Oscilloscope Philips - time base not working

[krajsek12345]

Hello!

I have a problem with my Philips PM3240 oscilloscope. If i select channel A and use external x deflector function, then there are two dots on the screen if i put the probe in calibration source, if i select time base, then there is nothing on the screen. I have noticed that channel B is not working at all (no dots on screen). The scope has two time bases, if i select the one that has "off" position on scale where you turn the big rotary switch then it shows two dots that go through the screen very quickly. Could someone please help me? nobody wants to help me, because noone knows how to fix it.
Sorry for my bad english, i am from Slovenia.

 

[BradtheRad]

This sounds as though you have more than one problem.

I have noticed that channel B is not working at all (no dots on screen).

Both beams appear under control of channel A?
But neither beam is under control of channel B?

Can you check to be sure the switch is making contact at all positions?

The scope has two time bases, if i select the one that has "off" position on scale where you turn the big rotary switch then it shows two dots that go through the screen very quickly.

The beam is on free run, fastest speed.

For this I am also thinking it is caused by a poor switch contact.

By selecting time base 'Off', it should allow you to input a signal, which causes horizontal movement. Have you tried this? Does it work?

The above problems are semi-related although not directly related. The cause might be a bad ground. It might be a microscopic fracture in a trace on a board. Or one bad wire which connects ground to two boards. It might be where a switch has been twisted and stressed many times. Etc.

 

[D.A.(Tony)Stewart]

The calibration signal is a square wave so with no defection in X it will appear as 2 dots.

Using internal Sawtooth time base deflection for X axis will show the square wave, where one wave of sawtooth is your time base for full width., followed by blank on retrace.

Using external X defection requires calibrated gain and offset control of X beam to be useful.

Most people use Ch2 for Y and Ch1 for X then select XY mode so that you can control X with gain and offset.

Until you learn how to use this, external X is not meaningful until you know the input level to deflection levels required.