[Arran]

I think we need to go back to the original post, one was a Zenith (Zenette) that definitely had a 25 Cycle transformer, the other was a Sparton where it is not clear exactly what it had, the only thing that they definitely had in common was a high B+ voltage. My understanding of power transformers and line frequency is that they are similar to resistors with regard to wattage, but the inverse. A higher frequency transformer does not like to operate at a lower frequency, but the lower frequency ones can operate at a higher frequency. I have owned several radios with 25 cycle transformers, it was the standard in my area and many others until the late 1950s in Canada, and not one has had a defective power transformer, I have repaired many for other people and never encountered a 25 cycle set with a smoked transformer. I have a collection of spare transformers, but have yet to have needed any for my sets, the last one needed for a radio was sold to someone else, a 60 cycle post war RCA Victor. Most manufactures offered both 25/60 and 50/60 cycle power supply sets in Canada except for one of the largest companies, Rogers-Majestic, every set they made had a 25/60 cycle transformer regardless of where it was sold.(I should mention that sets usable on 25 cycle cost more, hence why most companies offered 60 cycle sets as well) This is not a case of a ticking time bomb that has not gone off, it has been fifty years since 25 cycle power was offered for domestic use so if it were in inevitable almost all such devices would have developed this problem, much like the late 1920s RCA Radiola sets with type 50 tubes. I have also known several retired servicemen who were around in the era of the changeover, plus I have known probably a dozen long time radio collectors, and I have much of the factory service data from the same, the issue has never been mentioned with anything aside from electric motors.
I can't comment on the Mexican TV sets, although I wasn't aware that they used 50 cycle mains in Mexico, 50 cycle usually goes along with 220-240 volt power systems like they have in Europe, and the last radio I saw from there was a 110-125 volt Hotpoint tube set. But TV sets are a much more complex device then a radio, a number of component failures could cause these problems, without an autopsy it's hard to say. Maybe they were defective to begin with? It isn't unknown for people to sell broken down electronics at a garage sale just to save a trip to the dump. Even just the length of time the TVs were out of service can be a factor, electrolytic condensers don't age well and poor storage conditions do not help.
I think perhaps the higher line voltage may be more of a factor then line frequency, it may be worth carrying out a few tests on my own 25 cycle sets to see what is going on. Many were originally rated for 115 volts, it only takes a voltmeter to see what is going on, if anything. Strange enough I have never run across any Philco with a bad power transformer, unlike Zenith whoever manufactured them for Philco never cut any corners with their 1930s models.
Best Regards
Arran