It's not voltage issue, you never reach battery wirering from 1/4 phone.
If you want to use this battery tester, you can change switch jack to stereo jack (TRS jack).
I did it my Hamer B12-S with activator circuit.
I don't use this tester, but stereo jack wirering is working fine.
Series instrument are different because stereo wirering.
It means tip and ring for signal. There is No way to covert.
P.S.
Anniv. Circuit is stereo wirering for signal as Series inst.
It needs switch jack.
Eiji
(Message edited by room037 on September 13, 2015)
(Message edited by room037 on September 13, 2015)