Jackson 707 crt tester problem

[FRANK J.]

I have (2) Jackson 707 CRT testers with the same problem! With a little as 32 line volts applied via variac (enough to warm the 5Y3 LV circuit tube) , test switch in"merit" position , and merit switch in "grid" or "beam" positions meter pegs right and in "gas" position meter pegs left. These testers have separate 650V HV and 150V LV DC power supplies with independent power transformers. Pulling the 5Y3 for HV supply does nothing so problem surely originates in the LV circuitry. The 6SL7 bridge circuit seems to be the culprit. Note that one tester had this problem at purchase and the second tester was working when identical problem suddenly occurred. All caps replaced as well as any out of tolerance resistors. Many hours spent but unable to remedy. No service data or parts list available anywhere. Only have the schematic in the instruction manual. Any help greatly appreciated!