[Amps] A76pa glitch on warm up

[Charlie Young]

Jeff, hopefully you won't find the problem I had on a friend's Alpha 91b.  I suspect this problem is rare, but might be worth noting.  These amps share a power transformer design which uses multiple windings on a common core.   
 
A step start resistor was open.  After replacement, on startup the Alpha 91b would immediately blow the step start fuse.  I checked all the usual suspects in the power supply and on the HV side of it, with no problem found.  
 
To make a long story short, the filament winding and HV winding became connected inside the transformer.   With the HV winding connected to the diode board, the transformer behaved  like it had a shorted winding on startup, immediately blowing the step start fuse.  If I lifted the HV winding off the diode board, the amp would power up fine.  The no load voltage across the disconnected HV winding was good.  Only when it was connected to the diode board did the transformer act shorted.  
 
Again, this is hopefully not your problem with the 76, but I will never forget this issue, since it took me a while to figure out what the heck was going on. 
 
I also have a 76PA.  It has not been on in two or three years.  This reminds me to power it up slow and blow the cobwebs out of it. 
 
73 Charlie N8RR   
 

> From: keepwalking188 at yahoo.com
> To: km1h at jeremy.mv.com; amps at contesting.com
> Date: Tue, 1 May 2012 23:32:01 -0500
> Subject: Re: [Amps] A76pa glitch on warm up
> 
> Finally got back to looking into this after some travel and found the 
> blasted amp is DOA. Murphy's revenge!
> 
> Well, not quite DOA. Power relay K1 is functional. But K2 offers some 
> interesting observations... To wit, the red wire side of the step start 
> relay K2 seems to be shorted. And fuse F3 - the step start fuse - is blown 
> open...
> 
> I've not lost F3 before that I can recall. But the amp is 25 years old so 
> I'm not sure how to read the short on relay K2... Is the 25 years catching 
> up to my sweetheart - or is this a sign of some kind of trouble down the HV 
> supply path?
> 
> 73/jeff/ac0c
> www.ac0c.com
> alpha-charlie-zero-charlie
> 
> -----Original Message----- 
> From: Carl
> Sent: Friday, April 06, 2012 9:46 AM
> To: Jeff Blaine ; amps at contesting.com
> Subject: Re: [Amps] A76pa glitch on warm up
> 
> Leaky caps and out of tolerance resistors on the PC board would be the first
> place Id look.
> 
> Carl
> KM1H
> 
> 
> 
> 
> ----- Original Message ----- 
> From: "Jeff Blaine" <keepwalking188 at yahoo.com>
> To: <amps at contesting.com>
> Sent: Thursday, April 05, 2012 11:43 PM
> Subject: [Amps] A76pa glitch on warm up
> 
> 
> > My beloved A76pa in the last month has started to glitch during the 
> > warm-up countdown cycle. And on a couple of occasions it has shown a few 
> > hundred mA of Ip when sitting at idle.
> >
> > Wanted to see what the group thought may be at the root of it...
> >
> > 73/jeff/ac0c
> > www.ac0c.com
> > alpha-charlie-zero-charlie
> >
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