soundcraft spirit m4 mixer troubleshooting

[petehall347]

Re: soundcraft spirit m4 mixer troubleshooting

resistors fail high resistance . if reading in circuit you are reading the circuit as well . so can read lower in circuit .
deal with the ground mod later if needed . i strongly suspect it was working ok like it is .
for being lazy and my number dyslexia i use things like this ..do not trust it fully though ...http://www.ohmslawcalculator.com/ohms-law-calculator

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

i'm lost in the mountains, I won't be able to get anything till I get back home monday.
But I will try and get the most juice out of the tools I have here, if I can't test it live, that is tuff. I'll just try and figure out as much as I can while I'm here.

I've been testing resistors from mixl/r to the end of monitor and phones and more than half read low resistances compared to what they should, some are almost half of what they should be. all that in circuit, but I am willing to change them all if necessary when I can get hold of them.
I also found the two 75ohm(r37,r62) resistors missing that are replaced by that green wire that goes to sleeve at monitor out through the back to a gnd.

I still wonder how much power in watts is being drawn at r666, shall I multiply the amperage by the power set, or by the voltage drop?

Also, at what percentage of resistance loss do you think I should start replacing resistors?

And should the variable resistors(vr35/vr87) respond to the turning action at pins 1-2-3?

[petehall347]

Re: soundcraft spirit m4 mixer troubleshooting

this is why i said measure voltage across the resistor .. saying that it seemed good when you measured it so there might well be an intermittent short .
i am thinking you could add a light bulb with the resistor or in place of it for an instant visual of the short without burning up more resistors .. trouble is i am not knowing a correct value of light bulb to use ,
that was just an idea and something i have not tried myself . then again i specialise in audio amplifiers not mixers . much less complicated .

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

also, I just saw the resistor I put on this morning, is half burned out, there was this moment(I think it was just before testing and seeing most outputs gave signal) where I turned it on, saw this thin layer of smoke, switched it back of and realized i had turned it on while it was unscrewed to the chassy, and probably had a small short somewhere, thus, the resistor heated up very fast. And I know, it's naive to think nothing else happened.

But it seemed to work fine when I screwed it back in place. I WILL make a lightbulb-inseries plug before I attempt to test it live again, I promise. I will also try and find an other 10ohm resistor for that toasty r666, but that seems quite more difficult.

I'll keep on following the map back, i'm half way there anyway.

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

sweet, but I still wonder how much power in watts is being drawn then, shall I multiply the amperage by the power set, or by the voltage drop.

tracking down from the end back I found the pots for headphone and monitor are doing something funny. You see, they have 5 pins, 3 are closer and two more over the first and third. resistance measure on pins 4-5 are 0ohm to 9.7k. That seems correct and is true on both pots.

then I go to se what happens at pins1-2-3, and they look like a solid resistor, no change when I turn it up or down. 1-2 shows 9.7k, 2-3 shows 4.5k, and 1-3 shows 4.7k. That DON'T seem correct to me, but is also true on both pots. Is that normal? or should they also vary when turning up and down?

[Khron]

Re: soundcraft spirit m4 mixer troubleshooting

Exactly - only use values you can measure, or which you know from the get-go (like resistor values and voltages, in this case). The rest you can figure out, if needed.

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

i'm off at a friend's house, but I took my homework with me, allthough I forgot the speaker home, or in the car, ... if it's there I'll fetch it tomorrow. I also forgot the lightbulb-in-series plug.

In either case, i've only got my multimeter here, and the headphones, plus some basic hand tools and soldering kit, for if I need to build something, which looks like I will.

Yes, I DO have signal on the MIXL/R output, tracking I had a fine sound on the 3d pin.
So now I'm facing trouble from mixL/R forward. Allthough I hope that it gets a bit more exciting than a missing resistor this time :P

Since I only have the multimeter I'll just go ahead and follow the page 11 of the schem looking for bad or missing components/voltages for now since it's narrowed down to that page, and tomorrow I'll see what I can build to use the signal tracer.

Then Khron, if I can't rely on power in the equation my guess is that that resistor will be drawing 0.04amps, so to get the power drawn do we multiply that by the voltage or by the voltage drop?

[Khron]

Re: soundcraft spirit m4 mixer troubleshooting

That's not how Ohm's law works - just because a resistor is rated for 0.25W doesn't mean it dissipates that much no matter what

Both the monitor and the phones outputs get their signal from "MIXL/R", as i see on page 11.

If the main outputs work, then at least in theory, you SHOULD have signal on the "MIXL/R" (at R277/278). You can use your signal tracer again, and see where it "ends".

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

if you turn it up it don't do nothing either

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

whooooa, I have master outputs working!! somewhere along the way they started working, I found that tracking towards aux outs, seeing the mute switch(sw10) under return master pot(VR13) was on, pushed it out and AUXouts, MONO SUM, MIX L/R started working!
Now I still have Monitor out and phones out that aren't working just fine. Monitor sounds distorted with a loud hum, while phones still sounds like it's frying the phones, from the second you plug it in even with volume all the way down...

[petehall347]

Re: soundcraft spirit m4 mixer troubleshooting

there is a 40ma current draw according to voltage drop .
or 0.016 watts 16mw
what voltage is at the resistor ? it should be same for both resistors except r666 negative .r667 positive .

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

ok.. voltage drop is 0.4V, resistance is 10 ohm and power is 0.25w, and ohms law says I=P/V or V/R, so that would be 0.25/.04=0.625A or 0.4/10=0.04A.
What am I doing wrong?

[petehall347]

Re: soundcraft spirit m4 mixer troubleshooting

negative side of c836 goes to r666
positive side to r667 .
you could measure the voltages at those resistors .
if one side of r666 is low voltage measure voltage right across it .then work out the current draw using ohms law

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

c386 is in page 15 middle right end, the others are page 10 middle bottom

[Khron]

Re: soundcraft spirit m4 mixer troubleshooting

Hmm, i might've mis-read something. What page can those caps be found, in the schematics?

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

one end of c386 goes to ground?? I see it goes through c370/c242/c298 on the positive side to AGND; and through c371/244/299 on the negative, to AGND again.

does that count as being connected to ground?

[Khron]

Re: soundcraft spirit m4 mixer troubleshooting

He might've been asking whether you measured that voltage with the black probe on a ground point, but since one end of that capacitor is connected to ground... i'm pretty sure you did

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

sorry, I don't quite understand the question as "reference ground" is new to me...
But I can't see any direct link to gnd from there, if that is what you mean

[petehall347]

Re: soundcraft spirit m4 mixer troubleshooting

is that at the negative side of c386 reference ground ?

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

I installed it and it still seems quite the same.

I tried resistance from gnd to pin4 and it gave me 1.74kO. Then I switched it on and tested between c386 for voltage and I get -12vdc.