Grandson's original amp board fried.
Was able to use the HiLetgo amp fed by the AC/DC buck converter with a current keeper cap as mentioned above. Using an 8Ω speaker (original) it sounded great and no interruptions going over switches. Adjusted the pot on the amp to reduce the level feeding the speaker (no pad on the input from the LC engine amp) and it was plenty loud. Better than ever. So I ran it for awhile.
Then it derailed at a switch and shorted out popping the Sensata breaker connected to the Z1000 (usually not a problem). Just reset it. There are also a TVS diode attached to each end of the two loops.
Except now there is no sound at all even when put back at the stock configuration. No Bluetooth chirp - nothing except a hum. When Bluetooth remote is turned on the engine moves forward and backward, but the sounds are gone. Tried the Bluetooth app on a tablet - no sounds with that either. So engine controls work, Bluetooth works, sound does not.
Unless there is some miracle anyone knows about I am assuming I have to open up the Lion Chief engine and replace the entire amplifier board.
I knew the risks when I started this project. My grandson is coming tomorrow so I'll let him know what happened.