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

[Jay Hennigan]

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.

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