Check the batteries!!!
I am renovating an ST701 for a relative & came up with this problem too. I replaced the mercury 1.3v cells with Duracell D394 1.5v & used the very good 'battery fix' guide posted here:
(tip: Do remove battery cover before disassembly & line the battery chamber with waxed card cut from a juice carton)
I discovered, to my suprise, that the batteries must be inserted in the reverse of the more obvious direction, the + being the base of the battery chamber. The meter now works fine, after tuning the amplifier as described in the guide.