Gould 4035 DSO blows fuses

Discussion in 'General Electronics Chat' started by gould4035, Aug 16, 2014.

  1. gould4035

    Thread Starter New Member

    Dec 22, 2009
    4
    0
    This scope blows the rear fuse by the power cord and soon as it is switched on.

    where do I start looking for problem? any circuit diagrams available?

    should I just sell it on ebay and get something newer?
     
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
    611
    120
    There you go.

    It took all of five seconds to find!
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,442
    3,361
    That's usually an easy one. There is a capacitor shorting across the supply line.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    In the case of older test equipment, it's most likely that the aluminum electrolytic capacitors are going bad. If they sit around too long unused, the dielectric separating the plates "goes away", and acts practically as a dead short when power is suddenly applied.

    The best thing to do is to replace ALL of the older aluminum electrolytic capacitors with fresh ones from a high-volume reputable supplier; if you try to use "bargain basement" caps, you'll likely be installing new problems instead of solutions. Get good quality capacitors, rated for 105°C, same or somewhat smaller physical size, same or higher voltage rating than those that were originally installed.
     
  5. gould4035

    Thread Starter New Member

    Dec 22, 2009
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    bear with me...
    looking at the schematics it's not clear to me how the main fuse is used. I see several 120v primarys but the fuse connection is unclear.

    secondly, in order to check supply voltages I need the fuse to not blow. So does that mean I have to disconnect all supply lines, replace all caps in the PSU and then see if I get correct supply voltages without any loads?... then connect supplys to each section and test again?

    safety question. what sections to be wary off and how to be sure I have discharged any high voltages? I plan to be very cautious
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,442
    3,361
    Let me introduce you to the fine art of trouble shooting electronic equipment.

    Step 1: Visual

    Don't skip over these initial steps. Do a thorough visual inspection of what ever is visible.
    Look for discoloration of components, printed circuit board, traces, cables. Look for bulging or leaking capacitors. Check for loose wires cables etc. There is also a slight chance of short circuit by foreign objects, loose nuts, bolts, hardware, dirt and dust.

    Smell.

    If you catch a fault early, you may be able to see smoke or smell the remnants of lingering smoke from a burnt component.

    Step 2: Study the symptoms.

    In your case, your symptom is a blown fuse. You cannot do a power-on test or take voltage measurements if the fuse keeps blowing. You have to eliminated the overload condition first.

    There are different approaches to take here.

    Approach #1 - Look at all suspect capacitors across the power supply lines. Look for bulging and leaking capacitors.
    Use an ohmmeter and measure the resistance across the power rail. If you are on the right track you will find a dead short across the power rails. Start unsoldering suspected capacitors and test for the short on the capacitor and no short across the power rail.

    Approach #2 - Disconnect all sections of the power supply and see if the fuse does not blow. Then systematically find which section of the power supply is blowing the fuse. This approach means that you will blow a lot of fuses. I use an electronic circuit breaker for this purpose which I can assume you don't have.

    I'll stop here for now and wait for your progress.
     
  7. gould4035

    Thread Starter New Member

    Dec 22, 2009
    4
    0
    started to remove the power supply board but there is a connector which requires me to reach way in and snip a tie wrap which is close to the rear of the crt. I also need to unplug a connector close to the connector at the rear of the crt. I am wondering about residual voltages in various areas such as crt and other boards. service manual says high voltages may remain for a minute after power is removed. what about the next day?
    attached some pics
     
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    Last edited: Aug 19, 2014
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