Our home has a Malibu 300 watt transformer for landscape lights. The lights are on during the day. I have checked the photo sensor and have replaced it but the lights keep coming on regardless of what I set the transformer to except for "off". There is a delay of about 15 seconds. Malibu is out of business. Am trying to see if there is a fix to this or if I need to buy a new transformer.
Old photocells do age. Sort of by convention, they are often designed to fail "on". For instance most street lights and discharge loghts use $15 replaceable hockey puck photocells, designed to fail "on" so city staff can distinguish a failed sensor from a failed bulb, and go up in the boom lift with the right part. The manufacturer will happily sell you a special model designed to fail "off", if you really want that.
New photocells have a different "problem". They have a microcomputer and a clock, and are able to observe light levels over 24 hours and "learn" what night and day look like. This makes them resistant to shadowy locations or slow accumulation of dirt. However they also need 24-48 hours to get acclimated, and if you cut power to them, they may forget and have to learn all over again.
That last bit is important because some people like to put their photocell lights on a switch or mechanical timer too... their modern sensor will never perform right for them.
The transformer is probably solid, the sensor not so much. You also have to watch voltages, you need a different sensor on the 120/230V versus the 12/24V side. You may be better off bypassing the sensor on the transformer, then using a modern photo sensor upstream to switch the entire system on and off.
Typically the sensor 'sees' light and triggers a transistor that turns on a relay. In your case that transistor can be an ic op amp that has a gain setting (how bright it has to be to turn on). Then you have a delay circuit. Obviously it is seeing everything DARK. Meaning the Photo cell is dirty, the cover over the cell is dark, the cell does not connect to your amplifier, your adjustment for brightness is not correct or that potentiometer is broken.