MK120 always alarming

I have a MK120 kit that Ihave assembled successfully.
The transmitter works OK. I can see the LEDs light up when viewed through a digital camera.
The receiver is always in the alarm state, even when the IR transmitter is pointing at it.
The only response taht I see is a slight flicker of the alarm LED and a change in tone of the buzzer whn I switch on the transmitter.
I have checked the voltages shown in previous posts.
I can changed the IC.
I have tested both T1 and T2. They seem to be OK.
But this has no effect.
Does anybody have any ideas what to try next.

Make sure transmitter oscillating. You can check this with a multimeter set to ‘freq’ or an oscilloscope.
If this is the case, then an assembly problem is very likely at the receiver end.
Common problems: inverted phototransistor, diode, wrong resistor values, bad/loose/forgotten solderings, shorts, …

What frequency should I be getting?

a couple of KHz.