Don't Miss

Leaky Diode

By on December 15, 2009


There was a Monitor that came in with no display but with high voltage. The heater was only 2 volt, by right it should be 6.3 vdc. The filter cap checked to be okay with ESR meter and left only the output diode. The output diode was tested with analog meter with the X1 and X10 K ohm range and found it to be good. As usual, if could not find the bad component, the next step was to direct replace the component and retest to see if there is any component that breakdowb when under load. Guess what i found, the output diode broke down when under load.

So, i retest again with my analog meter (set to X1 Ohm) to see if there is any erratic movement in the pointer. At first glance, you thought the output diode was good, but when i applied a little bit pressure on the diode with the help of the probes while still testing the diode, i saw that the pointer have erratic movement. That was the cause of the problem! In future, if you would like to test any diode, i suggest that you apply a little bit of pressure from the test probes and slowly see the result. Don’t test it fast as you may miss a bad diode. Hope this helps!



  1. Roberto Salerno

    December 15, 2009 at 1:30 pm

    Hi Jestine,
    may I put this repair in my Magazine?
    But this diode was in the Power Supply or in the EHT stage?
    I understand this diode is in the circuit that power the heater in CRT.
    Brand of the Monitor? Model?

    Bye from Roberto Salerno, from Italy

  2. ravendra

    December 16, 2009 at 6:17 pm

    nice tip !!!

  3. admin

    December 16, 2009 at 9:26 pm

    Hi Roberto,

    It is a 15" Compaq Model S5500 CRT Monitor. Yes, you may put this in your Magazine. This is an output diode at the SMPS that power up the heater circuit.


  4. Roberto Salerno

    December 17, 2009 at 1:00 pm

    Hi Jestine,
    ok, thank you for this information. I will put in my Italian magazine.
    I searched the schematic in the Web about this monitor, but I didn't found.
    Please, have you got a diagram of this SMPS?

    Bye from Roberto

  5. admin

    December 21, 2009 at 12:48 am

    Hi Roberto,

    Sorry, i can't get the information too. Perhaps there is no one uploaded this schematic into internet.


  6. Roberto Salerno

    December 22, 2009 at 10:59 am


    Merry Christmas from Italy !

    Roberto Salerno

  7. admin

    December 22, 2009 at 11:13 am

    Same to you Roberto Merry Christmas


  8. Steve Hearns

    December 24, 2009 at 1:23 am

    Hi Jestine:

    Yes, I saw the same problem way back in 1996 with a 13 inch TV made by RCA/GE. The diode was feeding the base of the HOT. Nudging the leed with the needlenose made the set work fine. Since then other techs have seen the bad diodes anywhere else the same type diode may be, so it may be worth replacing them all to avoid a redo. We use new 1N4006/1N4007 and never had more problems.

  9. admin

    December 24, 2009 at 10:17 am

    HI Steve,

    Long time didn't hear from you and hope everything is well. Thanks for sharing!


  10. Kingsley Loo

    August 15, 2010 at 1:51 pm

    Hi Jestine,
    I'm a beginner in electronic sector. I would like to ask u a question about diode.

    So the condition is I'm going to use a Transistor H-bridge circuit to control the rotary direction of my moving motor in my Final Year Project(FYP)-Water Gun Robot (which is a base with 4wheels and 2 of the wheels will be controlled by the motors).

    Since the total weight of my project is around 10kg(each motor withstand 5kg). So, through the experiment of testing a motor with 5kg load, the current for the motor to drag up the 5kg load is nearly 10Ampere.

    Thus, I hope to know that any diode that can sustain the 10ampere current flow(some thing like the rated ampere for a diode if excess the rate value of current flow through the diode, the diode will get burn) and will not get burn? Hope to get your response as soon as possible. Thank you.

    Kingsley Loo

  11. admin

    August 17, 2010 at 8:04 am

    Hi Kingley,

    Sorry i do not have such high ampere of diode info. I suggest that search from the internet especially You may need to look out for the diode Peak reverse voltage rating too.



Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.