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

[Don]

Bill,
Good Morning Bill,
Without a schematic for your amp, I am at a disadvantage here.

However, careful reading of your latest info has me wondering....

I think you have just told us why the fuse is blowing.

You seem to say that all along, you have been firing up the amp without 
having the speaker attached....
>"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."

>From my experience you should always have the speaker attached to any amp 
you are powering up. Try that, and see if the fuse stays in.
You don't want to power up the amp without having that power transformer's 
High Voltage winding center tap "grounded" in the way its designers 
intended.

After tomorrow I'll be off list for a while.
Cheers,
Don


----- Original Message ----- 
From: "Bill Taylor" <bt_2627 at yahoo.com>
To: "Jukebox mailing list" <jukebox-list at lists.netlojix.com>
Sent: Thursday, October 05, 2006 4:24 AM
Subject: Re: [Jukebox-list] wurlitzer amp blowing fuse...


> 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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