The Tube Tester was needed for some tube testing recently and I had to get in there and troubleshoot a few problems. There was B+, but no bias voltage or heater voltage. This machine uses two separate power supplies to generate the needed DC. A 5Y3 Rectifier tube had finally gone bad in the low voltage supply. Replacing this tube as well as the AC cord (brittle) and cleaning all sockets and switches (particularly the NORMAL/LIFE TEST switch) restored proper functionality and the machine can be used again! Basic maintenance after 60 yrs.