Every time I go into the room where the M-__ sits, which is nearly every day, the torn open power supply silently glares at me; "Why...why do you just leave me ripped open like this?" It doesn't understand being spread too thin or being burned out. I'd like to be interested in putting it back together but that interest, or the energy to have that interest just hasn't been present for months now. Lately I've been thinking I should just go through the motions and force myself to spend even 30 minutes on it.
I did that this evening.
I got the zener diodes installed on the power supply PCB with the intent of dropping the ultra high headroom line amp power supply from the +/-40V power being produced with the add-on power transformer down to something closer to the specified +/-35V power.
Here are the zeners...the little guys sticking up:
I got them installed along with the new slo-blo fuses and powered it up without a load to make sure nothing blew up or otherwise caught on fire, and then from there to check and make sure the voltage was correct and clean.
No *BANG* or smoke...The voltage was running at around +/-39V. Grr. And my Fluke meter was reading about 20mV AC on each rail. Grr again.
I hooked up the scope to have a look and I got a solid trigger on a nice AC sine wave at about 120mV on each rail. Grr, grr.
Then something flashed in my memory...power supplies have the tendency to run a little wild without a load. So I pulled all the modules from the frame except for one, hooked up the umbilical, crossed my fingers and hit the power switch. No smoke...no drama. And the DC voltage settled nicely at about +/-36V with the load, and AC voltage dropped to something less than 2mV. Much better. And when I looked at the supply outputs with scope again all I could see when AC coupled was environmental RF noise, and very little amplitude even when zoomed all the way in.
So it was time to see if it was passing audio. I injected some music to one of the line inputs and hooked a set of headphones straight to the BUSS OUT jack (the high headroom line amp is the output driver for the BUSS OUT jack when switched to +4dBu output level, and, yes, audio passed, and it's clean, and the noise floor is (subjectively) very good. That was with the level switched to -10dBV. I switched in the line amp (-10dBV to +4dBu boost) and still clean and happy...and that already good noise floor got better by nearly 20dB.
So this is a bit of a historic moment here...this is likely the first time this console has been fed the full compliment of power for which it was designed since it was sitting on the bench in the R&D department in Montebello back in the early 1980s.
The sh1t works.
I loaded another I/O Module...again, no drama, works just like it should. And now we're having fun...I just like pushing the buttons...there are so many nuance features on this console and I like the fact that, somehow (since I've actually used it so little), I know my way around the control surface pretty well in spite of all the bells and whistles. It's a fun mixer to use.
It came time to load the Control (master) Module...did that...same thing, no drama...Supply output voltage now down to just under +/-35V and that's with 6 of 16 of the line amps powered. So I'll see where it sits with all of them loaded and maybe swap in some lower voltage zeners to maintain something closer to +/-35V with the full load if there's much more voltage drop...maybe not. Anyway, Control Module works like it should as well, except I had a little setback...I'm only getting the left channel in the headphones when monitoring the the Control Room buss...it took me about 15 minutes, but with all the controls and features on this thing I was able to pretty quickly narrow down the issue to something in between the output of the Control Room buss source select switchrack and the input of the headphone amp. The Studio buss works fine in the headphones and there is proper signal at the output jacks of the Control Module. So, I didn't yet fix it, but it should be fairly easy to find the specific problem. I know my way around the Control Module guts pretty well.
Time to put the power supply back together!