In recently got an old Cal Spa PS-4 hot tub, and after replacing the heater solonoid switch, and the thermostat, I got it working and enjoyed for awhile. I hadn't used for a few weeks and now find the heater won't cut in. The switch won't activate. Everything else works. I have removed the filter and tried running but still no heat. I can get the switch to cut in if I bridge from the pole on the bottomto the one on the top right, but don't know nwhere to go from here.

We would need more information as many of these controls while similar under the hood, will be quite different electrically depending on the year they were built.

A digital photo posted here of the inside of your ps-series control box would help a lot.

Here (hopefully) is the photo of the inside of the box. Really having trouble loading it. I have tried removing the filter buit the switch still doesn't cut in. The heater was working fine but then just stopped between uses. Can get the switch to cut in if I bridge between bottom pole and top right pole on switch.

Whoops! Having trouble getting good photo up. Sorry for the hassle.

Don't really know what you're talking about by 'bridging the poles', but all I can assume on that is that you're referring to the heater contactor in the top center, you're applying power to the heater contactor coil to get it to energize manually.

Assuming that your thermostat is working properly, I'd suspect that your flow switch is causing this problem. I'm not there and only going off of your photo and I cannot verify everything exactly... so here goes, use at your own risk.

The power for the heater contactor goes through the big flow switch to the right of the cabinet. If that switch does not close, then it will prevent the contactor for the heater from working. You need to be sure that you've got voltage at the plug for the flow switch (black line), and that it's coming back to the white line, which would energize the contactor.