I added an article to BallyAlley.com called A Power Transformer Substitution for the Bally/Astrocade Computer System by Michael Matte (MCM Design). Michael wrote this article in April of 2018.
If your original Bally power supply fails, and you have experience in electronics, then these detailed instructions with schematics and picture explain how to build a substitute power transformer.
You can read the article in various formats, here:
Here is an example of the finished power supply (more pictures are included in the article):
Thanks to Michael for writing this article. Enjoy!