Tascam 688 recording problem

[televisionman]

Hi, everyone! I've got a problem with recording on my 688. I put a cassette with prerecorded stuff, then select channels. When I pres rec - diodes become solid so it show that the record is on, signal comes to all of tracks. But in fact when I try to listen to what I recorded - there remains prereocrded stuff. So, it doesn't even erase the tape. Can anyone help me with that thing? No visual damage on chips on boards. Where and what should i check?
Many thanks!

 

[SkywaveTDR]

First a prerecorded tape should be bulk erased before use in this propriety head system. Next clean the erase head. This head in service is usually not considered for cleaning but when the tape does not erase that can be the problem. Next is a scope probe to the erase head to see if it has oscillation to it. These decks are very complicated and can several points of failure. Not a unit that a beginner will want to work on.
People get the impression that electronics components will jump out or show you where the problem is. In 45 years of working on equipment maybe 3% of the repairs had visible signs with other parts damaged that did not so- use of visual inspection is not a big tool for repair.