Jackson 637 Dynamic Tube Tester

[edison64]

Hello: sorry if this is the wrong forum to put this, but it doesnt fit any whwer else.
I am in urgent need of a schematic for a JACKSON MOD# 637 dynamic tube tester. I have used this tester for about 15 years, it's like new. the problem is, it stoped testing tubes, no heater voltage, only on some tubes, then nearly all tubes. but heater v ok on all sockets and trans. cleaned all push buttons, still the same,weird ??? any ideas? its really packed and cramped on space so a schematic is a must. any updated tube data would also be great. thanks for your help.