So just in case it is helpful to anybody reading this thread now or in the future, here’s what just happened...
Brad had a problem...none of the LED indicators or VU meter lamps were working on his M-312B.
He (understandably) came to a conclusion the audio wasn’t working because the channel “ON” lamps weren’t lighting. He executed a commendable measure by getting out his voltmeter and checking to see if the power supply was working, because everything was dark. He had power, but he was measuring the +/-15V audio power rails, and they don’t power the lamps. I’m not saying this to be disparaging. My knowledge is pretty limited, but this was a problem with which I could help using some basic knowledge and troubleshooting ideology, and you can too...help yourself or somebody else.
Always remember:
* ELECTRICITY KILLS. It only takes milliamperes of current to kill...no joke...and you will find that power in any of the devices we talk about here. You proceed at your own risk always and if you’re not comfortable you should not proceed.
* Face it: you might roach your own gear trying to fix it. You might roach it bad. I’ve done it. If you’re not comfortable with that you should not proceed. Minimize the potential fatal roaching by setting up your work environment with good light, physically stabilizing the unit under test, and using the right tools for the job! Need a different meter, trimmer adjuster or an additional piece of diagnostic equipment to do the job right? Stop! Get the right tool. Don’t like spending the money? Or you’re thinking “mehhh I don’t really need to get that [insert tool/device name here]...” Why waste your time trying to solve a problem if you don’t have the tool to do it? Or worse yet putting your device or yourself at risk? Obtain the proper tools and equipment first...borrow if you can or if that necessary.
I like to troubleshoot using these steps:
#1 Define the problem... Before you do anything clearly define what the problem is. That may mean testing and checking things to identify when the problem occurs or what conditions have an effect on the issue. In this case we now know the +6V power rail was disconnected. But before we knew that we could have narrowed the problem down by checking to see if the mixer passed audio. We didn’t get that far because I told Brad to check the +6V rail first at the Input PCB (and it was dead), and then told Brad to check at the source, terminals 20 & 22 of the power supply itself...BUT WE ALSO didn’t get that far because an assumption was made audio didn’t work because the “ON” LEDs (and others) weren’t working. You...me...anybody could make that same assumption. I mean, never underestimate your ability to be a complete ignoramous, okay? No offense, Brad. My signature line is about me in memory of my mint Tascam 58 I messed up badly because I installed an amp card on the extender card offset by one pin...*pffft!* [smoke]...but stop and think...define the problem first...what is happening? Just what exactly is not working and when and can I identify anything that influences the problem? The lamps weren’t working. Does anything work? See if it passes audio. If we had checked we’d have found audio was fine. Phew! Okay...so the problem is limited to the lamps. So then...
#2 Check the power supply... Always do this next. Again, to Brad’s credit this is exactly what he did! He checked for power, but he has checking for the wrong power, only because we skipped step #1. It happens. Because Brad said none of the lamps were working and I knew they were powered by a separate power rail from the audio power I asked Brad to check and see if the console was passing audio, but in the same breath (I’m an impatient cuss) I told him to check the lamp power rail. Small tangent here...typically in an analog console from this era (and others) you have a set of DC power rails, “bipolar” rails (a positive power rail and a negative power rail with a common “0V” ground reference) that power the audio circuits. Sometimes there is also another set of DC bipolar rails with a higher voltage that power specific audio circuits. In the M-312B the main audio power rails are +/-15V, and there is also a set of +/-18V rails that power the output balance amp and level boost circuitry. Then, if the console has phantom power there is (typically) a +48V “unipolar” DC power rail, and lastly there is a power rail for the lamps...sometimes more than one. If the console has LED lamps then there is typically a +5V or +6V unipolar DC supply because individual LEDs can’t really handle anything higher by themselves...incandescent lamps, like in VU meters, can handle more, but often those are also powered by the same power rail as