Electric issue - can’t start engine and no headlights

[TR6-ATX]

The engine started right up but it died a few minutes later when I turned on the headlights. I have a battery that has 12.8 volts and a rebuilt alternator that showed charging on the ammeter. Checked the battery with a charger and it was at 90% capacity. With the ignition key in the on position I checked the fuse box and there is no power to the top fuse or bottom fuse. The middle fuse shows 12.7 volts and all the interior lights work fine. Any thoughts on what to check next? It is a 1970 TR6. I did install a headlight relay a few years ago but never had a problem. Could this be an ignition switch issue? If so, how do I check it? It is one of the locking steering wheel type. I welcome any suggestions.

[RatRidgeRoadster]

I would say the brown/white wire from the ammeter to the ignition switch is not delivering power to the ignition switch. This might be because of a loose terminal at the ammeter or the switch. Another possibility is that the ammeter coil has burnt out, but under normal circumstances, I don't think that would take out the ignition coil causing the engine to stop. Either scenario would take out the headlights because there is another brown/white wire on the same ignition switch terminal that extends to and powers the headlight switch.

Do you have a wiring diagram to follow along with what I've described?

You can easily check if the ammeter coil is not burnt out by verifying that you have a reading of ~ battery voltage at the positive terminal of the alternator (large brown/white wire).

[TR6-ATX]

I do have the electric chart for this year model. I checked the voltage at the alternator and it was good. I will check the brown/white coming off the ammeter today. I remember seeing the ammeter fluctuating when accelerating right before the engine stopped. Thanks for the tips!