I have a Dayton Wright SKS MK3 preamp. I recently restored my turntable to working condition. When I hooked it up to the phono input I didn't get any sound. Then, I held down the button that selects the phono input. While doing that, it sounded fine. The non-phono inputs work fine, so I took it apart. I didn't see anyway to actually do anything, so now I don't know what to do.
Try some contact cleaner on the switch. Use only as much as needed to get the job done, do not swamp the inside of the pre amp with cleaner. If you still can't get the switch to work properly, you can, if you can gain access to the entire switch, use the flint from a book of matches and very lightly try to scrape off the oxidation between the switch contacts. All this must be done with the pre amp unplugged from the AC outlet.