Troubleshooting a Philips PM3240 Scope

Discussion in 'The Projects Forum' started by BelgianEngineer, Jul 15, 2014.

  1. BelgianEngineer

    Thread Starter New Member

    Jul 15, 2014
    6
    0
    Hello everyone, :)
    I'm a poor engineering student who doesn't have enough money to buy a working scope so I bought for 20euro a defect Philips PM3240, hoping I could repair it.

    I began troubleshooting today and on first view the screen seem to light on, it means axis are lit up but there seem to be no signal apearing (should I not have a near to 0V signal in AC mode if no probes are plugged in?). :confused:

    So I wanted to play a little bit with the buttons to see if I could get signal but before I could do this I got a smoke generator made by a blowen cap. In fact it is the main input cap, the one before the Graetz bridge.

    So my first question for you is with what can I replace this cap and do you have an idea why this happened? :p

    This is the info of the cap:
    220nF // +-10% // 250V rated // polyester foil type capacitor

    Here is a picture of it and his placement in the scope

    http://www.noelshack.com/2014-29-1405463818-capture.png
    http://www.noelshack.com/2014-29-1405463866-dsc-1807.jpg
    http://www.noelshack.com/2014-29-1405463825-dsc-1813.jpg

    Is this okay?
    http://www.ebay.fr/itm/X2-Metallise...al_Components_Supplies_ET&hash=item1c42b8023a

    Thank you in advance for your answers :)
     
  2. BelgianEngineer

    Thread Starter New Member

    Jul 15, 2014
    6
    0
  3. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    These capacitors are specially made to connect across the Mains in this application.
    They have higher ratings than conventional caps,though when they do go "bang" they do so spectacularly!!

    Google for X2 capacitors.
     
  4. BelgianEngineer

    Thread Starter New Member

    Jul 15, 2014
    6
    0
    So are the caps from the links OK (they're also rated for 250V) or do I necesserally have to look for X2 caps?

    And like asked above, is there any disadvantage to use a bigger capacitor?
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,364
    The capacitor on the power lines is for HF filtering. Increase the working voltage, yes,
    but don't increase the capacitance value.

    Can you post a link to the Service Manual?

    Edit: I found the manual here.
     
    Last edited: Jul 16, 2014
  6. BelgianEngineer

    Thread Starter New Member

    Jul 15, 2014
    6
    0
    Thank you for your answers. :)

    So today I went to the shop to buy a new cap, I replaced the faulty one on the scope and there the scope magically works again. And I seem to have signal so I'm verry happy. :)

    Altought I noticed a strange thing which is that the intensity of the signal on the screen changes when I change the timebase. The strange thing is also that it doesn't change proportionally to the timebase, it's more like intensity is low on two distinct timebase choices.

    Someone has an explanation for this, is it risky to use the scope like this?
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,364
    This appears to be normal.

    Which settings are bright and which are dim?

    Generally, as you increase the sweep rate (i.e. shorter TIME/DIV) the trace becomes dimmer. Also it may be related to the triggering of the trace, the more often the scope triggers, the brighter the trace.
     
  8. BelgianEngineer

    Thread Starter New Member

    Jul 15, 2014
    6
    0
    The trace becomes dimmer when I go from timebase 0.5s to 2us, then it becomes very bright again and it becomes dimmer again when I go from timebase 2us to 0.5us.
     
  9. BelgianEngineer

    Thread Starter New Member

    Jul 15, 2014
    6
    0
    So? Does it still seem normal to you? :)
     
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