Oscilloscope PM3240 Time base not working, showing only dots

Discussion in 'General Electronics Chat' started by Matic Krajšek, Mar 1, 2016.

  1. Matic Krajšek

    Thread Starter New Member

    Mar 1, 2016
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    0
    Hello everybody, I am somehow a beginner in working with oscilloscopes, and I bought my self an old 50MHz Philips PM3240 scope. Everything seems to work fine except time base. The knob doesn't seem to respond no matter the settings. If I select to use eternal x deflect, then there are two dots on screen (representing the calibrating signal). I can also get two horizontal lines one upon the other representing calibration signal on one setting, not rely shore on witch right now, but at least I know the screen is working, but I can't "zoom in" to the signal. I don't know where to start, I looked at all of the capacitors and all seem to be fine, looked the connectors and switches but can't seem to find the right spot. Can anyone point me in the right direction please?
     
  2. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    2,803
    1,237
    But that's to be expected!:) -- Inasmuch as external x-axis deflection is selected but not supplied, only y-axis deflection (via. the Cal signal) is present -- Hence the 'dots' represent the 'peaks' ('plateaus', actually) of the 'square' calibration signal -- the transition times are so brief as to produce little to no intervening illumination...

    The calibration signal may appear as a pair of horizontal traces if internal x-axis deflection is selected and a triggering threshold setting preclusive of synchronization is in effect. --- Of course 'free-running' in a 'dual trace' mode with the y-axis signal absent or 'grounded' will likewise produce such a display.

    1) Selection of x-axis magnification has no effect when x-axis deflection is set to Ext. and no external x-axis deflection signal is supplied... --- Ditto for 'delayed sweep' magnification techniques...

    2)
    Where x-axis sweep is present - unsynchronized display of a square-wave (CIP the calibration signal) may remain unsynchronized (and, hence, present as two parallel horizontal traces) despite x-axis 'magnification'. --- IOW no change to the display is not necessarily indicative of an inoperative x-axis magnification feature... --- Similar considerations apply should a free-running 'dual trace' mode be selected sans y-axis deflection...

    Based upon the supplied information, I am by no manner of certainty that your scope is malfunctioning...

    If, having considered this post, you yet feel the instrument is indeed faulty - please follow-up with further detail -- We'll be happy to assist your troubleshooting efforts!:):):)

    Best regards
    HP:)
     
    Last edited: Mar 2, 2016
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
    745
    Have you got a scope probe, connect it to channel 1/2 and the cal socket, set X gain to 0.2V div, input to AC, and timebase to 1msec, you should see a square wave of 1khz,?
     
  4. Matic Krajšek

    Thread Starter New Member

    Mar 1, 2016
    3
    0
    Thanks for feedback, I have tried what you have said, but there is no wave on the screen. This is what I am trying to tell you, I know the basics of how to get a square wave on the screen, what is eternal x deflector, we did it lots of time in school and collage, the problem is that I don't know how to fix it. I only fixed a couple of old TV sets in my life...
    Here is how I set it up:
    Press A for channel A
    Press AC for channel A
    Set it to 0.2V/div
    Select MAIN TB
    Set trigger to Auto
    Set it to channel A
    Set time base to LF (low filter I think)
    set time base to 1ms


    According to the manual something like this should work and a wave should be displayed, but it's not. I have tried to set the position and so on. The scope was working fine for long time, and onetime I had it switched on for about 15 minutes, did nothing to it, took a random look on the screen and the signal was just gone. That is when I noticed the problem. No smoke, nothing. The LED that has a sign saying NOT TRIG'D is turned off if I put the probe in calibration socket
    I must say that I am not 100% sure that my scope is malfunctioned because of the lack of knowledge on using one, but I have tried EVERY and I realy mean every possible combination with the buttons and can't get a wave. What are the chances of that? :p
     
  5. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    2,803
    1,237
    In the above instance (assuming the instrument is set for a single y-axis input channel/display trace) I believe there is "a [square] wave on the screen" -- However, with with an improper trigger threshold (level) setting and/or incorrect trigger source selection, the presentation will be that of two horizontal lines...

    Please try this:
    Set the instrument up for the (apparent) two-line display described in your OP - then, having made certain that the trigger source is set to the appropriate channel (i.e. the channel to which the Cal signal is applied) 'twiddle' the trigger threshold/level control...

    Selection of external x-axis deflection will, of course, effectively disable the time base function!

    You might be surprised!:cool:

    Good luck and please keep us posted!:)

    Best regards
    HP
     
    Last edited: Mar 2, 2016
  6. Matic Krajšek

    Thread Starter New Member

    Mar 1, 2016
    3
    0
    Hello again, long time no see :D

    I figured out the problem. Transistor TS857 and TS858 - the BFS17 was bad. The funny thing was that when I took it out and messured it, it showed up fine, worked for like 45 minutes when I returned it to the PCB, then it died again. replaced it with a new one and now it works like a charm. I think it has to do something with the trigger circuit, not really shore what.

    Thanks everybody for the help.
     
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