the LEDs, and if not it is a low voltage AC power rail. The M-500 series console’s are like this, using an 11VAC rail just for the incandescent lamps. Anyway, the M-300 series consoles utilize a +6V DC rail for all lamps. So if all lamps aren’t working, like, it’s a global problem, what is the common denominator to a global problem with the lamps? Every solder joint is bad to every lamp? Every lamp suddenly fried? How about the power source? Yes. So when we check the power rail, if it’s DC, we want to measure for DC volts and ensure it is at or near spec (i.e. a 15V rail should be about 15V...maybe it’s 14.8V or 15.3V...that’s okay...but 8.7V is not), and we also want to check there is little to no AC artifacts on the DC rail. That’s right...measure for AC on the DC rail. A power supply’s job is to take the AC power from the mains and convert that to DC (if applicable), ensure the proper amplitude (voltage level), and regulate the power so it is clean (no AC in the DC, no DC in the AC...this is not a Reese’s peanut butter cup arrangement), and stable (the amplitude should be steady). So check to see if there is any AC in the DC rail and vice-versa if that applies. Normally I jump right to checking the power at the supply, but I had Brad check the +6V supply at the Input PCB first because I knew it would be easy to access. I assumed it would be 0VDC...nothing...dead...and it was. So then armed with that I told Brad to check for +6V at the power supply, terminals 20 & 22 on the power supply PCB. How did I know where to tell Brad to measure? The schematic for the power supply shows, along the right side, all of the power outputs and there are numbers for each one that correspond to numbers printed on the PCB. Brad bravely wielded his voltmeter and to my surprise found power. I was expecting it to be dead. I’ve recognized a (faint) pattern of failure in this vintage of equipment of the JRC 7806 +6V regulator IC. I was wrong. Brad had power, and before I could say anything he jumped on the next step...
#3 Trace out the fault... Use your voltmeter and the schematics to follow where the power goes, measuring for power along the way until you no longer measure power. That’s where the fault is. It’s really that simple. The solution may be simple or complicated, but these simple steps will help you narrow down where the fault is. In this case Brad started following the circuit path and was able to discover a connector that was disconnected. Yes! I love those kinds of solutions, and kudos to Brad. Time for a beer. NOW...what if the power supply is not the problem and you are having a fault with, say signal loss (like its not a problem with the lamps)?
#4 Inject test tone... connect test tone of a known frequency to the relevant input and probe until you lose the signal. That’s the location of the fault. This is where it is helpful to have a meter with a frequency counter. You can inject a 1kHz tone, set the meter to measure frequency, and probe along the signal path...you’ll know you still have signal when your meter reads 1kHz. When it reads nothing that’s the location of the fault. Again, what the actual fault is and what the solution will be will depend on a great many things, but, in general, this will get you to the location of the fault and my goodness many times that is ALL the battle!
Remember:
A. Be safe.
B. Be slow and methodical...fools rush in. Just do one step at a time. Don’t be acting on multiple hunches at the same time and changing things as you go...you’ll end up chasing your tail and nobody will be able to help you because they won’t know what the hell you’ve done or are doing, and neither will you! Come up with a plan, the kind that is a process of elimination rather than a wild goose-chase...have a reason that is part of a plan for what you are about to do.
C. Keep in mind your problem may have more than one cause. Keep an open mind. Otherwise you might miss the little bugger that is the actual cause of your problem because you thought you knew what it was.
Sorry to hijack, Brad...and, seriously, good job tracking down the fault. I love it when we can fix stuff.
I hope this is of some general help. Sometimes problems have simple solutions...sometimes not, but I find I have better success when I try to step back, define the problem as much as possible, check the power supply, and go from there. Learn how to read Block Diagrams, and from there start looking at schematics. And try not to get discouraged. Try to see the problem as a game or puzzle to solve rather than an impossibility. It’s hard many times to have fun when your gear isn’t working, but try! It will keep your head clear and give you the wisdom to “sleep on it” at times.