[Amps] Alpha 91b problem

[K9FD]

I have a 91B that has the exact same symptoms,  and it appears its the 
transformer.
So you can add another to the list.   Also have a 99 and it has just 
went south,
hoping its a shorted tube and not the tranny on that one.
I would sure be in the market for a transformer if any one has one.  Not 
sure I will
spend 1000 for a new one to keep the 91 on the air.
73 Merv K9FD
> 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<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows
>
> From: Alek Petkovic<mailto:vk6apk at bigpond.com>
> Sent: Sunday, November 20, 2022 1:59 AM
> To: amps at contesting.com<mailto:amps at contesting.com>
> 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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