TRV138 Audio only on playback.

The video heads are clogged or broken. Remove the cassette cover and in the middle back of the mech is a cylinder, this is the drum assy. You can gently spin the drum and look for a black piece of carbon (tiny). This is actually one of the heads. You can take a swab (Q-tip) with liquid head cleaner and very gently clean the heads (more than one) wipeing horizontally, never vertically to clean the heads. Inspect for broken heads as well.