchiu

 

[E-design]

What make of scope is this?