1. The screeching noise.
Here is a video of using RCA - RCA cables between the Board and 48. When I having D Out & Tape In depressed on a channel strip, this is the loud shrill noise that emits & channel needle pins. This only happens when 48 & 1516 are connected, both In & Out, as well as the 48 Track is Armed to Record.
Walk through this with me. You have a mixer channel direct out connected to a tape machine input. Then you arm that tape machine track to record. When you do that the input of that track is present at the corresponding tape machine output...it does this so you can monitor the source you are going to record and set levels and such. You have that tape machine output connected to the tape input on the same channel that has the direct out connected to that tape machine input right? So you just setup a feedback loop. Mixer channel signal goes out to tape machine, that signal passes through the tape machine when you arm the corresponding track, and the same signal returns to the same mixer channel at the TAPE IN jack which goes where? Yes...the direct out jack...and around and around we go...meters peg, ears get covered, and sometimes opamps fry. The squeal sound also goes away when you lower the channel fader right? Because that's what feeds the direct out jack. You are not supposed to monitor your tape outputs using the same channels that are feeding the tape inputs. You have to use separate mixer channels or a monitor mixer or something to do that...looking at the M-1516, this is why they included the TAPE IN jacks as one of the sources for the DUAL buss. So, when tracking and overdubbing, do NOT source the TAPE IN jack as the source for the mixer channel. Set the DUAL source on that channel to TAPE IN, set the level and pan controls to taste, and then in the master section raise the level on the DUAL MASTER rotary control and then on the far right of the mixer in the monitor select switchrack latch the DUAL button, and this will allow you to monitor any or all TAPE IN jacks as an independent stereo cue mix without sending any of those signals back out to the tape machine causing a feedback loop.
2. The 48 Outputs
Rather than open up the back of the machine during this busy time, since it was only two of the RCA outputs ( 1 & 7 ) that were being finicky, I would order some simple Unbalanced XLR-RCA cables.
I went from XLR on the 48 Output to RCA Tape IN on the 1516 & RCA from the 48 Input to RCA Output on the board.
Not only did it work perfectly recording and playing back from the machine, but with the mix of the cables, I could not reproduce the screeching sound found when using only RCA - RCA cables depressing the Tape In & D Out buttons on a channel.
I can't explain this. I would need to see how you actually had things setup at the time. But the facts remain as presented above, it is not intended that you feed a direct output on a channel strip with an input that comes from the same device the direct output feeds. This creates a feedback loop or risk of it if you forget what you are doing and arm the tape track and **skreeeeeee**. There are lots of ways to screw up on most mixers and create this problem. You have to understand your console and how signal flows and know how to avoid it. It won't do it for you (not saying you are saying it should, just underscoring that it happens sometimes because it *can*, and you have to be aware of that and actively avoid it with your setup and operation).
I figured I would try this phenomenon on other channels. Yet suddenly… none of my XLR Outputs BESIDES 1 & 7 were working…
As it stands now on the 48
Only the XLR Outputs 1 & 7 work
Only the RCA Outputs 2,3,4,5,6, and 8 work
What are the chances of that?
Possibly really good if your feedback issues blew a bunch of opamps. I would do exactly what [MENTION=80733]lonewhitefly[/MENTION] suggested. Take the console out of the chain for now, test each track
one at a time on the tape machine using a simple source device and simple monitoring solution, use the same simple setup for each track and, again, just do one track at a time. Use the same known good cables for each track's test, and test all 16 inputs and all 16 outputs (8 RCA + 8 XLR). Make a chart or something to keep track of the results. Report back. This will tell us what is working and what is not, and then we can dig further into the tape machine depending on the results. Once that's squared up we can then do the same type of testing process on the mixing console, one channel at a time to sort out what might roached there if anything. THEN we can marry the two together and try it again. But please, if something doesn't seem right, avoid the temptation to just plow ahead and (figuratively) bang on it with a hammer to see if THAT works. I'm telling you from painful personal experience that's a bad idea. Here are some of the fun things I've experienced by rushing ahead without a plan or the right equipment/resources/support...these all happened at different times...yes...I'm a slow learner...:
Stuff exploding
Big sparks flying
Bad smelling smoke
Electrical shock
*Completely* roached my mint-condition Tascam 58 well beyond my ability to repair it.
3. Channel 6 on the 1516
I think you were onto something with the Insert jack Sweetbeats. Plugging in and out seemed to get a little extra signal coming into the meters & onto the tape.
It seems to me the real culprit is the Fader on Channel 6. If I move it and put just the right amount of pressure on it, the proper signal comes in and out fuzzily. Definitely needs to be cleaned somehow. I need to order contact cleaner
DeoxIT F5 is what you want for that fader type, but you can jet it in there all day and it won't likely fix the issue unless you can actually get the crud OUT...who knows what was spilled in there. So, the right way to do this, assuming the faders are the low-profile ALPS faders commonly found in a number of 70s, 80s and 90s Teac mixers, is to disassemble the fader and clean it, and also carefully reposition the fader wiper according to the instructions in this video here:
YouTube
The trick with the M-1500 series is I *think* the faders may be mounted directly to the channel cards rather than mounting to the chassis and being physically separate from the card, which complicates things...you have to desolder the fader from the channel card first before you can disassemble the fader. This is the kind of thing I was talking about much earlier when we were talking about how the M-1500 mixers have a lot of useful features and they aren't "bad", but as you get into the mid to late 80s and into the 90s Teac prosumer products had more plastic, and other design features centered around cutting manufacturing costs, but made them less durable, and harder to service. Every company did this...not singling out Teac per se. And Teac generally did a good job at passing some of those savings on to the customer with lower prices than other companies. I personally would take an M-300 console over an M-1500 any day for this and other reasons, but you've got that M-1516 for now, and its a neat mixer that can nicely serve the needs of your setup...has way more routing features and flexible input/output faculties than ANYTHING you will find in that bracket today or for the last couple of decades and beyond, really, so let's love on it and figure out what's going on...great bang for the buck...I've always, for some reason, wanted to lay hands on an M-1508 and fiddle around with it.
Strangely, when plugged into the 48, this fader also interacted with the Tape In channel causing a loud noise when touched and moved. That was a first… video below
Fader on Vimeo
Hopefully you understand now you are creating a feedback loop in your video just like we talked about in point #1 above, only the janky fader is saving your ears and your gear from certain destruction. You'll have to open the mixer up and take a look at the fader. Hopefully its not mounted to the channel card but I'd bet money it is. But it is either extremely dirty inside, is broken, needs the wiper re-positioned, or all of the above. You can also look for compromised wiring/connections (if the fader is not mounted to the channel card), or bad solder joints. I think I would start by opening the case up, sending some signal through that fader and gently manipulating the fader body to see if it is a bad solder joint or something before tearing into it...and looking for general trauma or damage first also. Somebody could have dropped something down onto that fader knob and broken something.