Oscilloscope with a fault PM3200

[Sel]

I got this old Oscilloscope which seems to have a fault.
It powers up  but all I get is a brief line going vertical which then turns into a wide (1cm)  faint vertical block moving left to right.
Other controls move it back and forth a little and change the speed from left to right but that's about it.

When I first got it and turned it on I did get a dot in the center that I could move left right up and down.
But before I could do anything else I turned it off and when turned on later all I get is the above.

I found a few circuit diagrams on line for the Philips PM3200 all in Dutch, I should point out that I know nothing much about electronics I just like seeing how stuff works.
Before I take it apart and totally destroy it is there any area I should look at given the poor description of the fault.

Thanks Sel.   

 

[MosherIV]

Hi and Welcome to the EEV Blog forum

I should point out that I know nothing much about electronics I just like seeing how stuff works.

Before I take it apart and totally destroy it is there any area I should look

If you really know little about electronics - be VERY careful when looking in any CRT based equipment.

Oscilloscope have around 10 to 15KV (thats 10,000 to 15,000 Volts) in them to accelerate the electron beam from the back of the tube to the front (If it was an old colour TV - that would be around 35KV).

It might help if you post a picture of the scope, showing the controls. If you are lucky, it may just be the controls are set to something stupid.

[PA0PBZ]

Does the knob marked "Focus" mid bottom change anything?

[Sel]

I'm not at home right now so will have to check and take photo later.

I think the focus did work when it was a dot and now just makes the beam wider or slightly narrow.

My old man was an electrician so he warned me of the dangers of CRT's so I do try and avoid where possible.

Thanks again I'll report back later.

[rf+tech]

Hi Sel,

I got this old Oscilloscope which seems to have a fault.

From your description of the scope working initially and the present failure - old aluminum electrolytic capacitors in the power supply are a quite suspect. Minimal response to the position controls and focus being way off point with certainty to a power supply problem.

Post up a few photos of the power supply when you can. I have attached the power supply board layout and schematic for reference.

Also, let us know what you have available for a meter to measure voltages with. The PM3200 manual shows -1500 Volts to the grid/cathode and +210 Volts to the deflection amplifiers, so the voltage extremes encountered are known. This oscilloscope does not use a Post Deflection Accelerator and therefore does not contain 10 kV to 15 kV.

Caution is still advised, none the less so as not to be

The power supply schematic shows that the scope uses a DC to DC converter to generate the various DC voltages. Most, if not all five outputs are likely to be low. Please do not attempt to measure the -1500 Volt output, have been from attempting this.

If you are comfortable measuring the +210, +90, +12 and -12 Volt outputs, please report the results.

RF+ Tech

[oldway]

http://www.qsl.net/p/pe2jeb//foto/Philips_meetapparaten/PM3200/
Don't take it apart and totally destroy it ....
Ask for help to somebody who has more knowledge and experience than you in electronics. 

[Sel]

Hi guys, thanks for the responses and the info.
I took a couple of external photos with a very poor camera I'll take an internal look asap.
Most of the dials have some effect apart from Y-amp and Y-pos, they did work the very first time it was switched on.


I do have a couple of cheap meters and a capacity tester type thing I picked up off Ebay.
I have replaced a couple of electrolytic  capacitors that were visually blow on an LCD TV and PC power supply recently.

I'll get back this weekend with internal pics time permitting.

Thanks All.
     

[MosherIV]

Hi

What happens if in 'Triggering' the switch is set to 'Int' and the Tim/Dic is set to any us setting ?

What happens when you sdjust the 'Y Position' knob ?

[Sel]

Hi

What happens if in 'Triggering' the switch is set to 'Int' and the Tim/Dic is set to any us setting ?

What happens when you sdjust the 'Y Position' knob ?

Hi,
 Y-Pos     has no effect.
 Int         makes the bar move left to right Ext freezes it.
 Tim/Div  anything less than 1ms I see the bar moving anything greater than that I can't see.

Cheers.
 

[MosherIV]

  Int         makes the bar move left to right Ext freezes it.

That is good. Normally you would be using the internal timebase.

Tim/Div  anything less than 1ms I see the bar moving anything greater than that I can't see.

What happend if you turn V/div to 50V?

[Sel]

Tim/Div  anything less than 1ms I see the bar moving anything greater than that I can't see.

What happend if you turn V/div to 50V?
[/quote]

I'm not sure which knob  V/div is,   If I turn Y-pos to 50 nothing changes if I turn Tim/Div to 50 it goes so fast the whole screen lights up a little.

[MosherIV]

I'm not sure which knob  V/div is

Sorry, the knob under 'Y Ampl'
Turn it all the way anti-clockwise.
What does the beam look like?

We have established the timebase is working, this is how fast the beam sweeps across the screen.

I am trying to work out if all that you are seeing is niose, this can hapoen if the gain is turn up too high.

[Sel]

There's no Knob under Y-amp just a selector switch 0-ac-dc which has no effect.
Focus makes the beam smaller or larger and Int makes it slightly brighter.

The thing to remember is that when I first switched it on there was a nice bright dot  which responded to X-pos and Y-pos, Focus and Int.
At which point I had to turn off and go leave it for a while, when I later turned it back on without touching any settings there's nothing just this very dim large block.
I've looked at a few web pages and set it to what I've seem but it made no difference.
If I can find some time this week I'll take a look inside and see if there is anything obvious that could be an issue.


