Embarked on the next wave tonight, which would be deep-cleaning and recapping modules 9 and 10...BUT...there was some remedial work to do first.
I've never been comfortable with how one of the capacitors is fitting in the chassis...it's crammed in there and pushing right against the side of the metal chassis framework. I don't like it. So I did the equivalent of blistering the hood of your favorite muscle car in order to make the air cleaner sittin atop your new performance carb/intake manifold fit when the hood is closed...just a little tweak...see it?
And here you can see it with the boards in place.
Why not just get a shorter cap? I could probably find one...I mean the stock cap was shorter, but that's a pretty critical cap. It's the input coupling cap for the mic amp, and I wanted to use a good quality part there...so I blistered the hood. Anyway, had to go back and do modules 1 and 2 which was fine because there were some other things I wanted to do to those as well.
Modules 1 and 2 were the first two I recapped and also socketed all the opamp sites for most of the audio path, at least those that would be part of tracking and mixing. I put in mostly OPA2134 amps but I used some racy amps in a couple-three places. I don't like how the modules sound compared to stock. I have a hunch it's the racy amps not being supported by the right components. You can't just buy the fastest bestest chip and stuff it in where you had a 4558 and expect magic, and to do it right there's analysis to be done. Yes you can get away with putting something like an 072 in place of a 4558...that's a good and conservative "upgrade", or even 072 with 2134 for instance...but the performance of an opamp is dependent on the quality and values of the components upstream and downstream and especially within it's feedback loops etc. I was not so savvy to any of that when I did the work on those modules. And the result? They sound harsh in a way compared to stock. And I betcha with some relatively advanced analysis the problems could be found and values tweaked blahblahblah...not worth it. I like how stock sounds. So what I did is put the 2134s in place of the 3 locations with the "racy" amps. Not "stock" but I do still want to compare this more conservative upgrade with stock. And I also realized I'd used a non-ideal value for the caps I used to bypass the power rails at the upgraded opamps. Oy. My advice? Find something you like and leave it stock. There are simple things you can do to improve the noise floor and such...maybe those are worth it. But wholesale opamp "upgrades"? Nah. Each on their own.
Anyway, here are the new bypass caps (the little orange dots) and below that pic a shot of the flip-side of that board showing the socketed ICs supported by those bypass caps:
So with the chassis tweaked to fit the mic amp coupling cap, several opamps "downgraded", opamp bypass caps replaced, and upgraded knobs and color caps ala modules 11 and 12 in place, modules 1 and 2 are back together and ready for action again.
I also noticed another problem...remember how I said module 12 must have been the guinea pig module as it's fraught with all sorts of hinky jumpers and cut traces on the boards etc? Well I noticed there's a ceramic cap missing from the eq board on that module...see here...finger pointing to part on a "normal" eq board, thumb pointing to where it should be on the eq board from module #12:
So I'm going to have to order one of those caps...hair splitting? Maybe...but that's in the feedback loop for the lo eq gain control...I'm betting it's important. They put it on all the other modules. Do it right do it once.
Another hair-splitting thing was bugging me. On the backplane there is a switch for the nominal level of the PGM BUSS OUT jack, either -10dBv or +4dBu. Well modules 1~11 have the same switch...the switch on #12 is taller. It's in the back. Nobody is going to look at it much less be able to SEE back there. I couldn't let it go. Swapped out the switch for one the same as the rest of the modules. Humor me.
So with all that remedial work done (except for that feedback cap for the eq board on module 12), I disassembled #10, got it deep cleaned and got the caps all prepped for both modules 9 and 10. Maybe tomorrow those two will be done.
I still wonder what the intent was for all the empty component locations on the BUSS PCB. See? Look at all the unfilled component locations:
I'm still thinking it is maybe a balance amp for the group associated with the module...that would make sense because there was room for more large jacks on the backplane...
Fun mysteries.
Anyway, that's all for now. More maybe tomorrow.
