The 552 has inputs which are mappable i.e. the inputs on the back can be altered to match to the buttons on the front panel.
Of course, if none of the inputs are ‘active’ when fed by the Supercap, this suggests no signal.
See below, section 4. If your 552 is on factory settings, input 3 (per diagram) should be tuner, which is how my Supercap to 552 is hooked up.
If not, act per instructions to allocate the input as required. IIRC, you will need good light/access to see the flashing indicator on the rear of the 552.