Greetings!
What I'm recommending at this point is, rather than going through the trouble of trying to recreate the plug-in type lamps like I did (since they are unavailable at least as far as I've found and it was a
PITA to do what I did), to just get these and solder them in:
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=070-275
They
should be the same or similar level of lumens.
Opamp mods? I can't really offer any strong recommendations as I've become a bid jaded to upgrading opamps unless there is really a reason that you NEED to do it. If you are just having fun tinkering then that's something different, but if you are thinking you're going to make a race-horse out of it its not worth the risk...opamp upgrades are not a silver bullet. I upgraded opamps in a couple of channel strips on a rptotype Tascam mixer I have from the same era as the M-500 series mixers and I actually like the stock channels better. I now own a Soundtracs MX 32 x 8 mixer and what I've heard from folks who have sorked with these mixers for decades in repairs and mods is that
customers who had their mixers modded actually reverted them back to stock because the "improvements" actually took away the character they liked in the board.
There is no such thing as a true drop-in upgrade for anything in my opinion. An operational amplifier does not operate in a vacuum or on an island. How it behaves is directly linked to many factors of the incoming signal, the components in the feedback loop(s) as well as the circuitry on the output side, so upgrades are circuit dependent in terms of performance
or problems. What I've learned is that I'm nowhere near smart enough to be able to quantitatively assess a stock chip's performance and then compare the upgrade and be able to diagnose and suggest how to address issues...and I'm not even sure my ears are good enough to throw all the quantitive stuff aside and say "I don't care what the scope says...It
sounds better!" Quite the opposite I guess because on my prototype board I liked the stock channels better.
While there is no true "drop-in" upgrade for anything (again, IMHO, and what I mean by "drop-in" is that you can swap the chip and get guaranteed positive results with no issues or additional changes needed in the circuitry), there are conservative upgrades that are relatively safe. On the M-500 series mixers you can use OPA2134's in place of any TL072's. You'll find the 072's used for the mic head amp (U1) as well as the EQ (U5, U6). The biggest performance improvements might be had in the summing amps and in the monitor mixer since everything has to go through that stage during mixing on the M-500's, but I never got far enough into research to tell you what you could use. evm1024
MAY get into that at some point but I couldn't tell you when if ever he will. I'm pretty sure it is a very low priority for him at this point. The big question is related to anywhere Tascam used the NJM4556 chips...they used those, for instance, on the input channels for the output amp U7 which drives every output path and so Tascam spec'ed the 4556 there since it has massive drive (70mA IIRC) and the question is how much of that 70mA is really needed? If all of it is needed (potentially) then there isn't much you could put there. If it was just grossly over-engineered in the event you were feeding all 8 busses, all 4 auxes and the solo buss wide open simultaneously and there wasn't anything in between the lower power chips and the 4556 then there are lots of options there, including the OPA2134 which has (IIRC...no spec sheet in front of me) something like 35mA output, but where we left it was that we'd have to put a 2134 in there and crank everything up and scope the output for distortion. I hope that helps.
I flush the pots with DeoxIT F5 (use the F5 stuff since it is designed for resistive element potentiometers) using the holes in the pot body...you have to completely remove the CPB from the mixer to do this properly...squirt it in there, exercise it and then flush with another healthy shot and set the PCB so the excess can drain out of the holes. With the faders my favorte thing to do is actually disassembly the faders and hose the element down and gently wipe up the yuck with a soft cloth, hose down again etc. until it comes clean...spray the wiper and gently wipe till clean and then at least just wipe the element with a cloth wettened with the DeoxIT. The faders on my M-520 would not have been easy to remove. Get all the yuck off the top of the fader. Gently blow out yuck from inside with a little compressed air, and then spray the DeoxIt up in there and exercise it.
I'm assuming you are asking because you are having trouble with your pots and faders? If they are not scratchy leave them alone is my opinion.
