The blinking red light is the battery charging circuit protection, the battery needs to be functional and chargeble along with the DC input power.
If you have missed any caps on the board then you may also get the blinking red light. You could say anything which does not power on
has a design flaw, I wouldn't say it was a flaw as it works with all the correct capacitors and batteries replaced.
I've repaired 2 units with the blinking red light and the blinking led that you are talking has other conditions in which it blinks to show
the diagnostic code. There is also an led code output on the LPT port which are all listed in the repair manual with the correct procedure
to remedy the fault. I guess that you did not use an LPT diagnostic connector on the LPT port as the blinking red led suggests there was a fault
code available and the error could be something other than the battery or power.
You see, almost all Toshiba laptops from that era had an integrated diagnostic circuit and ROM which allowed the use of a LPT dongle to diagnose
any faults with the system through the printer port (of course you'd be stuck if the LPT port failed). These dongles are easily made with just a handful
of leds and some wiring. I have used the for all sorts of diagnostics on Toshiba's and you can even reset the CMOS chip and any registered passwords.
If you are serious about repairing and restoring vintage computers, you would have one of these dongles.
I guess you were lucky to find 6 working laptops, that's not to say the non working Toshiba's had design flaws is it.
Thinking about it, you could say that all electronics from that era had design flaws when you compare them to the advancement
in electronics today
I'm only trying to point out that you may have missed something else when diagnosing the board and you may have focused all your
attention to the power supply when the fault could of been something as simple as a jumper switch or corrupt CMOS memory.
Sorry for the lenghty post, but I always like to detail my response with as much information backing up my statements rather than
one-shot comments.
What has it got to do with a Zenith center-negative plug ?