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How to estimate transformer voltage for audio preamp B&K cs117

Discussion in 'Electronic Design' started by powerdoc, Mar 13, 2007.

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  1. powerdoc

    powerdoc Guest

    I have a B&K CS-117 preamp which has a defunct power transformer. The
    B&K folks won't sell parts and the cost of the parts, shipping and
    labor will be more than the worth of the unit. I'd like to keep it in
    service but the transformer is unmarked as to manufacturer and the
    manufacturer of the unit won't give me the specs. I guess that I see
    their point, in some ways, but at least they'd profit from selling me
    a transformer and I could speak well of them. Is there a way to
    estimate the output needed (it looks like a center-tapped output from
    the 3 wires that go to the circuit board)?
  2. Phil Allison

    Phil Allison Guest

    ** Groper alert.

    ** The voltage rating of the first filter electro is a good guide - allow
    about 20 % under that figure.

    Similar size and style trannys have the same VA capacity.

    Also, trannys with the same primary resistance have close to the same VA

    ........ Phil
  3. Phil Allison

    Phil Allison Guest

    "Phil Allison"

    ** Of course, also allow the peak transformer voltage to be 1.41 times the
    rms value quoted by the maker and allow for the quoted regulation percentage
    too, if the load condition is not constant.

    ........ Phil
  4. What's the voltage rating of the main input filter cap?
  5. powerdoc

    powerdoc Guest

    The filters are 40v and the regulators are 15v so I figure a 36v ct
    transformer is the ticket. There is a model in the Allied catalogue
    that has the same form factor and maybe the same manufacturer although
    the original is just marked with cryptic numbers.
  6. Phil Allison

    Phil Allison Guest


    ** Long as that means an 18 - 0 - 18 tranny.

    ......... Phil
  7. powerdoc

    powerdoc Guest

    Yes it does. Thanks to all. dk
  8. There's no power tapped off before the regulators? If so you are close
    enough IMO. Try that tranny, hook up a variac, drop the voltage until the
    post regulator voltage drops slightly, then go back up 10%. If you're back
    at close to 18 - 0 - 18 you're set.
  9. powerdoc

    powerdoc Guest

    Unfortunately, there is something else wrong. After hooking up the
    transformer, the output of the vr's was 14.88 v but one ic got really
    hot. I guess there was a reason for the transformer to fail as the
    power is always on on this unit. More looking I guess; IC's are
    relatively cheap to replace
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