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

[Don]

Hi Bill,
Real interesting problem.
After reading this post twice, I noticed that you say on the one hand that 
NOW your fuse blows with the measured voltage across the new electro hits 
550 volts.
On the other hand you mention the new cap is 47uF at 450 vdc

Can you see the problem?
..... if what you wrote below is true.... then you may be having high 
current problems because the new electro has not got a high enough voltage 
rating.

But,  I wonder why the charge voltage hits 550 vdc.  Would this be normally 
this high?  Or is it because you removed all the downstream loads on the 
power supply.  I would hazard a guess that if the normal loads were in 
circuit they would tend to keep the voltage on your new electro from peaking 
that high on start up.  I know this type power supply will peak considerably 
higher than normal running voltage because the tubes don't make their full 
current demands at start up.... but with all loads attached there should be 
enough other load resistance to help keep the startup peak voltage on the 
new electro down under its MAX rating.

So.... do your test with all loads connected...
Then if you blow fuses at 550 VDC you could try to get another electro with 
a higher MAX voltage...

Otherwise it is something else, along with that old electro that is taking 
out the fuse.... :-)

....my 2 bits worth
Good Luck
Don




---- Original Message ----- 
From: "Bill Taylor" <bt_2627 at yahoo.com>
To: "Jukebox mailing list" <jukebox-list at lists.netlojix.com>
Sent: Friday, September 29, 2006 7:48 PM
Subject: Re: [Jukebox-list] wurlitzer amp blowing fuse...


> To Justin and everyone else,
>
> Thanks for responding to my problem.  I looked at the
> primary transformer off the output stage and it looked
> ok.  193 ohms off the center tap, and 215 ohms off the
> other side.  Then ohm'd out from the center tap to
> ground and was open.  So, at least without power the
> windings seem to be OK.
>
> Now, I've been checking out a few other things.  I
> pulled out F2 and F3 fuses too (low DC power) just to
> make sure there wasn't any other draw.  Then with
> nothing left except my 5U4 (by the way, I replaced
> that just in case), C35A, R69A/B high watt resistors
> in circuit (even with that primary winding pulled out
> from the center tap), still she runs for a couple
> seconds then blows the fuse.
>
> Now I understand I could use a 30 amp fuse to find
> what's bad, but I've used this test in other problems.
> Yeah, once in awhile it works.  Others, plenty of
> smoke, tough to breathe, and that terrible pop sound
> when a cap goes up!  With all my options gone I might
> do it, but not quite yet.
>
> So, I'm backing up a little.  I thought C35A was bad
> when I troubleshooted a week ago.  Pulling out a lead
> from that cap, the fuse stays up and I had about 375V
> at the rectifier, although with no big cap to filter
> the DC smooth.  So, I went to buy a replacement and my
> part source said he couldn't get a 30mfd 500v cap (by
> the way the original is a can cap with 4 caps in it,
> one being the 30mfd).  Said he has been using a 47mfd
> 450v electrolytic that works just fine.  I believed
> him.  Does anyone know whether the've used that cap as
> a replacement of the original 30mfd 500v?  It just
> seems to be that the cap could be my problem.  Now
> even with the cap replaced, the symptom is still about
> the same, using just 3 or 4 seconds before blowing the
> fuse.  The only difference, is that the old original
> blows once the voltage comes up to about 250v. With
> using the new one (47mfd) it charges up to 550v before
> blowing.  Does this tell us anything?
>
> Thanks so much for everyone's replies.  And I did make
> sure I had the 47mfd cap at the right polarity using
> negative to ground.
>
> Thanks, and will be looking for replies!
>
> Bill Taylor
>
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