Transformers voltage specifications are based on the rated current draw, at AC, at no load or a quiescent state, it will be much higher. You can expect a 12.6V transformer to actually test at about 15V with zero load directly at the secondary before the rectifier, at least this is my experience. Additionally, boomboxes don't use AC, they use DC and the type of PS circuitry makes a difference (full-wave, half-wave, capacitor or inductor filtered, etc. The final voltage peak will be different from the AC voltage once rectified into DC. For example, if it uses a full bridge rectifier and is capacitor filtered, you can expect final DC output to be approximately 1.41 the original voltage. Unregulated with no load. That brings the DC output from a 12.6V transformer probably somewhere around 19V which is about all that most 12V boomboxes can handle. If you start with a 15V transformer (or 14.4V which is probably more common), you're going to almost certainly end up with well over 20V unregulated after rectification and filtering.