MK 120 IR Barrier short circuit

Kits Mini Kits Range
1

It’s my first time making a kit - any kit.

The receiver seems to be working fine. But the transmitter seems to be shorting. The battery grows hot, the IR LEDs are not lighting up.
I have observed all the polarities. I have matched all of the components to the photograph of the assembled unit on the instruction leaflet - everything is in the right place.

I have been juggling the instruction leaflet of my newly-bought multimeter and the schematic of the kit to figure out what I should be measuring and how but it is not making much sense yet.

I have measured the resistances of all resistors - they are fine.
I have done a continuity test - which has very limited results. The two negative terminals of the power jack SK1 were closed circuit. So were the right legs of all three transistors, all the rest are open.

Having done this I haven’t a clue what is going wrong or how to proceed. Any tips from more experienced members would be appreciated.

2

First of all, IR leds never ‘light’, as human eyes cannot see IR light. However, if you point a digital camera at them, you can see them light.
As the battery heats, this indicates a dead short. As no other components heat, we suspect a soldering short on the board. Measure between the + and the - of the battery connector. It probably measures close to 0 ohms. Next, check the board carefully. Look for shorts between tracks, touching solderings, etc…