 

   

[Sel]

I took a look inside and everything looks fine to the eye, it seems clean with no signs of anything burnt or bulging.
 

[rf+tech]

Sel,

Good observations. Unfortunately, not all component failures provide visible signs.

For such situations when there are no visible signs of what has failed, it becomes necessary to make measurements.

By comparing measurements with the schematic, clues will guide you to the failed component.

This may seem intimidating for your skill level. If you are not sure where to begin, just ask.

RF+ Tech

[Sel]

I don't know if this will add to the information or not but when I first turned it on I didn't see anything on the display, now the tube may have just been warming up but when I adjusted the Y-Pos that's when it came on,  the Dot that is.
   
I think I had to turn it a few times to get it to stay on, however it might have been the X-Pos.

Now of course it's doing nothing, if that pot was faulty/dirty could that account for the vertical bar?
 
Thanks for the replies..

[tautech]

I don't know if this will add to the information or not but when I first turned it on I didn't see anything on the display, now the tube may have just been warming up but when I adjusted the Y-Pos that's when it came on,  the Dot that is.
   
I think I had to turn it a few times to get it to stay on, however it might have been the X-Pos.

Now of course it's doing nothing, if that pot was faulty/dirty could that account for the vertical bar?
 
Thanks for the replies..

If you're just getting a vertical line with say a cal input it'll be because the sweep circuit is not working....presuming everything else is set correctly.

[Sel]

Thanks, it's not really a line it's a block about 1cm wide  top to bottom, I tried posting a picture above but it's quite faint.
Only after it's wormed up and I switch it off an on do I briefly see a line go upwards, vertical if set to init  or  if ext has been selected then it's more diagonal.

[MosherIV]

Based on what you have described and the observations that you have done  it sounds like either the x amplifier is playing up or the beam focus is playing up.
The x amp should be ok to probe around (I think but I could be wrong, I have not looked at the service manual).
The focus is more dangerous. I believe they use a few hundred volts on a ring near the beginning/back of the crt tube.

Check the schematics and try to understabd how the circuit works so that you have an idea of what to look for whe  probing.

Be careful 1. Not to electrocute yourself 2. No to slip with the probe and short something out and do more damage.

[David Hess]

Wow, the service manual is pretty nice for this oscilloscope.  There is no PDA (post deflection acceleration) so the highest voltage you need to worry about is the -1500 volt cathode acceleration voltage.

I would not continue with detailed diagnostics without checking the low voltage power supply outputs.  The symptoms are consistent with worn out aluminum electrolytic capacitors allowing excessive ripple on the power supply outputs.  The power supply uses an inverter so the ripple will be high frequency.

[Sel]

Thanks guys, I'm a little short of time for a while so can't really advance this right now and I also don't trust these cheap tester leads.
They're already beginning to crack and expose the wires inside, fine for arduino/low voltage stuff but not for mains.

Even if I do test the voltages I'm not sure I'd be able to associate that to a particular failed part.
Just one other thing when testing capacitors an I right in thinking that one end needs to be un soldered.   
 

[tautech]

Thanks guys, I'm a little short of time for a while so can't really advance this right now and I also don't trust these cheap tester leads.
They're already beginning to crack and expose the wires inside, fine for arduino/low voltage stuff but not for mains.

Even if I do test the voltages I'm not sure I'd be able to associate that to a particular failed part.
Just one other thing when testing capacitors an I right in thinking that one end needs to be un soldered.   

Do get some new leads. 

Yes, to be sure of cap values unsolder one lead otherwise you are also measuring any other caps on the same net/rail too.

[rf+tech]

Sel,

Even if I do test the voltages I'm not sure I'd be able to associate that to a particular failed part.

That's what we're here for and ready to share our experience. We can interpret the measurements and explain how they relate to the failure. Consider class is in session.

Something basic that is always worthwhile doing with old test equipment is cleaning all potentiometers and switches with a good quality contact cleaner. If you can find it in your country, Caig De-Ox-It is a great product for this purpose. A little goes a long way, no need to drown parts with it. A quick squirt and lots of twisting back and forth works it in well.

From your description of turning the Y and X position several times to get the dot to show up (a few posts back), maybe, just maybe the extreme out-of-focus and other symptoms are due to oxidation. Doing the cleaning before anything further could eliminate some and resolve some problems.

RF+ Tech

[Sel]

Cheers I'll get some contact cleaner and try it, if that doesn't work I'm just going to have to store it for a while real World issues are getting in the way.
 Thanks again, I'll definitely have a go at fixing it later.

[Sel]

It's been a while but the current situation is this, I tried contact cleaner and even bought an ESR meter to test capacitors while in the circuit but had no joy. I also pulled the transistors and reinserted them  but it didn't make any difference. In the end I just left it lying around in the shed constantly having to move it out of the way.

Today I plugged it back in and surprisingly it now works perfectly with a nice bright clean display.
I can only think there must be a bad connection or switch, maybe the contact cleaner took time to work.

[rf+tech]

Congratulations Sel,

I've seen just this very behavior, where something cleaned with Deoxit just needed to sit awhile.

Cheers and thanks for the follow-up.

RF+ Tech