I think my friend got a Ebay special.. [Repair]

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From: Jamie on 4 Oct 2009 21:14

My friend was a wining bidder on a guitar amp, it was posted as not
working, stating that it blows the fuse instantly. So he
knew that it was broke from the start, didn't pay much for it.
THe seller even removed the shipping cost ($25) to be nice..

How ever, he asked me to look at it for him to see if it can be
repaired. I don't know much about the cost of this type of equipment
these days since I really don't buy it how ever, he stated it goes
for $400 bucks new..

Any ways, I opened it up, noticed that it had already been looked
at, cause some one was nice enough to mark the secondary xformer leads
color on the circuit board. So I guess it's been unplugged from the
board before I got there.
I finally ended up doing the same to remove any doubt of shorts that
could effect the loading that was taking place. As it turns out, even
with all the secondaries disconnected, the xformer still loads up heavy..
This unit only extracts no more than 250 watts max from the line
(2.5) amp fuse when working properly!

After performing some test on the primary of the xformer, I come up
with Lx = 1.5 (4 mh) and 3.5 dc ohms..
Z = ~3.8 ohms. which = ~ 31 amperes at 120 volts, 60hz..
I guess this would stand the reason why it blows out a 2.5 amp fuse :)

When all said and done, I think this defect was already known about
since the primary had already been disconnected and most likely a load
test was performed to verify a bad xformer..

Ok, just thought I would pass that along if any one was interested. :)

From: 1PW on 4 Oct 2009 21:31

From: Dave M on 5 Oct 2009 02:18

Jamie wrote:
> My friend was a wining bidder on a guitar amp, it was posted as not
> working, stating that it blows the fuse instantly. So he
> knew that it was broke from the start, didn't pay much for it.
> THe seller even removed the shipping cost ($25) to be nice..
>
> How ever, he asked me to look at it for him to see if it can be
> repaired. I don't know much about the cost of this type of equipment
> these days since I really don't buy it how ever, he stated it goes
> for $400 bucks new..
>
> Any ways, I opened it up, noticed that it had already been looked
> at, cause some one was nice enough to mark the secondary xformer leads
> color on the circuit board. So I guess it's been unplugged from the
> board before I got there.
> I finally ended up doing the same to remove any doubt of shorts that
> could effect the loading that was taking place. As it turns out, even
> with all the secondaries disconnected, the xformer still loads up
> heavy.. This unit only extracts no more than 250 watts max from the
> line (2.5) amp fuse when working properly!
>
> After performing some test on the primary of the xformer, I come up
> with Lx = 1.5 (4 mh) and 3.5 dc ohms..
> Z = ~3.8 ohms. which = ~ 31 amperes at 120 volts, 60hz..
> I guess this would stand the reason why it blows out a 2.5 amp fuse
> :)
> When all said and done, I think this defect was already known about
> since the primary had already been disconnected and most likely a load
> test was performed to verify a bad xformer..
>
> Ok, just thought I would pass that along if any one was interested.
> :)

Jamie,
You can do this approximation with speakers because of the small number of
windings having very low resistance and inductance. However, you can't
depend on the DC resistance of a power transformer to resolve the current
draw. The inductance of the power transformer winding is the limiting
factor here. In order to calculate the normal unloaded primary current in
your transformer, you need to measure the inductance at 60 Hz with all
secondaries open. Calculate the total impedance using the inductance and DC
resistance. Then you can calculate the current that the transformer should
be drawing.
If your transformer blows the line fuse with all secondaries open, that's a
pretty good indication that there is an internal short. It might be in one
of the secondaries. A shorted secondary winding will cause the primary
current to go very high.
You have two choices here... try to find a replacement transformer from the
manufacturer (someone here might be able to help if you post the brand and
model of the amp), or attempt disassembly of the transformer to try to
locate the short, hoping that it's near the outside of the winding stack.

--
David
masondg44 at comcast dot net

From: N_Cook on 5 Oct 2009 03:11

Dave M <masondg4499(a)comcast.net> wrote in message
news:XNGdnY_OeO1VElTXnZ2dnUVZ_hmdnZ2d(a)giganews.com...
> Jamie wrote:
> > My friend was a wining bidder on a guitar amp, it was posted as not
> > working, stating that it blows the fuse instantly. So he
> > knew that it was broke from the start, didn't pay much for it.
> > THe seller even removed the shipping cost ($25) to be nice..
> >
> > How ever, he asked me to look at it for him to see if it can be
> > repaired. I don't know much about the cost of this type of equipment
> > these days since I really don't buy it how ever, he stated it goes
> > for $400 bucks new..
> >
> > Any ways, I opened it up, noticed that it had already been looked
> > at, cause some one was nice enough to mark the secondary xformer leads
> > color on the circuit board. So I guess it's been unplugged from the
> > board before I got there.
> > I finally ended up doing the same to remove any doubt of shorts that
> > could effect the loading that was taking place. As it turns out, even
> > with all the secondaries disconnected, the xformer still loads up
> > heavy.. This unit only extracts no more than 250 watts max from the
> > line (2.5) amp fuse when working properly!
> >
> > After performing some test on the primary of the xformer, I come up
> > with Lx = 1.5 (4 mh) and 3.5 dc ohms..
> > Z = ~3.8 ohms. which = ~ 31 amperes at 120 volts, 60hz..
> > I guess this would stand the reason why it blows out a 2.5 amp fuse
> > :)
> > When all said and done, I think this defect was already known about
> > since the primary had already been disconnected and most likely a load
> > test was performed to verify a bad xformer..
> >
> > Ok, just thought I would pass that along if any one was interested.
> > :)
>
> Jamie,
> You can do this approximation with speakers because of the small number of
> windings having very low resistance and inductance. However, you can't
> depend on the DC resistance of a power transformer to resolve the current

> draw. The inductance of the power transformer winding is the limiting
> factor here. In order to calculate the normal unloaded primary current in
> your transformer, you need to measure the inductance at 60 Hz with all
> secondaries open. Calculate the total impedance using the inductance and
DC
> resistance. Then you can calculate the current that the transformer
should
> be drawing.
> If your transformer blows the line fuse with all secondaries open, that's
a
> pretty good indication that there is an internal short. It might be in
one
> of the secondaries. A shorted secondary winding will cause the primary
> current to go very high.
> You have two choices here... try to find a replacement transformer from
the
> manufacturer (someone here might be able to help if you post the brand and
> model of the amp), or attempt disassembly of the transformer to try to
> locate the short, hoping that it's near the outside of the winding stack.
>
> --
> David
> masondg44 at comcast dot net
>
>
>

Probably bifilar winding so the failure at point of maximun voltage stress
which is right in the middle of the winding on the inner layup, most likely.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

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