Re: Estimating transformer voltage for B&K CS117 preamp


<meow2222@xxxxxxxxx> wrote in message
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On 14 Mar, 12:41, "powerdoc" <dkingl...@xxxxxxxxxxxxxx> wrote:
On Mar 14, 4:05 am, "N Cook" <diver...@xxxxxxxxx> wrote:
powerdoc<dkingl...@xxxxxxxxxxxxxx> wrote in message
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I have a CS-117 with a defunct transformer. The factory won't sell
me
parts and the value of the unit will be exceeded by the repair costs.
Is there a way to estimate the value of the ac output (it looks like
a
center - tapped output with 3 wires) so I can get this back on the
road?

The voltage regulators are 15v and electrolytics are 40v.

I remember asking the type of Vregs, without this you cant work it out
accurately.

Lets guess at 4v drop, which is on the generous / safe side, so that
means you need minimum 19v dc input.

Its a preamp so low current supply, so say 2v BR drop and perhaps a
25% regulation transformer. Allow for 15% mains drop. So your
transformer ac voltage will be

(19 + 2 ) /.85 x .707= 17.5v.

You'll need to allow for copper loss too, so 18+18 wont be enough.

Top voltage limit will be dictated by margins & cap rating.
If 25% transformer regualtion and 10% mains overvoltage get us to 40v,
running loaded v = 40/1.375 = 29v.

25v ac would be a good guesstimate. If its a split secondary with 2
diode rectifier, 25-0-25.



A tranny of nominal output 25v RMS is going to push those caps VERY near
their limits. If it's just a preamp, the current demand on it is not going
to be any more than a couple of hundred mA tops, which will not load any
reasonably rated transformer anywhere near into copper losses IMO. In fact,
a transformer rated at 25v nominal will likely have a low / off load output
of up to perhaps 28v RMS. Multiply that up by 1.4 to get to the peak voltage
and you will be just about at 40v across the caps. Also, I would suggest
that it being a preamp, it will have opamps in it, requiring split rails, so
it will be a bridge across the outer limbs of the winding, with the CT
grounded. 15v regulators, in the plural, would tend to confirm this
surmisal, one being for the positive rail, and one for the negative. Those
regulators should have 20 to 25v going into them, which would be a sensible
figure across 40v caps. Multiplying that figure by 0.7 to get to an RMS
value will result in a required transformer nominal voltage of 14 to 17.5v.
The correct transformer will therefore be one with a quoted nominal RMS
output voltage of 15-0-15 or possibly 18-0-18, so the OP's guess at 36v CT
seems to me to be about right ( as below )

Arfa



I figure
that the transformer is prob 36v. c.t. with a fractional amperage.
Will try that with a variac and see how it works.




.

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