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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