No, don’t chase. No ghosts. That always creates more ghosts. Sweep the ghosts away with methodical objectivity.
The signal from the tape head goes through wires to the head connector, then to the motherboard, then to the R/P amp cards, then to the motherboard, then to the dbx cards, then back to the motherboard, then back to the R/P amp cards, then to the motherboard again, and from there to the TAPE OUT jacks where it also fans out to the input cards and BUSS A PCB (where it goes to the monitor mixer and meter amp assembly). You swapped cards (both R/P and dbx), and the problem didn’t follow. So it’s not the cards. You flipped the tape and the recorded signal is actually fine, so it’s not the record electronics or anything upstream of the record electronics all the way to the input…and you tested this redundantly by recording to track 3 and flipping the tape…reproduced signal was appropriately on track 6, not tracks 6 & 8…it’s happening on playback. There is no logic circuitry carrying audio signal between the R/P head and TAPE OUT jacks, so it’s not a logic issue….like, there’s no way for signal to short across channels through logic circuitry. Logic circuitry controls the state of the channels on the R/P amp card, between record or reproduce, but logic doesn’t allow those channels’ audio signals to cross paths as a result of faulty control signal, which would allow the problem to occur regardless of which card is installed. But this isn’t the case, and any problem like this would be a fault of the card and would follow a swap. It doesn’t. So it’s not. And the symptom is the same across all 4 destinations after the output of the R/P amp card (the TAPE OUT jacks, input section on RMX setting, monitor mixer channels and track meters)…it is a “global” problem at that point, so the problem is upstream of that point and either somewhere between where the R/P amp card plugs into the motherboard, and where signal splits at the TAPE OUT jacks on the jack PCB, or it is between the head cabling and the motherboard where repro signal is input to the R/P amp card. So now is the part where we start tracing signal and I don’t know how easy that is for you. You’ve done a lot of work so far so I’m assuming you have some skill and some equipment.
At this point you need to be able to record test tone to tape, and you need a DMM to probe for that signal. 400Hz or 1000kHz test tone is fine. Record tone to a blank section of tape. Longer is better so you don’t have to keep stopping and rewinding. Record tone, and start at the end of our identified block where we’ve determined the issue could reside, which is the TAPE OUT jack. We’re going to start there and work our way backwards to where the fault stops. The TAPE OUT jacks 1-4 plug into the MOTHER (1) PCB (the big one that mounts the R/P amp cards, dbx cards, etc.) via a white 4-pin connector J117 for tracks 1 & 2, and a red 4-pin connector J217 for tracks 3 & 4. Unplug J117 & J217. Set your DMM to AC volts, and clip the common (black) probe to the 388 chassis. Reproduce your recorded test tone. Probe P117 pin 1 on the motherboard, which is track 1, and P217 pin 1 on the motherboard, which is track 3. You should only have signal on P117 pin 1. Is it only there or is it also at P217 pin 1? I’m guessing it’s on both.
Report back.
Ask questions if you have them.
