Philips 3226 oscilloscope vertical position starts low, then rises

[jpwolfe31]

I have an older scope that works fine, but the vertical position when powered on is so low that you can only see the very tops of the waveforms.  After 5 to 10 minutes, you can see the entire waveform.  After 15 minutes the waveform is centered and the scope works normally.  Any experts that know where the best place to start on this problem would be.
I have the service manual but have never worked on a scope before.

 

[cowasaki]

I'm not an expert with old scopes at all but have you dropped, moved or banged it?  Have you taken it apart to have a look if the fault is obvious? The vertical deflection is faulty whilst cold but as it warms the deflection starts to work.  This sounds like a dodgy connection.  Is the board on the end of the tube loose? Connected properly?  I had a similar issue with an old scope and pulling the board off the back and pushing it back on fixed the issue.

I'm sure someone with far more knowledge of these old scopes will be along shortly.......

[nsrmagazin]

I am not an expert also, I hope we find such a person with knowledge of old scopes. At some point the voltage or current should be rising. What is responsible for the axis? How is it powered. The display seems to be ok, so something back in the line is causing the problems. Can you take some readings?

[Chris56000]

Hi!

The PM3226 is a straightforward 2 channel 15 MHz basic oscilloscope with no frills or complicated bells and whistles!

Begin by referring to the circuit diagram for Unit 1, which contains the two vertical amplifiers. My recommendation to start with is to replace the four or five small 68uF electrolytics on this PCB – two of them, C269 and C309 provide emitter decoupling for the long–tail pair immediately before the final Y amplifier transistors, and leakage here will cause the operating points of the transistors to vary at random, resulting in their collector currents varying!

Because the transistors forming the Y amplifier are direct–coupled, any random variations in operating currents will cause the voltage–drops across the various load and feed resistors to vary and with this, the potentials at the Y plates of the CRT, resulting in unwanted vertical movements of or inability to centralise the trace, which is the symptom the OP is experiencing!

If replacements of the 68uF electrolytics on Unit 1 don't resolve your fault, there could be thermally defective transistors on the Unit 1 Y amplifier, and the best method of tracing this is to use a can of freezer–spray sold for this purpose!

Chris Williams