# Estimating transformer voltage for B&K CS117 preamp

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

1. ### powerdocGuest

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?

2. ### Guest

For a preamp, look at hte electrolytics. Also look at the volt reg,
both voltage and type, if it uses one.

NT

3. ### Arfa DailyGuest

Agreed. Take around 70% of the rating of the main filter electro(s) that
follow immediately after the rectifier, and then take again about 70% of
that figure to arrive at the AC RMS winding voltage. So, if caps rated at
say 40v, reckon on the DC that's normally across them to be around 28 - 30v,
and the RMS output voltage of the tranny to be around 20v.

Arfa

4. ### N CookGuest

Before throwing out the original , excavate under the covering and you may
find an o/c thermal fuse.

Connect a dual ps to the 2 main DC rails and slowly up the volts until
reliable operation and then add 20 percent or so and divide by 1.34 or
whatever for AC volts

Totally demolish the transformer, counting turns/ weighing /gauging the
wire, then using the formulae to discover the likely output V and I.

5. ### powerdocGuest

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

6. ### Guest

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.

NT

7. ### Arfa DailyGuest

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

8. ### powerdocGuest

The 36v ct transformer gave me about 25v on the high side of the vr
and 14.88 on the low but there is something else wrong as one of the
ic's got really hot. Time to start looking at other ic's in the
circuit but since I don't have a schematic it may be replacing all of
them.

9. ### Arfa DailyGuest

Well, that's all looking good so far. Those voltages seem just about spot-on
for that type of equipment, so the guesstimate math we have done is
right.Before getting carried away with replacing ICs, there are a few simple
checks that can be carried out.

Firstly, is there a measurable short or low resistance on the back end of
the regulator that's getting hot, compared to the one that's not ? Bear in
mind that the pinning is different between the positive and negative regs
( I'm assuming a 7815 and 7915 here ? ) If there is a measurable short or
low resistance, it's worth first checking the regulator's output decoupling
components, likely to be a small value electrolytic, and possibly a ceramic
cap in parallel. These will be located physically very close to the reg
device. If there is no measurable short, what is the output voltage of the
regulator whilst it is still cold eg before it goes into thermal foldback?
Although it is most common for these regulators to fail open and cold in my
experience, I have known them to fail low voltage and run hot.

If there is a measurable reason for the reg to be getting hot, it's worth
just letting it run,making sure that the tranny and reccy are not getting
too hot, to see if you can detect something else getting hot on the board,
such as an IC. Failing that, if you have a very low ohms meter, sometimes
you can chase a short down, by measuring at each of the ICs' supply pins,
looking for the lowest resistance reading, which may be only a few
hundredths of an ohm lower at the bad IC.

If none of these methods turns up the bad component, I usually then resort
to following the print traces from the regulator, looking for wire links,
which there almost always are, because the rails have to go to so many
places. Lifting one end of these, one at a time, allows at least the general
area of a short to be run down very quickly. If there aren't any links, then
it's time to get out the scalpel, and indelible felt tip pen to mark where
you've made the cuts, lest you should forget ... !! Good luck with it d
:~}

Arfa

10. ### Guest

no, near their ratings.
yes, probably less.
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.
12% regulation, maybe, or maybe higher. We really dont know.
You're ignoring the effect of loading plus winding resistance. R has
more effect on a peaky waveform.
It does appear to be what we have, though plenty of opamp ccts run on
single rails.
If they see 20v then 15% mains sag and the regs drop out of
regulation. 78 series require a 4v overhead.

NT

11. ### Arfa DailyGuest

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%

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.

Which potentially makes the situation even worse ...

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.

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

12. ### powerdocGuest

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.

13. ### Arfa DailyGuest

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

14. ### powerdocGuest

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.

15. ### Arfa DailyGuest

That being the case, I would expect it to be the phono preamp, which is
often separate and near the input jacks because of the high sensitivity
required. It is probably an ordinary 8 pin dual opamp, yes ? I would start
by just removing it. If it has developed an internal short, you may have two
faults - or even 3 ...

