Got an old oscilloscope, some display problems

[allanw]

I got an old Philips PM3256 75MHz scope as I mentioned in my last thread. Everything's mostly working fine, except it seems like the trace sometimes gets "stuck" at either the 2nd from the left or 2nd from the right vertical line. When this happens everything seems to get scrunched up (bright dot), as if it had hit the edge of the screen and stopped drawing. The problem randomly comes around and goes away, but it's mostly stuck like the second picture I'll attach. Here's some pictures (intensity levels changed as I took pictures):

Everything good:


Scrunched up 20% from the right (note bright dot where it stops drawing):


More scrunching:


Sadness:


It's not just me being dumb and having the horizontal shift on wrong or something, it simply won't draw past a certain point sometimes. Also, it isn't permanently stuck, as sometimes I see it flash and actually draw the whole way through. Other times it'll actually stay at 90% or 100%, which is usable. Also, it happens no matter what trigger mode or channel or anything that I set.

Anyone have some advice? I did obtain the service manual for this so I'll take a look through that.
edit: here's a video which will work after it's processed:

[Pyr0Beast]

Dry or cold joint somewhere probably, not sure tho. Does it do that when it is still cold or when it warms up ?

Might be even something with deflection. Won't hurt to open it up

[mkissin]

If it's an old CRT scope having deflection problems, my first port of call would be changing all of the electrolytic capacitors in the CRT drive circuitry. Check for any that look suspect first, and if that doesn't help, replace the lot. Even if they aren't dead, it's good insurance.

[MoJo]

I have the same issue with the same type of CRT, by that I mean from Phillips/ Fluke ... mine is an old PM3094. The CRT is old and worn out ... it has burn-ins and is quite dim on default settings. Now I asses the issue to be a couple of causes, one being an old CRT, second being that these solid Scopes have an achilles heel being electrolytic caps which over the years are not good anymore. As another person suggested it would be wise to start their. Third cause is that these Phillips scopes tend to run hot, over the years it may have done heat damage to the board and it's components. I opened mine and saw that the board is browned/ baked at certain areas, ie. the power board, and the CRT control board ... two areas that deal with high voltage.

My scope at times has perfect horizontal tracing, and at other times has 2/3rd tracing. It doesn't seem to be temprature related as I have started and restarted at times with high working tempratures, and the result still varies ... sometimes perfect and others not so. My suspicion is that this is a charge issue, relating to old worn out caps. Lastly replacing the screen is quite difficult on the Flukes, as they are quite hard to find and sometimes expensive ... requiring the buying of an entire non functional unit just to salvage it's screen. Facing this I bought the Rigol, and consider my options ... I suggest you do the same.

[allanw]

It seems to start and stop randomly. If I'm lucky it'll be perfect for a long time.

I'll try recapping the power and CRT drive sections. It's actually pretty cheap to do, and hopefully I won't break the scope in the process.

At least my screen is very good. Who knows how long that'd last though. Question: does keeping it on lower intensity settings preserve the life of the screen longer?

[Pyr0Beast]

I'm sure you won't burn your CRT tube with somewhat normal use in many many years.

[Kiriakos-GR]

From my experience with CRT's  , the best ones could stay alive the max of 25 years,
by having in mind  an operational cycle  of 4-5 hours per day. 

But the point is , that if you use the instrument for productive ways,  it pays out for it cost by it self,
so replacing it with a new , its just an expected and not painful process.   

[alm]

Scope CRT's don't usually burn in unless you really abuse them. Don't set intensity higher than necessary to get a good trace, and don't leave it in XY mode without any inputs (it will create a single dot, which will heat up the phosphor fast). Of course phosphor does have a finite life, so don't leave it on overnight, especially with a straight line, and use the vertical position control once in a while, but the CRT isn't usually the thing that dies first. Plenty of CRT's much older than this that are still fine, although the amount of usage it saw is usually unknown.

I would start by checking the power supplies (as usual), check for correct voltage (with your DMM), they should at least be close (14.5V instead of 15V is often fine, but 8V isn't). Also check for ripple (your scope will still work as long as you only look at the first 8 div's or so, right?). Check any connectors or sockets for bad connections. Check for the already mentioned bad solder joints. Cracks in PCB tracks. Heat make things expand, so it can easily make a bad connection worse. Anything that looks burned?

