Home › Forums › Electronics Restoration › Voltages drop after about one minute

Tagged: 

Viewing 3 posts - 1 through 3 (of 3 total)
  • Author
    Posts
  • January 26, 2021 at 12:06 pm #16120
    Les Dickson
    CVRS Member

    I’m working on a Deforest Crosley model York/Mayfair. I’ve recapped everything and repaired an IF transformer. It plays well for a minute or two then the volume slowly fades down.

    • I’ve checked the voltages on the plates of the plates of the tubes and they drop as the volume fades
    • The voltages of the heaters are all steady and where they should be
    • The transformer is putting out consistent and adequate voltage so the drop is occurring somewhere in the circuitry
    • My first thought was it must be a resistor that overheats so I tried spraying resistors with a Super Cold spray to see if brings the volume back up, but – NO

    What should I be looking for?

    Attachments:

    1. sch-de_forest_crosley_ltd.jpg

    January 26, 2021 at 1:42 pm #16122
    Alistair Thomson
    Forum Participant

    “The transformer is putting out consistent and adequate voltage..” so you start there. Check the voltage at the cathode of the 280 rectifier (I don’t know the pin numbers). My guess is that this is dropping, and if so, the 280 is faulty if the three 8µF caps are good.

    January 26, 2021 at 5:38 pm #16123
    John Greenland
    CVRS Member

    Evening Les,

    I would check what voltages that are on the center tap of the power transformer when it works and when it doesn’t.
    This radio raises the center tap above zero volts to generate the negative bias voltage for the output tubes in particular.

    Perhaps something is amiss there as well.

    As well, a type 80 tube is the same as a 280 according to the powers that be all knowing in this sort of thing.

    Good Luck with it.

    John G. VO1 CAT

  • Author
    Posts
Viewing 3 posts - 1 through 3 (of 3 total)
  • You must be logged in to reply to this topic.