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Re: [Amps] Alpha 91b problem

To: Alek Petkovic <>, "" <>
Subject: Re: [Amps] Alpha 91b problem
From: Charlie Young <>
Date: Sun, 20 Nov 2022 16:04:49 +0000
List-post: <>
I have a defective Alpha 91 transformer in my junk pile.   WD8CCC’s amp started 
blowing primary fuses probably 8 to 10 years ago now.  The problem was an 
internal winding to winding short.   The transformer would make full AC voltage 
on the HV secondary, until the HV rectifier board was connected.  I forget now, 
it has been so long ago, which winding was shorted internally to the HV winding.

I had Ben order one from Alpha.  They were kind of condescending about it; they 
did not believe it could be the problem.   Finally they sold him one (they had 
a used one), while stressing they would accept a return when it didn’t fix the 
problem.  I had the amp running already using an external HV winding.  
Apparently at the time it was not a commonly known issue.  It was a new problem 
to me, but I only occasionally repair amps for friends so don’t see as many 
cases as folks who do this all the time.

Sorry to hear 91B transformer failures are becoming more common.  Ben’s amp is 
still going strong.

73 Charlie N8RR

Sent from Mail<> for Windows

From: Alek Petkovic<>
Sent: Sunday, November 20, 2022 1:59 AM
Subject: Re: [Amps] Alpha 91b problem

Yes. I have a shot transformer from a 91B here. Similar problems to yours.

The owner bit the bullet several years ago and bought the Hammond
replacement transformer and the hv/screen supply board. The amp has been
sold to another friend and it is in daily use. In other words, well
worth the cost.

73, Alek, VK6APK

On 20/11/2022 8:38 am, Terry Baxter wrote:
>        All,
>        Recently I wrote about a problem I had with my Alpha 91b. It popped
> the rear panel fuse F3 (2 amp). The problem turned out to be a bad
> power transformer. In my case the secondary winding for the 105V power
> across pins 1&2 on connector J2. This should read approx 3 ohms. My
> transformer read 10K ohms. In addition it also read 4.5K ohms to the
> HV secondary winding pins 1&2 on J1. A good transformer should read 3
> ohms across pins 1&2 on J2. It should read 10 ohms across pins 4&5 on
> J2 and less than 1 ohm across pins 6&7 and 7&8 on J2. I measured 500
> ohms across the high voltage secondary. Do this quickly as it can get
> interesting if you leave the ohmmeter on the primary. There should be
> a wide open reading between any pin on J2 to J1. There should be no
> resistance between any pin on J1 or J2 to the transformer body. All
> resistance readings are with all primary and secondary connections
> removed from the Power Supply board (in other words all connectors
> related to the power transformer). This quick check should confirm
> whether your transformer is good or bad.
>        I had several responses asking for my findings on this issue. From
> what I learned a lot of the transformers in the 91b are failing. A new
> transformer from Hammond is currently selling for $1000 plus tax and
> shipping. I was fortunate to find a replacement transformer at a very
> affordable price.
>        Sorry for the length of this note. I hope it will help answer any
> questions on this failure.
>        Terry/N6CW
> _______________________________________________
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