First the chip itself, then it may have done something in on the feed to its
opposite rail's regulator ( "there is no input to one of the regulators" )
and finally, it may have hotted up the transformer so much before anything
else failed, that it gave up the ghost too.

When the IC is out, check the ins and outs of the two 15v regs. Remember
that on a 79xx reg IC, the input and ground pins are reversed, compared to a
78xx reg, so don't just measure on the left hand pin of both looking for
input volts, and find that there are none on the negative reg. Its input is
the centre pin as I recall. Both types use the right oin for output. Once
you have gotten + / - rails restored, just try the thing out on a different
input. Chances are, it'll work. All you have to do then is replace the phono
preamp IC, if you're ever going to use that input. If not, just don't bother
even putting a chip back in.

Arfa

16. ### Guest

If you see high failure rates at 80% rated voltage, something's wrong.
Perhaps the circuits are such that voltage fluctuates. Or maybe theyre
low grade caps.

The load is a bridge rec + reservoir, so it only charges the reservoir
caps at the peaks. Most of the time i=0, and at peak i= several times
average. Copper losses have a bigger effect with peaky waveform on a
low power and thus poor regulation transformer.

averaged over each cycle yes, but instantaneously its the other way
round.
no... thats a 9w transformer. Why would one fit a 9w tf to a 3w app?
an old fashioned inefficient way to do things. Cheap volt regs make
such practices unnecessary today.

12% regulation is a rephrasing of what you stated there. Unless you
mean 28v due to some other cause.

exactly. If you work through the theory + numbers you'll see what
youre doing creates results that work fine until mains sags, then they
go wrong. A designer has to make circuits that tolerate the usual
overvoltage and undervoltage limits, whereas when repairing this is
optional in practice.

Surely it should be as expected, else you've miscalculated.

Here (EU) all new goods can be expected to function correctly with
real world mains over- and under- voltage.

NT

17. ### Arfa DailyGuest

It's something that I've seen for 35 years with all grades of caps ...

I think that this is a highly debatable way of looking at it. If the cap is
of a sufficiently large value, the charging 'peaks' on each cycle should be
small, once the cap has gone through the initial charging phase over the
first few cycles after power up, otherwise you have significant ripple,
which I'm sure you would agree, is not the case with most properly designed
power supplies. The cap does the averaging, so the current demand on the
transformer, is pretty much constant rather than 'peaky'.

See above

OK, maybe an overkill, but we already agreed that the curent demand of this
item is likely below a couple of hundred mA, so maybe a 5 or 6 VA tranny,
which is a typical size that would likely have been fitted originally. Even
at this level, I still contend that on a reasonable quality tranny, copper
losses won't be significant.

I wasn't suggesting that this was a good thing. What I was trying to say is
that if a designer decided that say 18v AC was required to arrive at the DC
level he needed on the back side of his bridge or whatever, then he would
have to take account of the fact that a cheapo small tranny with poor
regulation, would be likely to produce a significantly higher level than
that calculated and, because of the very light loading, it would be unlikely
that this value would drop to what was actually required, as a result of the
copper losses that you are fond of ... Cheap voltage regs by no means
mitigate the potential problems of this as, first off, we come back to the
level of voltage that you are throwing across the resevoir cap before we get
near the regulators. Secondly, these monolithic voltage regs are quite
inefficient, being shunt types, so dissipate quite considerable amounts of
power, which is why it is important to keep the input voltage as low as is
practical, above the required overhead for correct regulation. If a circuit
is designed for a particular input overhead, based on what the calculated DC
*should* be, and then that DC turns out to be 15 or 20% higher due to poor
transformer regulation, this is going to significantly increase the
dissipation in the regulator, which might mean that the calculated
heatsinking that was required, is no longer sufficient, which could lead to
the regulator starting to go into thermal foldback, which completely wrecks
any stabilization that it was bringing to a rail. This is another reason why
it is important that the OP gets it right, and why I doubt that the raw DC
was originally anywhere near 40v.

