Cooling Fan continuously running in JVC CA-UXS59

[OXAUDIO]

This midi audio system was brought to me with the complaint that 'the protection circuit trips as soon as the unit is switched on'(which indeed it did)
When I opened it up and checked it(it was still shutting down immediately although it had not been used for about 3 weeks by this time) , as soon as I disconnected the cooling fan
the stereo worked normally, with no sign of temperature rise or other problems.
The odd thing is that it continued to do so when I reconnected the fan, and refitted the covers, except that the fan runs continuously from switch on.
The Thermistor bolted to the o/p I.C's Heat sink falls in resistance as it's temperature rises, and other components in the sensing circuit seem to be O.K.,
plus there's no measurable rise in temperature even if the stereo is played continuously for 2 or 3 hours. Anyone have any ideas as to the cause of this
problem?