An analog mixer like the Model 5 Series…it boggles me how it could be deemed as irreparable…probably rude of me but I’m questioning the skill of the tech you contacted. With the exception of the the pots, mic transformers and VU meters, EVERYTHING on a Model 5 is standard through-hole components, and it is very easy to work on with its modular design…good gracious…IMO I’d work on an analog mixer any day over a tape machine…my hunch is the tech doesn’t have the patience to take the console through all the basic stuff you have to do with something of that type and age so they are just chalking it up to “can’t be fixed”. That’s my hunch. And if I’m right that is super-lame. For the 4 channels that aren’t working, step 1 basically check that power is getting to them. And in general the switches are terrible by this point. Every switch becomes a point at which signal may stop or be highly compromised. I would get some DeoxIT D100 and apply it to every switch and exercise throughly and then apply more. The contacts are brass and they oxidize. At the extremes you can replace the switches. IIRC it’s a pretty standard footprint. The backplane cardedge connectors for the modules probably also need to be exercised. Now, if a rodent got into the cavity between the module backplane and the jack panel and chewed up wires that could be not fun to repair, but it’s not at all impossible. So I guess I need more info about what’s happening to so more, but D100 on the switches and cardedge connectors, and F5 on the pots and faders. 9 times out of 10 those measures of applying appropriate treatment to the pots and switches and exercising things will bring it around so you can know what else that’s more complicated is going on. But straight up dead channels, if they are getting power, that can just be one dirty oxidized switch and you can usually get those working.
I’m not for sure the difference between the 5 and 5A…maybe that was the upgrade from the 4558 to the 4559. And then if that’s the case I think the only difference between the 5A and 5B is colored knob caps vs all putty colored. The service manual for the 5/5A applies to the 5B. You can use it interchangeably. No differences in functionality or circuit design that I know of. Somebody correct me if I’m wrong please.
Test equipment: in your situation I would, if you have a smartphone, get a tone generator app for your device and use that for test tones. You don’t need a low distortion oscillator or anything like that. And then look for a digital multimeter (DMM) that is “true RMS” measuring and is rated for audio bandwidth. Your garden variety multimeters are only accurate for AC volts up to maybe 400Hz or maybe 1000Hz if you’re lucky. But the audio band standard is 20Hz to 20kHz, so you want a meter that is rated to be relatively accurate across that frequency range. So as long as you have that, and can measure DC volts and resistance, and can make some tones, that’s kind of the basic level of tech equipment and not hard to get. You don’t really need an oscilloscope for what you need to maintain with an analog console like that. There are no distortion null trimmers or phase adjustments or anything like that. It’s pretty basic.
. Although I was planning to buy an oscilloscope and other testing equipment. I'm planning to learn electronics and make myself a DIY synth.
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