On Mar 15, 2:35 pm, "Arfa Daily" wrote:
"powerdoc" wrote in message

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On Mar 15, 7:45 am, "Arfa Daily" wrote:
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On 14 Mar, 17:22, "Arfa Daily" wrote:
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On 14 Mar, 12:41, "powerdoc" wrote:
On Mar 14, 4:05 am, "N Cook" wrote:
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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.

no, near their ratings.

I replace many electrolytic caps a week, and oftentimes, cheapo hifi's
will
have caps rated at only 10v across rails that have 8v or so on them.
Trust
me, they don't last long. For long term reliability, that rating figure
should be taken as an absolute limit. It is common design practice to
derate
electros by working voltage, by a factor of 30 to 50%

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,

yes, probably less.

which will not load any
reasonably rated transformer anywhere near into copper losses IMO.

Eh? A small lower power transformer is going to have poor regulation,
ie relatively high winding R, and you've got a peaky current waveform
being drawn. Vdrop in the copper will thus be significant.

Why a " peaky " current waveform ? With a low current demand circuit such
as
a preamp, the current drain from the transformer should be pretty
constant,
the main rectifier resevoir caps taking care of supplying any transient
requirements. Even a 'small' transformer at 18-0-18 is likely to have a
current rating of at least 500mA per limb, and with the low demand of
this
type of circuit, I would not expect to see barely a small drop due to
copper
losses. The poor regulation will ensure that the output voltage is high
on
the nominal design figure, and will likely remain so.

In fact,
a transformer rated at 25v nominal will likely have a low / off load
output
of up to perhaps 28v RMS.

12% regulation, maybe, or maybe higher. We really dont know.

Which potentially makes the situation even worse ...

Multiply that up by 1.4 to get to the peak voltage
and you will be just about at 40v across the caps.

You're ignoring the effect of loading plus winding resistance. R has
more effect on a peaky waveform.

I don't think I am. I am employing years of practical experience with
this
sort of thing. If we were talking about a power amp, then yes, factors
such
as transformer regulation and copper losses have to be taken into account
for voltage sag calculations, but in low demand power supplies, it's more
relevant to look at it from the opposite angle, and work out how much
higher
the output voltage will be than expected.

Also, I would suggest
that it being a preamp, it will have opamps in it, requiring split
rails,

It does appear to be what we have, though plenty of opamp ccts run on
single 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,

If they see 20v then 15% mains sag and the regs drop out of
regulation. 78 series require a 4v overhead.

So as I said, 20 ( a 5v overhead ) to 25 ( a 10v overhead ) is correct.
If
you were unfortunate enough to live somewhere with 15% sags on your
incoming
line voltage, I would suggest that there would be a lot of equipment in
the
house suffering odd problems. Taking the case of poor mains regulation,
if
it can go down by 15%, you'd have to reckon on it being able to go up as
well. That's going to take those 40v caps over their limit, or 'rating'
if
you prefer.

Arfa

Actually the IC that's hot is in in the input circuit not a vr. One
vr has no DC on the input side so I'm going to assume something in
front of it in the power supply is bad, a diode perhaps. Once again,
since I can't get the schematic it's hit and miss.

So what exactly is the IC that's getting hot ? Usually, all there is in this
type of circuit, is a tranny, a bridge, 2 smoothers and straight into the
two regs, perhaps via safety R's. There may be safety R's or fuses ahead of
the bridge. It should be very easy to find out why there is no input volts
on one of the regs, even without schematics. Just check across the two
smoothers for volts. If there's some there, but not at the reg, then there
must be open print or an open protection device. If the volts are missing
across one of the smoothers, then check the bridge, and any protectors that
there may be around it, and that you have wired in your trial replacement
tranny correctly.

Arfa

The hot ic is on the board with the input jacks. It may have
something to do with the RIAA equalization if there is such a chip as
it's near the phono input. Only one of the many projects for the
weekend.