Problem with Crotech 3131.

[erchiu1971]

Hi everyone.

I have an oscilloscope Crotech 3131.

Until this morning it has operated very well, but after a black out it has a problem. The trace on the screen shows a signal without a probe connected. This waveform has a 100 Hz of frequency and its amplitude changes with the position of the trace. If I connect the probe, it's added to this signal.

Somebody can help me for solve this problem. You have some schematics circuit of this oscilloscope?

Thank you in advance
Best regards
Erchiu1971

[ukcol]

Hi enchiu

You have probably got a ripple on the supply to the Y-amplifier.
PM me your email address and I'll send you the manual so we can follow component numbers.

[erchiu1971]

hi colin,
I sended you my email address in mp
Thank you for your disponibility
Regards erchiu

[ukcol]

Manual sent by email.

The first things to check are the following supplies.

+150volts check C322 and C323.

+24volts check C325, IC302 and C326.

-12volts check C340, IC303 and C328.

+12volts check C341, IC304 and C342.

I am not sure that the Y-amplifier uses the +12volt supply but check it anyway.

There is also a +5volts supply but I haven't located its source yet.

[erchiu1971]

hi colin,
sorry if i write only now, but when you sent me the schematic i was going to work and only now I was able to do the checks that you suggested.
first thing
on my oscilloscope, there are not capacitors in the output of the three voltage regulators:
C326 and C327
C328 and c329
C342 and C343
even if they are marked on the pcb.
I did the measurements with a multimeter (I do not have an other oscilloscope), putting the negative ground.
positive on the C323 I measured 119v DC (200v end scale), while input to the C322 I measured 175v dc
output to IC302 there are 24.5 vdc
output to IC303 there are -15.0 vdc
output to IC304 there are 12.0 vdc
I repeated the steps several times to be sure they were accurate.
I attached some photos showing the problem in the different position of the trace without probe connected
what do you think?
regards
erchiu

[maninashed]

100hz hum on trace is usually a smoothing capacitor gone bad or open circuit. If it was 50hz hum, I would check the bridge rectifiers in each of the the power supply outputs. If you don't have an ESR meter, disconnect each smoothing capacitor in turn and substitute a known good component and check results for each power rail.
Make sure you ground both Y inputs before checking results.
Bill

[erchiu1971]

hi bill,
I have an digital instrument for measurement the induttance and capacitor?
Can i use it for these check.
Sorry, but i not understand when you write: make sure you ground both y inputs before checking results?
Thank you so much
Regards
Erchiu

[ukcol]

Hi Erchiu

The 115 volt supply being slightly high is not significant because it is an unregulated supply but the 12 volt negative supply being - 15 volts is wrong.

I think you almost certainly have a fault IC303. Note it is a positive regulator wired for negative output, so use a 7812 for replacement and not its negative version the 7912.

[maninashed]

Erchiu, Colin is probably right about the 7812 voltage regulator.
When I say 'ground inputs' I mean connect centre input connector to chassis/earth, so the input does not 'float' and pick up hum. A capacitance meter is no use for checking the ESR, which means 'Equivalent Series Resistance', this requires a special instrument, but if you don't have one just use a known good capacitor of the same voltage rating and value and connect this in place of the original smoothing capacitor. Change the voltage regulator first.
Bill

[erchiu1971]

hi colin
i done the measurement again in this moment:
i connected the negative probe of voltmeter to gnd of oscilloscope and with the positive terminal on the follow points:

on IC302 (ua7824)
input 26,7v (first pin from left)
0 volt (central pin)
output 23,8v (first pin from right)

on IC302 (ua7812) connected in negative version

input 1.5v (first pin from left)
-15v (central pin)
0 volt (first pin from right)

on IC303 (ua7812) positive connected

input 17,5v (first pin from left)
0 volt (central pin)
output 12,01v (first pin from right)

i attached a photo of the three voltage regulators
the IC303 is ua7812 and is the central of three.
thank you
regards
erchiu

[erchiu1971]

HI maninashed,
read only now your post
thank you
regards
erchiu

[maninashed]

Yes IC 302 is faulty, replace it.
Bill

[ukcol]

Hi Erchiu

With reference to your second list of voltages as in post 10.

Make sure you have the negative lead connected to chassis (is that what you mean by grid?) when making measurements.

First the 24 volt regulator (IC302). The input voltage seems a little low which may point to a faulty reservoir capacitor C325.

The 12 volt negative regulator (IC303) the output of -15 volts is wrong and points to a faulty IC303 which I mentioned before. The strange input voltage is due to the fact that the positive regulator is wired upside down to give a negative output and the small voltage you are reading is the voltage drop across the regulator.

The voltages around the 12 volt positive supply (IC304) look healthy.


To sum up, I still think that a new 7812 IC in position IC303 will cure your fault. The slightly low input at IC302 is probably not going to give you a fault symptom.

[erchiu1971]

hi colin,
i removed the IC303 (ua7812 negative configuration) and i tryed on my workbench.
with any voltage in input, measure same voltage in output without regolation.
so i replaced it, with a good component .
when i turned on the oscilloscope, i not believed to my eyes.
my oscilloscope is come back as before.
i'm very happy for this and i thank you and everyone that helped me to solve this problem.
i hope that this thread can be to helped to other people.
i attached some photos done after the reparation :
one is without probe..gnd position (perfectly horizontal)
one is the waveform of my electric line 50 Hz
one is the component faulty (IC303=ua7812)
.......as we say in rome....you have a beer paid
thank you so much
best regards
erchiu

[ukcol]

Excellent.

I am glad you have got it going OK again erchiu, they are quite a nice general purpose scope. I have one that I use all the time. It let me down once when it had a fault due to a short circuit tant capacitor; apart from that it has been reliable.

My guess is that a spike on the mains, when the power went down, killed the 7812.

[erchiu1971]

hi colin,
Thanks to you if it is come back to operate.
I think that without your schematics i not will have solved the problem.
Thanks you so much to you and all excellent forum.
best regards
erchiu