I'm in a weird predicament (and it's kind of long-winded but it's that way for clarity's sake - please bear with me). I purchased a newly serviced Tascam 488 MKII off eBay at the beginning of September (with the 8-channel RCA output mod). It arrived & worked perfectly (by which I mean I was able to record on all 8 tracks, the motor speed was consistent, everything seemed to be in order). However, a couple of days ago I started working on my second album (the whole reason I bought the 488 - thought I'd forgo digital this time around), and while working on drums, I found that while input signal was both audible in headphones and visibly being ported to Track 3 on the visual monitor, upon playback the recorded sound was incredibly quiet, muddy and scratchy. Frustrated, I gave up for the day. I did some research yesterday and thought maybe it was an input problem, so I tried all the other inputs. While tracks 1, 2, 5, and 6 all can have audio recorded on them and faithfully played back at the same volume, tracks 3, 4, 7, and 8 do not have this ability - they all turn out to be scratchy and very quiet (unless the input signal is clipping the entire time - then you can kind of hear it). After about 3 dozen attempts to record on these tracks, I opened up the machine and disconnected & reconnected all wire clips but to no avail. I opened it back up again and decided that I'd test the Record/Play PCB (the top PCB located in the bottom shell beneath where the channel controls would go). To do so, I swapped out the channels as such: 1<->3, 2<->4, 5<->7, 6<->8. This means that audio read by the head for channel one was now being displayed and controlled by the slider for channel 3 and vice versa. I discovered that after the switch the same four channels (formerly 3,4,7,&8 and currently 1,2,5,&6) were now completely messed up - the erase head wouldn't function on them & no new sound could be recorded. I swapped the channels back to their original location (now 1 means 1 again) and played an 8-track recording I had made several weeks prior. I noticed that unless I ported channels 3, 4, 7, & 8 through the Group 1L/2R bus, they wouldn't play through the line output. So it's really starting to seem like a bus issue. My best guess is that the Group 3/4 bus isn't communicating with the record/playback head properly. Has anyone experienced a similar issue? I found this post on here describing a similar problem, but they said the Group 3/4 switch was not engaged on the channel strip in their case, but the Group 3/4 switches are definitely engaged on mine. I am not too well off financially & would like to do anything in my power to fix it myself, so any suggestions would be helpful. I have only minor knowledge of electronics and repairs, so please keep that in mind when responding with technical suggestions.