Its done...
Got the replacement opamp in. It started to smoke when I first powered it up.
Shut it off right away of course. I didn't double check before putting it in. Remember that the original opamps are a single row 9-pin chip. Couldn't find those but pins 1 and 9 on the stock chips are redundant (do the exact same thing), so 8-pin chips would work, but I didn't double-check which way to offset the 8-pin chip (i.e. leave pad 9 open or pad 1 open on the PCB). I
assumed pin-to-pin placement (pin 1 still to pad 1, pin 2 to pad 2, etc.), but that was wrong. Duh. If I'd
thought about it just a little bit or referred to the datasheets I'd have remembered that on an 8-pin DIP pin 8 is the ground, so I needed to put pin 1 to pad 2, pin 2 to pad 3, etc. That offset would have pin 8 in pad 9 and the inputs and outputs to the chip just right. The only problem is that pad 9 on the PCB is connected to
nothing (i.e. no traces go to pad 9, pin 9 of the stock chip was not connected). Fortunately I was smart and bought a couple extras just in case, so it was a $0.50 mistake.
Here's a picture of the new chip (top) and a stock chip (bottom):
To solve the pin 8 issue I ran a jumper wire from pad 1 on the PCB to pad 9 so that pin 8 of the chip would be connected to ground, which used to come in to the stock chip on pin 1 because, remember, pins 1 and 9 are redundant. So I pulled the abused replacement chip out and put another new one in. Here is what the jumper looks like:
Buttoned everything up and guess what? I now, at long last, have a 100% functional MX-80.
And the pots are much less scratchy now. This unit was the first unit on which I cleaned pots. I did it wrong and it didn't improve anything and the pots were horribly scratchy when I got it. They are silent as can be now.
This may seem like a small thing but I have to say this is an extremely satisfying thing to have accomplished; to have the MX-80 working. I remember now how this all came about...The MX-80 was, indeed, my very first vintage analog piece of gear since going to digital in teh mid-90's. My only preamps were on
my Yamaha 01X mixer/interface/control surface, and
my Yamaha i88x. The 01X has 8 preamps of decent quality, and the i88x has 2 preamps of very good quality plus 6 analog line inputs. I remember I was shopping for mic preamps to connect to those line inputs because the pre's on the 01X were relatively noisy at high gain. I was doing a search on eBay and there was this thing made by Tascam and I remember being amazed because, at that time, I didn't even realize Tascam made rackmount processors. I thought it was the coolest thing I'd ever seen with the stacked pots and everything. The seller disclosed that channel 7 wasn't working so I knew I was only getting 7 working preamps but that was fine because it was only $35. Done. I got it figuring I'd be able to fix it; that it was probably something simple. Well, obviously it was not simple for me though I quickly figured out that the preamp was working, but the circuit from channel 7 to the stereo buss wasn't working and that didn't matter because I was just going to use it as an 8-channel discrete mic preamp. It was ALL covered with labels when I got it and somebody had removed additional labels in the past with what I figure to be a screwdriver because the face has ALL sorts of scratches in it...so it was kind of neglected...it took forever to get those labels off because the adhesive was like cement.
So anyway, it has always been a symbol of the unfinished in my lineup of gear, and the MX-80 was, obviously, the first of MANY unfinished projects...I got the MX-80 even before I had gotten back into tape recording; I hadn't woken up to that yet. I remember getting the RS-20B next, then the 48 and 58...on and on andonandonandon...Its wierd but I feel like, between being really happy with the MM-1000 and settled with it as "IT" for me as a tape machine as well as having the Soundtracs mixer as a companion, between that and finishing my first unfinished project, I feel like I'm finally slowing down and settling in and catching up...like the marathon of discovery and madness is coming to a close and the MX-80 marks the beginning of the last chapter of this volume. Forgive me for waxing poetic but its a significant thing for me.
So, like I've said previously, the MX-80 is kind of a funny thing for me because, due to my sort of constantly shifting lineup as my knowledge and understanding has changed over the years, it has never really found a permanent place, but I've never quite gotten up the courage to sell it or part it...it has saved a situation more than once for me and has kept its place in limbo here. Now I have a real need with needing a temporary preamp and 8 x 2 cue mixer for the MM-1000. Yes, temporary, but I think even when that need is done the MX-80 will stay. It really is a versatile unit as, again, it isn't just an 8 x 2 mic mixer...it is really an 8-channel discrete mic pre and an 8 x 2 line mixer. There is also a 30dB pad on each mic pre so you CAN use it for balanced line inputs too, and, though not "ideal" it will work as an 8-channel balanced to unbalanced line converter as well OR mix and match!...the mic pre's connect to the line mixer via jumpers so those SEND and RCV jacks provide you with inserts. Add to it all a mute function and phase reverse, balanced AND unbalanced outs for the stereo buss with individual level controls, buss input jacks so you can cascade multiple devices together and a phantom rail (external supply required) with individual phantom enable switches and you've got a nice package. While the face is a bit scratched up on mine the jack panel is
really clean and the inside is clean too. Like all units from this era from Tascam it is solidly built. Lots of steel. So its here to stay...And the PSU is recapped, and there's the new phantom power input jack (now a standard 3-pin XLR...stock is a bizarre and obscure 2-pin XLR) and I have a nice Stewart Audio phantom supply to go with it.
So, that's it. This thread wasn't initially one of my "Story" threads but it really has turned out that way, and though it is in the shadows of a number of other more prominent and prolific "Stories", this is one of the most significant for me.
I'll leave you with some additional pictures of the unit's front and back overall, as well as closeups of the master and channel controls as well as the master and channel jacks. Enjoy!
I wonder if you could say "photo of actual item for sale" in the advert? I guess so. It's Cory's actual MX-80 just sexed up. 
and then do I just connect to the output of a standard in/out phantom power supply? Thanks for being a prolific poster sweetbeats 
