Toshiba BomBeat RT-S85 - no sound

I have a pretty BomBeat RT-S85 from Japan. I decided to see if I could run it on 240V using a transformer from a Pioneer which delivers the same 13.5V that toshibas such as this (all the RT-S8xxx series appear to be the same box with slightly differing looks and decks) run on.
So I gave up fiddling about with it as popped some batteries in for a test run. Hey ho - everything working, functionally and mechanically - except no sound at all. I put headphones in and I can hear the static pop as they are inserted into the jack as normal. If I turn the volume up all the way I can hear the radio very faintly - but interestingly not whilst it is switched to radio mode; just when it's switched to Line mode or a tape is playing.
Line out works as it should, using another box to test it out.
I haven't taken out the main circuit board yet, but inspection doesn't reveal any cracks so far.
I just wanted to know where I should begin looking?
All the controls have been cleaned out and lubricated with Servisol.
It's a very pretty unit, and I haven't seen another S85 since. Although they're basically all the same unit, this one seems like anorexic scarce design.
Thanks for any help.

It's your line in/tape/radio select switch, corrosion. Japanese boxes are famous for it, mine did exactly the same thing. Best get the cover off and pay attention to this switch.

Lots of de-ox and lots of toggling will fix it.

Mine is the S83, but like you say pretty much alike inside.

That's what I've been up to
But it's gotten me no-where so far. The best I've had is a sudden lick of sound when I flick it from power: off to either radio or line in, whereon I will hear the selected feature for a split second before it fades off to near-nothing again. You're right about japanese boxes doing that though, I've had a few which wanted a fiddle.

yep, that sudden burst of sound is what twigged me to the issue. Mine occasionally still does it actually, but most of the time behaves itself.