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yo,

im back already with a rundown of the thing ive been working on since i finished the logic module: repairing a mixer. i've needed some kind of mixer for a while so i can play my synth with other stuff (eg. my volca sample) and not have to have everything connected to separate speakers, but since the last 2 modules i've made for my synth have been ones that don't directly affect the sounds it can make i didn't really want to make one myself right now, so a couple weeks back i decided i was finally gonna just go ahead and buy one second hand. long story short i ended up picking up a yamaha mg8/2fx for £30 from cash converters. then spending another £19 on a power cable cos it didn't come with one and unfortunately it's got the weirdest power jack i've seen irl for some time now,

so once the power cable for it arrived i hooked everything up and pretty much immediately i found out what the conflicting claims of "working" and "faulty!" in the mixer's listing meant: it turned on just fine and the popping sound at startup lit the volume meter, but audio i put in it just would not come out the other end. i'd kinda expected this given it was the cheapest 8-channel mixer cashies had listed and was planning to crack it open anyway to give it a thorough cleaning, so i went ahead and disassembled it so i could check if anyone was home when the lights were on and ohhhhh buddy,

we got popped caps....

rust even on parts that arent metal.......

and to top it all off the led for the phantom power switch was broken too. i'm probably never gonna use it on account of not owning a single xlr cable but its the principle of it. insulting. also theres meant to be 5 plastic spacers holding the main board to the front panel but 3 were missing so theres no way i was the first person to open this thing up. this mixer has had a troubled life i think.

thankfully this thing is old enough that it's pretty much entirely analog (aside from the dsp board that powers the effects), so i took a look at the circuit diagrams in the service manual and noticed that the popped caps were all filter caps. which in theory means they shouldn't stop the mixer working. but.... they were also all on the -15v rail. i checked with a multimeter and sure enough, the -15v regulator was 100% dead. i didn't really get the reason for the weird all-resin package on the regulators tho, so i just swapped in a l7915cv since they're cheap and do the same thing.....

.....and replaced the blown fuse.......

....and i win!! turns out that was basically all that was stopping it working. buuuuuuuut. now the rest of the problems revealed themselves:

• absolutely foul ground hum
• effects not working at all
• pots crackling like fuck

i figured the ground hum was likely due to the 3 missing caps so i kinda just moved onto the broken effects. this shit truly drove me nuts cos the dsp board is mounted on the back of the main board with its components facing up and almost no through-hole components or even test points to poke with the multimeter, so after some failed attempts at doing "some clown type bullshit" with an ide cable i gave up and threw together the most disgusting "cable" i've ever made in my life just so i could poke at the board while it was plugged in and turned on:

to my frustration, i still couldn't find anything wrong with it on the analogue side, and my cheap digital oscilloscope only goes up to ~100khz (200khz?) so i couldnt really check the digital side properly. but i could see that the adc¹ chip was outputting something, and i knew the cpu was working cos it controls the light on the effects toggle switch. so i stared through the circuit diagram again and again and kept poking around the dsp board and eventually i noticed some things:

1. the yamaha mg8/2fx puts the adc/dac² and cpu on separate ground planes
2. the ground plane for the adc and dac seemed to be reading ~3v higher than the one for the cpu
3. the 2 ground planes are connected only by the (currently removed) front panel

pissed off as all hell at this point, i wrapped a piece of scrap wire around an input jack and an output jack and switched it on and heard this thing's charmingly shitty flanger effect for the first time. like i said i don't have an oscillatoscope with a high enough resolution to confirm this but i'm fairly certain what was happening was that the floating ground plane used for the adc was causing it to output signals at 3-6.3v instead of the 0-3.3v the cpu was expecting and the cpu just read the whole thing as a series of 1s. or 0s. i forget which way round the cpu takes input. either way, i understand why u would want the digital and analog grounds to be isolated, but having them connect only via the front panel feels unhinged.... like, as if they were specifically trying to stunt on anyone trying to service one of these things. thanks for whatever that was, yamaha

anyway it turns out this was also the cause of like 99% of the ground hum too so that just left the crackling pots to deal with, and since i figured that was probably just cos of how fucking dirty this thing was i got to work cleaning everything. drowning all the pots in isopropyl alcohol fixed most of it, but the level pot instead went from crackling to having a huge dead zone on the left channel. i didn't really want to desolder it, but i didnt want to shell out £5 + p&p for a single replacement 20k logarithmic stereo pot even more, so i begrudgingly pulled it off and cracked it open to see if i could just fix the pot myself

its weird in there but it turns out i was justified: all it took was scraping the gunk off the wipers for the left channel and gently bending them back out with a screwdriver to fix it :)

with all the major issues dealt with, all the rust and gunk cleaned out and the broken led³ and popped caps replaced (as well as the 2 puffy lookin caps between the rca jacks. dont think i didnt notice those.) i finally got bored and decided i didnt wanna spend another week or 2 trying to figure out a solution for the snapped pot stems. so i just put it together now right? well.....

when doing a quick sanity check with the multimeter after installing the new caps, i noticed that the heatsink for the voltage regulators was connecting to the center pin of the l7915cv i'd put in. so what, that's the ground pin, right? not on a negative voltage regulator - for whatever reason, the center pin on l79xx regulators connects to the input. so i checked the datasheet again and yeah i guess for whatever reason the metal tab is connected to the input too. meaning the entire heatsink was now sitting at roughly -26v. guess that's what the weird packages on the other voltage regulators was about. its ok tho i fixed it:

with the heat sink now "properly" isolated, i put it back together, replacing the missing spacers with chunks of surgical tubing. after a month of pissing around fixing things and waiting for parts, the cashies mixer is finally ready to use!! there's a bit of a hiss, especially on channel 3/4, but it's not too bad (hell, i kinda like it) and other than that and the snapped pot stems it seems to be fully functional. i guess with the extra cost of the power lead and parts this ended up being marginally more expensive than the other mixer i was considering, which was some behringer with similar inputs and outputs, but that one only had eq controls for like 2 of the channels and since i use eq so much when i make music on the computer i want as many eqs as possible

ok thats all this post was long as hell and i wanna do something else now. i dont think ive written this much in one go since i wrote a phd thesis⁴. the next post will be shorter. thx 4 reading all of it. if u didnt read all of it fuck u!!!

til next time,

– freya