LG 26LC2R No picture sound ok

[Mac4000]

Got an LG 25LC2R which has been working perfectly. Picture now disappears after 2 seconds but the sound remains. Once screen is blank, the onscreen menu is not see either. Can go into standby then back on again gives same result. However, if I switch to standby and remove all external tuners, then switch on it gives the usual 'no signal message' on screen but picture does not disappear. Then I can connect external tuner and picture remains on without a problem.

Have swapped over external tuners to eliminate that.

Any suggestions?