Problem with Tascam Porta02 cassette 4-track.

[Jurassic]

Hi everyone!
I know this subject isn't stompbox related, but hopefully people don't mind (moderators feel free to delete or move this one, if it's too much offtopic). This is one of my favorite sites and I know there are a lot of talented electronic wizards here, so maybe someone could help me out with this one.

I bought this nice Tascam cassette 4-track from ebay, but saddly tracks 1&2 don't work as they should. Track 3&4 play and record normally, but with tracks 1&2 there seems to be a problem somewhere in the mixer part of the unit. First (at playback mode) you don't hear anything and both tracks (1&2) seem to be totally dead, but when you put the volume all the way up and hold it there for a while, tracks suddenly start to play normally (track 2 first and then later track 1). After that the unit seems to work just fine, but when I shut it off and put back on the problem comes back and tracks 1&2 are dead again.




Hi-Res image:http://www.jurimusic.com/tascam/tascambottompcb.JPG

First I suspected that the problem could be the tapehead, but after switching some playbackhead wires I realized that the problem lies somewhere else. Wire of track 1 from the tapehead connected to track 3 input on pcb, makes tape track 1 play normally on mixer channel 3. Same thing with track 2 (See picture below).


Hi-Res image:http://www.jurimusic.com/tascam/tascamsignal.JPG

First when I got the unit I could get any sound from tracks 1&2, but after I opened it up and by accident touched lightly on one of the transistors (Q501) while the unit was playing, both tracks came alive. After that it has worked like I described earlier: tracks 1 and 2 are dead silent when the unit is plugged on but 'wake up' if volume on both tracks are cranked up for a while. I have resoldered the Q501 transistor but that didn't resolve the problem. Maybe I should change the transistor?


Hi-Res image: http://www.jurimusic.com/tascam/tascamtransistor.JPG

The problem doesn't seem to be in the mixer pots, because they don't crackle and measure ok. To me it looks like this is some kind of amplification problem because, when dead, tracks 1&2 don't make any noise at all, even when I crank the volume up. Hiss on tracks 3&4 on the other hand gets louder when volume is increased.

Here's the mixer pcb:


Hi-Res image: http://www.jurimusic.com/tascam/tascammixerpcb.JPG

and the whole thing:


Hi-Res image:http://www.jurimusic.com/tascam/tascamwhole.JPG

Maybe someone could tip me to right direction and help to locate the problem. I really love the sound of c-cassette and this Tascam would be a nice unit to play with, so hopefully I get it fixed. Thanks!

-Juri