[Jukebox-list] wurlitzer amp blowing fuse...

[Jens Hultgren]

Once after replacing a power xformer on a 530, I was astonished there was no
sound although everything had been double-checked before
applying power.

Tubes were dark???

Yeah, I had reversed the leads for the 3.2 volt filament winding.

 Jens


On 10/5/06, Jay Hennigan <jay at west.net> wrote:
>
> Don wrote:
> > Hello Bill,
> >
> > There seems to be a little confusion here.
> > Apart from the fact I don't have the schematic to refer to and so, am
> > taking certain risks here...
> >
> > You say you can measure from center tap to each side of the 5U4.
> > This presumes you have some point say "a" where you can access the
> > center tap of the winding.....  Actually you should be picking your
> > ground point at the place where the center tap wire is connected to the
> > chassis. This point is depicted by "b" in diagram.
>
> The center-tap of the HV winding is grounded via a jumper on the speaker
> plug.  Assuming that he is measuring with the speaker unplugged, it
> sounds as if there is a short to ground internal to the transformer
> about 44 ohms from the center tap.
>
> Bill, when you are making your measurements and when the fuse is
> blowing, is the speaker plug disconnected?  If you are measuring
> continuity less than several megohms between the chassis and any portion
> of the secondary with the speaker and 5U4 unplugged, you have a bad
> power transformer.
>
> Unless you can find a cheap 530 amplifier with a good power transformer,
> you should probably take Don up on the rewinding offer.  This is an
> unusual transformer in that it has a split winding for the filaments in
> series with another transformer.  A generic transformer won't work.
>
> Don, if you rewind this one, keep track of the start and end windings
> and color codes.  The phase has to be correct.  This amplifier has two
> power transformers.  One is powered continuously and provides 24 volts
> AC and DC via a selenium (Bill, replace this with silicon) rectifier as
> well as half of the filament voltage for the tubes other than the 5U4.
> This keeps the filaments "half-hot" when the juke is idle.
>
> The other transformer, which we're discussing here, and appears to be
> defective, has the center-tapped HV winding, the 5-volt filament winding
> for the 5U4, and an additional 3 volt winding for half of the filament
> voltage.  It is powered only when the jukebox is actively scanning or
> playing.  When a selection is made, this transformer powers up and
> provides high voltage.  Its filament winding, in series with the other
> transformer, raises the voltage on the tube heaters to 6.3 volts for
> normal operation.  So the phase has to match the other transformer or
> the heater voltages will cancel and drop to zero during operation.
>
> > -------------------------------- PLATE of 5U4
> > B
> > B   coil @ 51 ohms
> > B
> > B
> >> "a"--------C.Tap----- GROUND"b"chassis *** Via speaker jumper! ***
> > B
> > B
> > B  coil @ 51 ohms
> > B
> > ------------------------------- PLATE of 5U4
> >
> > So, I wonder, how can you get 44 ohms between "a" and GROUND...? Unless
> > the wire from "a" to GROUND is corroded or not electrically tied to
> > chassis ground due to a loose bolt or cold solder joint or.....
> > I wonder what point "a" it is that you are calling "center tap"....
> > I wonder if you are using the same "GROUND" point for all your
> > measurements.
> >
> > I suspect your 600 + volts is "floating" that high because something is
> > not grounded, either the CT of the power transformer's secondary or
> > something else such as the grounds for the electrolytic filter
> capacitors.
> >
> > If, you have no other option but to rewind.... I do that. After holidays
> > in next weeks.
>
> --
> Jay Hennigan - CCIE #7880 - Network Administration - jay at west.net
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