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

Thu Oct 5 09:47:35 PDT 2006

Hello Again Bill,
Sorry, I hadn't seen Jay's response when I just finished my last post to 
you.

I have worked on a 534 amp for a Wurly 2300S and its schematic shows the 
center-tap grounded near transformer, not through a jumper on the speaker 
plug. So that's fine, Jay's info really helps here.
Sounds like Jay is right on the money with his analysis (as usual.. 
:-).....)
The transformer winding insulation is leaky to ground.  And IF you try a 
huge fuse you will notice the transformer will get very hot, within 
minutes.... likely to the point of smoking. Suggest you not do this as a 
burned transformer is far more troublesome and messy to rewind. Hence more 
$$$.    ;-)

So, as Jay says, replace or rewind.

If you get a replacement, matching the colour coded winding leads is easy if 
you diagram things as you dissassemble.

If you go the rewind route, MAKE SURE you do a nice pictorial diagram of the 
pins and places where you unsolder the leads. On the diagram mark those 
places "A", "B", "C", etc and put matching tape tags on each lead of the 
burned out transformer.  Should I be the one to rewind it, I always wrap in 
new color-coded leads and will tag each of them to match the "A", "B", "C", 
etc.   That way we get on no problem with polarity. The other risk of 
reverse polarity is covered..... when I disassemble the tranny I carefully 
note and precisely replicate several things about the winding(s)....
its start
its stop
wire diameter
number of wraps per layer
number of total wraps
winding wire length
direction of wrap rotation
lead diameter and  colour and your marking
lead length (I add 2 inches )
I use your old core and case, send it all so I know the fit will work.

Pricing US$:
385.00    for primary and 2 secondaries (center-tapped or not)
add 95.00 per secondary (for 3rd, 4th...)
add 10.00 per tap on 3rd, 4th secondary
add 50.00 if transformer is cooked (smells burned)
add 28.00 for varnish triple dip (mandatory)
add 20.00 for shipping handling return
When I handle the winding wire my fingers are washed and degreased to avoid 
skin oil and acid contamination of the winding. In old transformers, I have 
seen winding failure due to internal corrosion and the only thing I can 
think of that caused it was a factory employees dirty fingers, handling the 
wire during coil winding.......  so I stay clean.

Such newly wound transformer will run cooler that the original (even as new) 
because today's wire is insulated (double or triple) with better material.
Hope this helps,
Don

----- Original Message ----- 
From: "Jay Hennigan" <jay at west.net>
To: "Jukebox mailing list" <jukebox-list at lists.netlojix.com>
Sent: Wednesday, October 04, 2006 10:18 PM
Subject: Re: [Jukebox-list] wurlitzer amp blowing fuse...

> 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
> NetLojix Communications, Inc.  -  http://www.netlojix.com/
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