VTR plays back sound for 3 sec only

[Jen2]

I have an old panasonic VTR (NV-FS90EG) which I have not used for a while. When I connected to the AC yesterday, not even the blinking clock appeared. After opening the box and checking the fuses, wiggling some of the wires from the power supply now it works correctly, except that it plays the sound only for about 3 seconds, then it goes to mute. If I pause and resume the play the sound works for another 3 seconds. The tape counter doesn't work while in play mode either, but works when I spin the tape fast forward or back. It has a selectable mono-stereo sound output but switching it doesn't make any difference. The picture quality is perfect.

Any idea where to go from here? What to check before seek commercial help? Particularly because this is a PAL system machine, however it is probably insignificant in this case.

Thanks
Jen2