I've got this long standing issue with my Tasacam TSR 8 channel 4 card, the signal plays back fine with DBX switched out but when DBX is switched on there is no output signal present. The other 3 DBX channels work fine on that DBX switch. I took the card out & swapped it over with another channel but the exact problem followed the card with the replacement card working fine on the same channel. I cleaned the card with PCB cleaner / isopropyl alcohol with no luck. I looked at the schematic in the DBX section of the r/p card circuit below. I assumed the problem lay inside the U10 switching circuit, so I removed & socketed the U3 & U4 op amps (NEC 4570C ) with equivalent LM833N but no change,. I removed & socketed U6 DBX VCA & also tested with equivalent THAT 2180 without any change. There's 5 transistors in the DBX circuit serction, Q17-Q22 (2SD1915 ), I've tested them all & some appear to be faulty, although I tested in circuit so perhaps not? Or the U10 (LC4966) 4way bipolar switch itself could be faulty? Anyone with any advice on what to try next would be very welcome!