Yeh, I swapped the wires at the lock-on this morning, remembering this is what was needed for the MTH locos when the whistle did not function. Now I have a bell sound, a good one, but still no horn. I am going to try the other suggestion and see if more juice gives me the horn.
I gather that swapping the wires at the lock-on one way will give the bell and the other way the horn?

I am going to swap away!

No, the unit was sealed and brand new. I will check for the horn and board in there. How difficult is it to install the horn and board if need be?

Hello, I just purchased a GP-38 with Tru-Blast II horn and with my MRC Tech II 027 the horn does sound when the horn switch is activated. It operates all my Lionel locos and MTH locos and their horns/whistles with no issues. All I get is a blink of the lights from the GP38.
The locomotive performs fine and pulls great so I am very pleased with it but I would like to hear the horn sound. Is there a bell also, that particular transformer does not have a seperate bell button, could that have something to do with the horn not working?

Thanks