Nope, it's not normal. I would recommend calling Audio Research. It sounds like you will be sending it in for repair. I hope you didn't pay too much for it. This is why a 9/10 rating on these older units is not accurate. The faceplate may be free of scratches, however, the electronics is 30 years old and failing. Also, if you want your preamp restored to original and sound the way it's supposed do not send it to anyone except Audio Research. It may cost a little more, but it is well worth it.
ARC SP-6 Warmup time
I just bought an Audio Research SP-6 preamp. It takes about ten minutes after powering on for the power lamp to stop blinking and the unit to start passing signals. Other than that it sounds perfect on all inputs. Is this at all normal? Does it indicate something is about to fail? Ten minutes to warm up is more than annoying.