Does it appear heat related, i.e. does it always occur after it's on for some time or always right after you turn it on cold? Does turning it off and on without letting it cool down for too long make the problem re-appear faster? Or is it completely random?

The main timebase and delayed timebase both have the problem, so that suggests something they have in common (eg. power supply or horizontal amplifier). It the problem went away for a few seconds when you switched from delayed to main intensified by delayed timebase, as if it had to re-stabilize or re-heat, is this consistent? If you have an idea where the problem might be, spraying cold spray (or an improvised version of that) can be informative, if the problem goes away, you've found the problem.

What happens when you change the horizontal sweep length and/or position control? Does the distortion compress/move with the control, or does it stay on the same place when the waveform moves?

The service manual should be helpful in at least localizing the problem to a certain part, and should tell you have to check the power supplies and similar stuff.

[saturation]

I was watching your video and based on it, its looks like the horizontal amplifier gain is clipping, that's why it only goes so far.  I also note when you make adjustments, a ton of noise occurs, it might be something as simple as dirt on the switch, and need cleaning, good contact spray.

I forget but there should be an adjustment for the horizontal width too, is it affected?  i.e., can you adjust it so the trace sweeps the entire length of the screen.

It seems to start and stop randomly. If I'm lucky it'll be perfect for a long time.

I'll try recapping the power and CRT drive sections. It's actually pretty cheap to do, and hopefully I won't break the scope in the process.

At least my screen is very good. Who knows how long that'd last though. Question: does keeping it on lower intensity settings preserve the life of the screen longer?

[alm]

Be careful about what kind of stuff you spray in delicate switches and pots, you don't want it to become a sticky mess after some time.

I agree that it does look like clipping in some stage, which could also be caused by a power supply rail that decreases in voltage for some reason.

[djsb]

Hi,
First of all you need to try to narrow down in which subsystem the fault lies by doing the diagnosis from the front panel switches.
Note down what happens to the trace when you operate the controls.
Some questions
1/ Does the trace complete it's travel all the way to the right without a signal being input?
2/ Can you move the trace completely to the right using the horizontal position control?
3/ Does the beam finder (if fitted) button work?

Note down anything unusual so that a picture starts to form of which area the fault could lie. Keep the fault finding at as high a level as possible otherwise it's easy to get confused. Once you've narrowed it down to a particular area then you can think about individual component replacement. Best to do this than start replacing parts randomly, youll be making work for yourself if you do.

It appears that the potential on the horizontal plates is not reaching the correct level (enough to pull the beam fully to the right) or the beam blanking is happening too early. The first fault could be in the horizontal amplifier or the sawtooth waveform feeding the amplifier is not reaching the correct level. Could be a voltage drop somewhere or a faulty resistor. Just a few possibilities.

It would also help to spray the switches with servisol contact cleaner. A lot of faults are down to oxidation and poor contacts.

HTH.

David.

[allanw]

Hmm, I just used it for about an hour and it didn't display any problems at all. Which means it's really random. (actually, I've since removed the large leather pouch that sits on top. The unit gets warm on top after usage, but not that hot, so I'm not sure that was actually the problem. However, that might have fixed it? I'll have to use it more)

Whenever it did occur, it occurred when I switched to XY mode, so that means it's independent of the time bases right? When the distortion does occur, moving the position control doesn't affect where the distortion occurs. In the video I adjust the horizontal position.

If the problem persists I'll open it up and take some measurements and check for heat damage. I think it had to do with the display circuitry.

To djsb's post:

1/ Does the trace complete it's travel all the way to the right without a signal being input?
No, it happens regardless of anything I try to do.

2/ Can you move the trace completely to the right using the horizontal position control?
No, it still distorts in the same absolute place on the screen.

3/ Does the beam finder (if fitted) button work?
I don't think there's a beam finder button.

[sigxcpu]

From what you are saying, this is the best explanation so far:

I was watching your video and based on it, its looks like the horizontal amplifier gain is clipping, that's why it only goes so far.

[djsb]

Hi,

Please download and read this tektronix training material.

http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

I have followed and used this manual on all my repair projects.

David.

[Pyr0Beast]

djsb, thank you for the manual. It seems to be very useful and applicable to all sorts of equipment.