Restoring an old Graphic Equalizer - Printable Version +- Makestation (https://makestation.net) +-- Forum: Technical Arts (https://makestation.net/forumdisplay.php?fid=45) +--- Forum: Technology & Hardware (https://makestation.net/forumdisplay.php?fid=29) +---- Forum: Technical Design/Build Journals (https://makestation.net/forumdisplay.php?fid=133) +---- Thread: Restoring an old Graphic Equalizer (/showthread.php?tid=3465)

Restoring an old Graphic Equalizer - SpookyZalost - March 18th, 2021 Finally decided to make a new Project Journal since I came across what should be an interesting challenge. I recently got for free an ADC S-1 MKII, vintage circa mid to late 70's, maybe early 80's. was pretty beat up, but was intact otherwise. side note, this unit goes for anywhere between $40 and up to $70 working. it's not even a full featured one as it has only 5 rather than 10 sliders per channel but it'd be a fun restoration nonetheless. Here's the unit as I got it. Pretty beat up but the internals are hopeful and the potentiometers for the frequency adjustments slide smoothly, likely just need a good cleaning and a bit of silicone grease to lubricate them. part of my plan is to replace the wood part of the case by fabricating a new one, it's just a simple 3 sided box with a lip on one side to attach to, should be able to come up with something and either paint it or leave it with a nice wood grain finish. also going to change a couple minor things to update it a bit and make it easier to service in the future. here's the power supply which I'll likely need to replace due to it's age. so a few things of note. several capacitors have started leaking though the traces are not on the capacitor side so that's good news, means there may not be any damaged traces. (edited, yes there's no obviously damaged traces, it's a single layer board, hand drawn too by the looks of it). at least 3 resistors are damaged, and all components are still available except for the transistors but I can get them pretty easily on ebay if needed. when I get the chance to go over everything I'm going to write up a component list and try to reverse engineer the circuit as I go along with my multimeter as well as note which components are faulty, that'll be the next update. I'm super glad I didn't plug it in to test it first given the state the components are in, but I can fix it for less than it's worth oh! and it has all the slider knobs btw! they just need a good soak in some warm soapy water and an air dry. Update: the wires are fairly short so I'm going to need to note which ones go to which pins (which are numbered!!!!), De Solder them, Then Remove the board The great news is, it's a single layer board, the components are mounted opposite the traces so nothing was damaged by the leaky capacitors near as I can tell. Definitely 1970's. the traces are not electronically drawn, they're curved instead of straight angles which is a pretty solid indicator. The good news is, that makes reverse engineering it easier since being single layer and hand drawn means you can visually connect what pins on what components go where. the pins themselves are marked on the side they sit on so this should be pretty easy once I de solder the wires holding it in place. RE: Restoring an old Graphic Equalizer - SpookyZalost - April 8th, 2021 So I finally had some time to sit down and start looking over this board and removing it from the wiring harness and front panel of the device. and I learned something shocking. looking at how the silkscreen was done on the component side of the board (circuit side and component side of a single sided board). this was clearly intended to be assembled by a person and not a machine. the wire sequence is easy to follow, the components are labeled by type and grouped by voltage/resistance/capacitance/etc, and the wires are color coded. meaning that not only was this graphic equalizer's circuit drawn by a person rather than a computer, it was assembled by a person. the last person to do anything to this device was a human not a machine, and to me that's huge! the other takeaway is that the wires were wrapped around pins, not soldered to the board, which is something else I'd never encountered before, besides paper capacitors I mean. Finally there's a patent listed on the back and I did some digging between my last post and now and well... it's expired, completely free to do anything with. so long as I don't expect to make a profit from it I can totally just reverse engineer it and post a board derived from studying it so other hobbyists can build one themselves. RE: Restoring an old Graphic Equalizer - Guardian - April 9th, 2021 Good stuff. Interested to see what you can do with it. RE: Restoring an old Graphic Equalizer - SpookyZalost - April 13th, 2021 So I did figure out how the circuit works. The Integrated Circuits on the board are 4558's which use this circuit. that's the chip diagram, if you look at the circuit it makes sense. Considering that's the underside of the board the diagram would be flipped. the power leads are connected to pins 8 and 4 and the inputs and outputs are setup in an amplification loop which outputs to the wire leading to the potentiometers on the front. the resistors create an inverse frequency to the one you're cancelling out rather than filtering it similar to how mufflers and helicopter noise cancellation works. it's very much analog and it obviously worked well given people swear by this make and model for it's era in any reviews I've found. think of it like how noise cancellation works by trapping sound so it doesn't echo, only with it setup to bounce certain frequencies filtering them out. it's actually kinda neat albeit not the standard approach which is just separating it out into separate channels and filtering the various frequencies then re-merging them instead. I should be ready to order parts and begin proper maint and repairs soon so I can create a video demonstrating all this. RE: Restoring an old Graphic Equalizer - SpookyZalost - July 17th, 2021 SO I'm working on a more detailed explanation... but I was incorrect with this last post and had to spend time studying and relearning stuff. first, the circuit listed here is a low pass filter, an active one. This is the Audio Amplification chip, each side connects to a low pass filter, both the same frequency and are mirrored and connect to each input (L channel, R channel). now here's what it does. First the sound comes in (red line), it goes through the filter circuit (blue line) which set's the filter frequency via the capacitors, this works by filtering out all but the specific frequency based on the capacitance of the circuit. finally it goes both to the amp and ground using the resistors to filter out any excess noise leaving a clean signal at that specific frequency, amplified to prevent a decrease in voltage from the filtering. this then goes back through the filter in a loop with the input/output going through the same wire to the potentiometer (slider control), on the front of the equalizer. this circuit is mirrored for each operational amplifier chip and each side services either the left or right audio channel before being mixed back and pushed out through another circuit I'm still working on decoding. each filter circuit has different capacitors at different ratings to change the frequency, the resistors are the same though. here's the schematic I came up with, I wrote down the numbers that were the same and left blank the capacitors because they vary depending on frequency. the capacitors are the same in the rest of the filter loop, the first two are replaced by a single capacitor however, not sure why this first one is in parallel, the rest is the same though. Change value to match, this circuit filters out all but 60hz frequency and the potentiometer changes the volume of that frequency. as a side note, the hertz calculations are factored in with the following. we know the values of R and C. R=220kohms C=12nF (nano farads) F=60hz meaning C4 and R2 are the frequency filter. this leaves the op amp, C3 and R3 as the filtered amplification circuit and C1 and C2 as some kind of feedback noise reducing circuit, as determined by their polarity, only certain frequencies can get through.