Another much cheaper alternative is to just use a separate bias or
screen supply depending on which one has failed. However since my
transformer has a 4.6k ohm short from the 105 V winding to the HV
winding what would be the results of this short? Would floating the
bad bias winding have an effect on the HV winding? Would the HV
winding be compromised with this short? Someone out there would have
the answer to that.
Terry/N6CW
-----------------------------------------From: "K9FD"
To: amps@contesting.com
Cc:
Sent: Sunday November 20 2022 11:34:19AM
Subject: Re: [Amps] Alpha 91b problem
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 [1] for Windows
>
> From: Alek Petkovic
> Sent: Sunday, November 20, 2022 1:59 AM
> To: amps@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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