Audio Research SP10 sudden shut off/mute while playing ??

I have an ARC SP-10 that seems to have a problem. After about 45-50min of playing, the signal suddenly cuts off. All sources. I am left with a faint hum, that disappears when I flip the mute/operate switch on the front panel. Only solution seems to be to turn it off, let it cool down, the power it up again. Then everything is fine....for 45-50min.

The circuit includes a turn on mute relay that delays full turn on for 2.5min. The power on LED flashes until the relay trips after 2.5min. This LED does not flash when the signal cuts off as outlined above.

Could the problem be a malfunction of the turn on relay or something else ?
Send to ARC asap this unit is 40 yrs old time for maintenance.
Send to ARC asap this unit is 40 yrs old time for maintenance
That's probably good advice if it hasn't had recent maintenance. The problem could also be a faulty tube in the power supply. Have you checked those?

It's not sophisticated preamp and could be repaired by local tech.
You can also chat with Chris from partsconnexion about possible upgrade of circuit elements of your preamp to give it newer and stronger life.

AR has the schematic on their website. They use a pair of LM555’s to time the on delay, pretty straightforward. Take that to a tech and they should be able to analyze what's wrong pretty quick.