Mixer PSU - Zero
Why?
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Why? 〰️
The Numark M3 2-Channel Scratch Mixer in the first stages of disassembly; the internals of the device seeing some late afternoon sun for perhaps the first time.
This is the mixer I used as part of my “hi-fi” setup for a decent while. I originally bought it purely for its phono inputs and the associated pre-amplifiers; interfacing between my record player (another Numark, the Pro TT-2) and some generic cheap power amplifier. In 2017 the mixer cost roughly £30 used from eBay. It was maybe ten years old at this point. I imagined it had seen some real use, as evidenced by the bevy of scratches, dings, and rust present. It duly took the signals off of my record player, gave them a requisite boost, and presented to the power amplifier without fuss for seven years.
The Issue
I quite like this design actually.
That brings us to 2024, where the mixer starts to make itself heard. It started as a little background noise on the master channel, the sound of static or fuzz that one would associate with a cheap old vinyl record ground out by a stylus. Over the course of a few months this noise grows louder. At first it becomes annoying, then unlistenable - any music sent through the inputs is lost, drowned. The noise is present whether inputs are plugged in, and irrespective of any volume controls on the mixer itself. One day I turn it on and it is silent, not miraculously fixed, but now an impractical mid-00s paperweight. Time to open it up.
From its behaviour, I had a strong feeling that something had given up within the mixer power supply. I had always been suspicious of this subsystem; I’m not one to read “INPUT VOLTAGE 10V AC~500mA” without raising an eyebrow. The power adapter supplied with this thing is a veritable brick. As they say; if it looks like a duck and has an output spec of 10V AC, it must be an over-moulded transformer. The brick heats up nicely (even with the mixer disconnected) to a temperature which reminds me of a disposable hand warmer. I am by no means an “old-school” electronics hater, however, it is 2025 and the aspirational adjectives of modern electronics do not spring to mind. Interoperable, durable, miniature, efficient, sustainable? Anyway, let us not pass judgement until we’ve seen the insides, innocent until proven guilty.
This won’t stop the components after our barrel jack from being first in line for interrogation.
Internals
I find the PCB labelled INPUT/OUTPUT and give it a once over. Big electrolytic capacitors? Check. Diode bridge? Check. A few TO-220 packages with names starting in 78xx? Check. To no-one’s surprise we’ve got a classic bridge-rectifier x linear-regulator. Among the things I notice immediately are the single sided PCB with its wire jumpers, the solid core ribbon cables for connecting to the other PCBs within the device (soldered and glued), and distinct singed area. Around IC952 and IC951 the brown substrate is noticeably blackened, and on the reverse side discolouration of the soldermask radiates outwards from some burnt gunk.
Some probing with a multimeter shows IC952 and IC951 are shorted through. These are a pair of 7812 and 7912 IC’s for regulating the +/-12V rails. No doubt providing a supply to the active audio processing circuits within the mixer. Replacing them will be very straightforward. The ICs themselves are affordable and ubiquitous, and the board is an accessible, single sided, and through hole only. Checkmate.
“I could have moved my king ... after which mate ... was inevitable, but from the aesthetic point of view this for some reason appeared unattractive to me.”
Quite right Mikhail. The inevitable win of replacing those ICs feels distinctly unaesthetic. So rather, I take this as an opportunity to redesign the whole power supply, and perhaps the rest of the I/O board too. In doing so I aim to have repaired and even upgraded my mixer while having “in my heart, remained faithful to myself”, as Tal puts it. The series of posts headed “Mixer PSU Design” will follow this effort.
