[AMRadio] Re: 500 Transformer

[russ dworakowski]

It is not  really  unusual for  these transformers  to go bad.  You must  
consider  the age and  how  they  were manufactured.  You can  blame  
Johnson for  using  cheaper  transformers,  they  could  have  used potted  
designs.  But  who  would  have thought that  you would  be using  this old 
stuff?  Old transformers do bad  things-  they  suck up moisture in  the air 
and  the windings viberate and  break down the
insulation.  I don't  think  you will find  anything  seriously wrong   with 
your  amp.    Heres an experiment:  got  an old  unsealed  transformer in 
the basement that has not been  run  in a few years?
Put it  in  the oven@ 200 degrees.  Just leave it in  for a few  minutes.   
Open  the oven door and  you are likely  to see it steaming!
Thats water.....Russ.






>From: Dave Aabye <w4qcu at prodigy.net>
>Reply-To: amradio at mailman.qth.net
>To: amradio at mailman.qth.net
>Subject: [AMRadio] Re: 500 Transformer
>Date: Mon, 10 Feb 2003 14:20:22 -0500
>
>Hi Gary,
>
>Yep, it's a bummer!
>
>WZ1M wants $260 to rewind the existing transformer.  Peter Dahl sells a 
>replacement
>for $285.  I think that Dahl offers a better deal because it is a hypersil 
>unit
>with a 500 ma CCS rating.  I believe the original is rated 400 ma ICAS.  
>Only
>drawback is the need for 4 new mounting holes.
>
>I replaced the tube recitifiers with solid state units shortly after 
>purchasing the
>500 (about two years ago).  I also inserted thyristers in the primary leads 
>to cut
>down on inrush current.  I wonder if you have any further protective 
>measures to
>suggest.
>
>73 de Dave, W4QCU
>Oak Ridge, TN
>
>Gary Blau wrote:
>
> > Dave:
> >
> > Ouch.  Sorry to hear that.
> > Sounds like WZ1M's offer to rewind might be attractive, unless you can
> > find another 500 xfmr somewhere (good luck!).  That's a great rig so
> > it's worth the investment.
> >
> > Now, to find out why it went bad in the first place...
> > Best of luck.
> >
> > Gary
> > W3AM
> >
> > Dave Aabye wrote:
> > >
> > > Hello Gary,
> > >
> > > Thanks for the suggestion, which I believe has proven to be 
>conclusive.
> > >
> > > I hooked my Variac to the primary and slowly increased the secondary 
>voltage.
> > > The caps were disconnect from the rectifiers, so the transformer was 
>"all by
> > > itself".  At about 700 volts, the voltage started to jump just a bit.  
>At
> > > maybe 750 v, the secondary emitted a puff of smoke with an 
>accompanying hiss.
> > >
> > > I think it is clear that the transformer is shot.  Time to call Peter 
>Dahl!
> > >
> > > Again, thanks for your input.
> > >
> > > 73 de Dave
> >
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