Fixing Philips 3233 oscilloscope Y-axis Jitter

[duckhunt]

I recently got hold of a Philips 3233 oscilloscope on ebay! Its my first oscilloscope and I am quite excited to use it to learn more about analogue electronics. My current background is pretty much in just digital stuff. Unfortunately, the oscilloscope is a bit older and seems to have a few issues. I was hoping someone on the forum might be able to offer a bit of advice on figuring out how to go about fixing it.

The main issue with the oscilloscope is that both of the Y channels have this pretty terrible jitter effect on the Y-axis while the oscilloscope is placed in auto mode. I am pretty sure it should be just holding a straight vertical line.

Here is a short video of what the the jitter looks like.
https://youtu.be/AaWK73lORtg

Some interesting things I've noticed about it are that both the channels seem to move up and down at the same time. There are some really long periods where the oscilloscope will hold a straight line and then some other periods where it tends to go a bit crazy.

My first thought is that maybe this kind of issue could be caused by something like a bad capacitor but that is just a guess based on the fact I think capacitors are commonly used to filter noise. Does that sound realistic?

Right now I have the oscilloscope hooked up using the power cable it comes with to an outlet. The oscilloscope also has a separate BNC power cable where it can be hooked up directly to a 24V power supply. Is this worth trying to switch over to the DC power supply to get the noise to disappear?

Is it possible that the CRT is just bad?

How would you go about diagnosing the issue?

[oldway]

Hello and welcome on this forum.

The Philips PM 3233 oscilloscope has the particularity of having a cathode ray tube with two beams .... the two channels are therefore totally independent.

Note that the PM 3232 is the same oscilloscope but without delay lines.

The only thing common is the power supplies.
Since the fault exists on both channels, it is therefore on the power supply side that the source of the fault must be sought.
No problem with the CRT at all !

You will find the service manual with the diagrams here:
https://ia801302.us.archive.org/24/items/philips_pm3233/pm3233.pdf

In an analog oscilloscope, there are high voltages (here 8.5KV), it is perhaps not recommended to try to repair such a device when one does not have sufficient knowledge and experience.

Be carefull.

Check all the power supplies for stability and ripple. (+12A, -12D, +5A, -5A, +20B, -6B, +80V, + 160V)

[Armadillo]

Also contact clean those noisy switches;

[oldway]

There is an essential rule in troubleshooting : if you want to achieve a result, repair the fault first before you look at maintenance issues.

If we look at the diagram, we see that there is no contactor or switch common to both channels .... so it is impossible that the failure is caused by a bad contact. 

To start by dismantling what has nothing to do with the breakdown is the biggest bullshit that we can do because we risk introducing new failures.

I have a PM 3233 and I know what I am talking about.

This is indeed an oscilloscope that has a lot of problems of bad contacts, but fix the fault first, you will worry about bad contacts later, if they exist!

However, you should also re-insert the power connectors and check the solders, mainly at the connectors, power resistors, and power transistors.

[Armadillo]

@oldway should learn how to respect others opinion and not be outright rowdy about his own opinion.

The English read ....."Also contact clean those noisy switches;" Which means I am not denying power supply could be the likely faults but while you are at it, Also contact clean those noisy switches;

we see that there is no contactor or switch common to both channels .... so it is impossible that the failure is caused by a bad contact.[/b] 

You can slam your head on the wall to realize that you cannot claim the expert you are when there is infact a common contact supplying common voltage to the 2 channels. See attached.

if you want to achieve a result, repair the fault first before you look at maintenance issues.

That's when you have found the faults and not while looking for the faults.

Relax oldway. Be accommodative of others 

[oldway]

@oldway should learn how to respect others opinion and not be outright rowdy about his own opinion.

The English read ....."Also contact clean those noisy switches;" Which means I am not denying power supply could be the likely faults but while you are at it, Also contact clean those noisy switches;

we see that there is no contactor or switch common to both channels .... so it is impossible that the failure is caused by a bad contact.[/b] 

You can slam your head on the wall to realize that you cannot claim the expert you are when there is infact a common contact supplying common voltage to the 2 channels. See attached.

if you want to achieve a result, repair the fault first before you look at maintenance issues.

That's when you have found the faults and not while looking for the faults.

Relax oldway. Be accommodative of others 

I do not understand your grievances because I have not made any criticism or personal attacks against you, I have only provided repair advice considering that duckhunt himself recognizes to have little knowledge of repair of oscilloscopes.

You try to justify yours advices but a faulty potentiometer is not the same as a bad contact ...

The recommendations to do are not the same ..... turn the knob a dozen times from one end to the other and especially attention .... do not inject any contact cleaner .... some dissolve the carbon track and permanently damage the potentiometer.

[Armadillo]

The drift or jitter can be describe as follows;

1] Speed of drift or jitter: Not fast, easy human perceivable speed looks like something mechanical or capacitor created.

2] Pattern and Repetition: Not oscillatory, irregular, not exactly repetitive, not exactly fixed pattern hence unlikely cause by capacitor.

Hence I think its mechanical created which is more likely as deduced above. Stuff like dry solder joints, poor mechanical/intermittent contacts or connections, loose cable, poor contacts, crackling carbon [resistor/potentiometer] etc...

I would first proceed to check all the voltages to be within tolerance and then check the mechanical type faults including proper contacts cleaning [including potentiometer contacts].

[oldway]

Turn and adjust the focus potentiometer and look if it has any influence on the fault you see on the screen.

Be carefull with brightness and focus potentiometers....they are at high voltage ! (-1.5KV)....do not spray contact cleaner in these potentiometers.

[Armadillo]

Since the identification is mechnical so don't just limit yourself to the focus knob.
Try others like R809, R812, R813 and etc....

Poor/Intermittent potentiometer contact is not uncommon in repair you know. 

[Armadillo]

. turn the knob a dozen times from one end to the other and especially attention .... do not inject any contact cleaner .... some dissolve the carbon track and permanently damage the potentiometer.

If you have found a faulty potentiometer, without doubt, replacing a new one would be the best option.

But, considering this is from an old oscilloscope, it would either be hard to find or not cheap, yeah! 
So the remaining option would be to dismantle/tear down the potentiometer to clean it [surely not an easy feat] OR
clean it with contact cleaner as many others have done and have moderate success with it.

@How would you have done it?
is a good piece for discussion.

[oldway]

WD40 became a brand....To clean contacts, never use the original WD40 product but theyr specialist product electrical contact cleaner.

https://www.wd40specialist.com/products/contact-cleaner/