One thing about JVC's that I hate, at least on the models with sliders is that they use Alps sliders that are just flat out crappy. They fall apart internally. I had a b**tch of a job on an RC-838 where 4 of the 5 sliders (not counting the volume) were bad, and by bad, I mean fell apart inside.
So had to search high, searched low and finally found someone with some NOS Alps sliders, same type except different tonal curve and value. I had to remove the old sliders, carefully disassembled them, took the broken slide and replaced them keeping the old carbon resistor base. Well, what bugged me was that on the new ones, one was already broken (same way) brand new out the box. This tells me it failed not due to wear but POOR design. I reinforced mine with superglue and finally fixed it but it was not fun.
Not sayin that your M70 has the same problem but I would not be suprised at all. Mine wouldn't respond to cleaning and I finally decided to take the sliders apart after all indications were that the problem (dead channel) was in the tonal control circuits and lead to the balance control. After fixing that, I checked the non-responsive tonal controls just in case and sure enough discovered that the same thing was occuring in those as well.
The problem I'm talking about is that the "feelers" that slide and contacts the resistor breaks off internally. No contact between feelers and resistors mean no change in tone. In the balance control, no feelers mean one dead channel (or both, if both are broken).