No, and again, you seem to misunderstand. I am perfectly capable of
calculating that the 3v I'm suggesting represents about 12%, and that is
exactly what I was intending. I just felt that in this particular context,
an actual voltage was more 'meaningful' than a percentage. I was in fact
referring to your " or maybe higher. We really don't know ... " What I'm
saying is 28v on a 25v nominal tranny output is bad, higher, if you think
that it might be, is even worse.

I agree, but there are limits, and sags of the sorts of level that you are
suggesting are pretty significant, and much worse than I would have expected
over most of the civilised world. I see many many group amps and hifis for
repair, all of which employ some kind of regulators, and most of which use
78 and 79 series ones, which as you rightly say, are cheap. Most group amps
have semiconductor front ends these days, employing opamps, run very
typically from +/- 15v rails, derived from 78 / 79 regulators. It is
*exceedingly* rare for the input to these to be in excess of +/- 25v. In
practice, even if the regulators did drop out of tight control for brief
periods of excessive power line sag, it is unlikely to have a significantly
noticable effect on the performance of the opamps, and I think that most
designers would be prepared to accept occasional poor regulation on these
occasions, as a trade against excessive regulator dissipation in the vast
majority of circumstances.

No, because the real world calculations will not match the theoretical
calculations, because I still maintain that in cases of very light
transformer loading, the copper losses will *not* be significant. This means
that you need to calculate for nominal output volts plus the overvolts from
the transformer regulation factor. Perhaps I should have made it clearer and
said " how much higher the output voltage will be than the nominal
transformer output voltage, would lead any calculations based on that, to
suggest ".

Being in the EU myself, I am aware of this, but I don't think that there are
too many places in the world where 15% sags are the norm. On a nominal 230v
supply, that represents around 35v. I would be pretty pissed with my power
supply company, if my house input was dropping below 200v on a regular
basis. In the U.S., such brown out events do occur, but I'm willing to bet
that not too many items of equipment with linear, rather than switch mode
supplies, are able to cope with a drop 15% in their incoming line voltage,
without showing some operational signs of it. Maybe I'm wrong on that.
Perhaps someone in the U.S. would care to comment ?

Arfa

18. ### Guest

I'm puzzled why I havent and everyone else here has.

ok, explanation time.

What you say is true for a large high power good regulation
transformer, but things are different with 6 watters.

Opening my catalogue at the 6w transformer section shows regulation of
25% for all the 6 watters.

What this means is that when delivering no load V_out is 25% high, and
at full load that 25% is dropped across the transformer, it is copper
loss. And this is true for a sine load.

Now, along we come with a BR+reservoir load, which only eats at the
peaks. Trouble is, peak current is several times tf rated current, and
thus copper loss V_drop is several times 25% of V_out_rated. What this
means is that peak i is reduced, and conduction occurs over a wider
angle than is ideal. It also means V on the reservoirs falls due to
copper loss.

The end result of this is even poorer regulation on the higher side of
the Vregs.

Quality is nowt to do with it. 25% regulation is standard for a 6w
tranny. If you build one with lower R wire, it can pass more i and has
higher power rating.
12VA 10%
100VA 9%

you need to read up on transformers & psu design
no, it worsens things, as today we use minimum power transformer with
bad regulation and sort it out with a low cost regulator.

sure, just basic cost and energy efficiency

If thats the situaion then the designer doesnt know what theyre doing.

I dont know you think that. You'd be hard pressed to find a 6w tranny
with regulation as good as that.

Whats the real load of an opamp based preamp? More like 10s of mA.

only if you screw up the calcs

Its not possible.

NT

19. ### Arfa DailyGuest

Well, I guess that we are never going to agree on this, and you seem
determined to take everything I say out of context. It has now got a long
way from the OP's original question, and FWIW, I personally think that he
has got it right with an 18-0-18 transformer, and you are completely wrong
suggesting by your calculations that he needs 40 volts on his raw rails, or
that his caps will stand up to that for long, just because they are marked
"40v". I have no desire to turn this into yet another ' mine's bigger than
your's ' thread, so for my part, I'll be leaving it at that.

Arfa

Arfa

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