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

[Bill Taylor]

Don & Jay,

When I measured ohm of the center tap to the windings,
I use the wire attached to the speaker socket pin 3
(where ground is found via the speaker plug) and to
the 5U4 side of the windings with the 5U4 socket pin 4
and then pin 6.  Between the center tap to 5U4 pin 4,
I got 51 ohms.  Between center tap to 5U4 pin 6, again
51 ohms.  Now with both the speaker plug (speaker plug
is needed for ground) and 5U4 out, there shouldn't
have been any measurement from the center tap to
ground. When I looked for a short to ground, I did
find 44 ohms.  The only place to get some continuity
would be via the transformer itself, shorting in the
windings.

Now things are starting to make sense.  Since I've
been working on the amp module on my bench I didn't
even had the speaker plugged in.  I shouldn't have had
any high voltage without having that center tap
grounded via the speaker.  Yet, I've been getting 600V
out of the rectifier, or at least for just a few
seconds before the fuse blows.  I guess whatever has
been bleeding into the rectifier has been keeping it
running until the C35A cap charged up to 600V then
blows the fuse.

Now for grins I jumpered pin 3 of the speaker socket
(which connects to the TX center tap) to ground to see
what happens.  As I would expect, the fuse blew
imediately.

If I'm getting all this correct, I think my problem
has been diagnosed.  For those you guys who thought
the transformer was the problem at the beginning of my
emails... you guys are pretty sharp!

Don, again if you think I got this right, I'll look
around to see if there are any old amps to be found. 
That will give me a few weeks.  If I can't find one,
after the holidays I'll take you up on that
transformer re-wind of your offer.

Thanks, Bill


> 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.
> 
> -------------------------------- PLATE of 5U4
> B
> B   coil @ 51 ohms
> B
> B
> >"a"--------C.Tap----- GROUND"b"chassis
> 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.
> 
> Cheers,
> Don
> 



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