Problem with a analog oscilloscope

[willkgin]

I have bought an second-hand analog oscilloscope.
It works fine but some problem in CH1.
When I use 0.1V/Div or above to measure the calibration port (2V square wave with 1k Hz), the amplitude of the waveform will be decreasing from 2V to about 0.6V in 5 minutes. The X-axis measure is fine.
After it decreases, if I tune to 50mV/Div or below for few seconds and then repeat the measurement in 0.1V/Div, the amplitude will be back to normal (2V).
However, even I doing so, it will have some change in CH1 DC gain and I need to calibrate it by opening the oscilloscope.

Such problem doesn't show in CH2 but just CH1.

The analog oscilloscope is MATRIX MOS-640CH (40MHz).
Does anyone know what's the problem with my oscilloscope and how could I fix this problem? Thank you!

[alm]

1) Try to find a service manual for either the same model or a similar model (possibly under a different brand).
2) Start measuring power supply voltages (including ripple). Since you have a functional and a non-functional state, you could try measuring immediately after power on and a couple of minutes later when the response has decreased.

Though it is always good (and easy) to start with the voltage rails, it does not really sound like it because the power supply will likely be common between the channels. It sounds like it might be a marginal part. Does anything get hot? Since you have a functional and a non-functional channel, you can compare. It may be easier to diagnose without a second scope if you feed the channels a DC voltage instead of a square wave (assuming that shows the same problem). It can also be helpful to heat (soldering iron) or cool (cold spray) components to see if that affects the problem.

This document by Tektronix is a fairly general document that may be helpful:
http://sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

[Armadillo]

Wriggle the volt/div knob, selection sliders and the time/div knob to see if it restore or affect the traces.

If it does, clean the rotary contacts with contact cleaners.

Could be as simple as that.

[tautech]

Check the Cal knob on the V/div control is in the locked position.

As others have said exercise all the controls that are giving trouble. Doing so cleans/wipes all the contacts clear of any grud that might be affecting the connections.

[Specmaster]

As has already been mentioned on this thread, double check all the controls such as switches, pots and any plug and sockets that are on channel 1. I'm not familiar with that scope but it could be just as simple as a poor connection to a pcb or switch, do the channels share a common PCB or are there separate?

You also don't say if you are using the other channel at the same time or not, if you are, try swapping the probes over in case you have a an intermittent fault in one of them?

[willkgin]

Yes, both channel shares the same PCB.
I use both channel at the same time and just Ch1 is malfunctioning. Ch2 works perfectly.
I tried to disassemble the rotary contact and clean it. The problem is still there.
When I use 0.1V/Div or above, the amplitude decays very slowly, behaves like a discharging capacitor. When I tune to 50mV or below, it's like restoring the charge immediately and the measurement in 0.1V/Div will be normal, followed by the decaying probelm after few minutes...

[Armadillo]

Yes, both channel shares the same PCB.
I use both channel at the same time and just Ch1 is malfunctioning. Ch2 works perfectly.
I tried to disassemble the rotary contact and clean it. The problem is still there.
When I use 0.1V/Div or above, the amplitude decays very slowly, behaves like a discharging capacitor. When I tune to 50mV or below, it's like restoring the charge immediately and the measurement in 0.1V/Div will be normal, followed by the decaying probelm after few minutes...

How did you clean the contacts of ch1?

[willkgin]

Yes, both channel shares the same PCB.
I use both channel at the same time and just Ch1 is malfunctioning. Ch2 works perfectly.
I tried to disassemble the rotary contact and clean it. The problem is still there.
When I use 0.1V/Div or above, the amplitude decays very slowly, behaves like a discharging capacitor. When I tune to 50mV or below, it's like restoring the charge immediately and the measurement in 0.1V/Div will be normal, followed by the decaying probelm after few minutes...

How did you clean the contacts of ch1?

Cleaning the rotary set with a small brush.

[Armadillo]

Cleaning the rotary set with a small brush.

Can you spray Deoxit or contact cleaner on the all the contacts [plus a pencil]. The video exactly replicate your problem.

[willkgin]

Cleaning the rotary set with a small brush.

Can you spray Deoxit or contact cleaner on the all the contacts [plus a pencil]. The video exactly replicate your problem.

Thank you! Let me buy a Deoxit and try.

[Armadillo]

Thank you! Let me buy a Deoxit and try.

If you are in Hong Kong, I don't think deoxit is available there. Actually any contact cleaner will do. "WD-40 Specialist Fast Drying Contact Cleaner" might be available in Hong Kong. Give it a good cleanout. Don't spare out CH2 and all other contacts.

[alm]

I would avoid corrosive contact cleaners (you are unlikely to be able to get rid of it all), contact cleaners leaving a sticky residue or contact cleaners that do not leave any lubrication. So I would not say "any contact cleaner will do". But since I am not familiar with the brands available in Hong-Kong, I can not give specific suggestions. DeOxit D and Cramolin ContaClean are safe, but I imagine neither is available in HK.