soundcraft spirit m4 mixer troubleshooting

[5inc]

Hello people

I have this soundcraft mixer, but I don't know what is wrong with it.
So I plug it in, and connect an input to first channel with a music source, and first tried the monitor L output. turned up all pots beyond the middle point (except pan) untill I managed to get a minimal output signal, totally distorted.
The same happened on the R monitor, and all other outputs.
Except for the earphone output, which throught my earphones, sounds like a synth machine cooking them. Not gonna try that for long.
Also I tried putting my headphones into direct out, and everything was working fine through there, all the EQs and everything, so I think the channels are ok,

I opened it up and had a visual inspection, nothing blackenned, bulged or burnt, at eyesight. I think next would be to check the PSU voltages, but I don't know what they should be... or if I should go ahead and assume they are ok since direct out is ok... I looked for a service manual with no luck, anyone have any idea where to point the finger?

Thanks in advance!

[Khron]

Re: soundcraft spirit m4 mixer troubleshooting

Seek and ye shall find




That being said, since there don't seem to be actual values listed, chances are good the opamps will be supplied with something around +/-15v.
The only "different" supplies are the +48v phantom power, and the possibly +/-5v(?) rails for the MC14053 chips.

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

Hi Khron! thanks, that was quite helpfull to start moving along.
I made live measurements with the front pannel off without unplugging any interior connections.
There are 2 connectors that go from the power supply to the main boards. The one that goes to the upper part has 10 pins, I read both DC and AC (just in case) in all pins, this is the layout:
1 2 3&4 6 7 8 9&10 pins
42,7 13,7 -13,7 VDC
90 30 (-30) VAC the -30 means I tested it backwards, even though it's AC

The lower board has a 12 pin conector with this layout

1 2 3&4 5 6 7 8 9&10 11 12 pins
13,7 2,1 -13,7 2,1 VDC
30 4 4 VAC

I also measured the headphone output, while I was at it, and got something like 12,1VAC in the red cable(tip), and 1,2VAC in the next(ring) and 0v at sleeve.

At the beggining I thought it was really wrong with all those cables with no voltage on them.
Then looking at the power page in the schematic, found what looks like the same conectors, saw that not all pins had voltage. But the values are a bit lower than you said, and that AC present there just creeps me out. Should I do further testing on the power supply or is that normal?

I also found this 14053 chip you said, just one of them, and has apparently been replaced before.
Googling around I haven't found an easy way to check if it is ok, appart from building a circuit around it.. haven't done anything like that yet. If it gets to that point I think I might just order a piece or two.

On the mean time is there anything else I can check with a multimeter?
Thanks again

[Khron]

Re: soundcraft spirit m4 mixer troubleshooting

http://medias.audiofanzine.com/image...-m4-894428.jpg

Were the AC readings you took in volts, or millivolts? And where did you have the black multimeter probe connected?

The 10-pin connector (CN6) should have the phantom power voltage on pin 1, +13.7 on pins 3-4, -13.7 on pins 9-10, and the rest are various grounds.

CN8 and CN9 (12-pins) should have +13.7 on pins 3-4, -13.7 on pins 9-10, grounds on pins 1, 2, 5 and 6, and the rest are audio signals.

The voltages i mentioned were just ballpark figures / examples. It's analog electronics, supply voltages don't need to be 0.000001% accurate to any arbitrary value

But it sounds like the opamps are supplied with those +/-13.7v, which is normal enough.

The (1)4053 chips are used as signal switches; in this mixer it looks like they're used to mute certain signals on the way (or rather, to select between pairs of signals going) to the LED meter drivers.

I don't suppose you have (access to) an oscilloscope, do you?

If you don't, you could try one of these:
http://musicfromouterspace.com/analo...naltracer.html

With that plugged into an amplifier driving a speaker, you could connect, say, a constant sinewave at each input (one at a time), and then follow the signal path on the board (following the schematic) and see where the signal begins to get screwed up.

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

whoa, I think I should have re-read my post after replying, it is a bit unllegible, sorry about that.
I was reading volts, and i had the black lead on the chassis holding the boards.

And yes, CN6 has a 42V at pin 1, +13,7v at pins 3-4, and the negative at pins 9-10.
And CN9 has correct voltages at 3-4, 9-10.

But I also get 2,1v on pins 7 and 11. I don't know if that is any sign of something wrong.

I don't have an osciloscope, I know a guy who has a hantek USB, I'm trying to get him to come along some day to try it out, then maybe buy one for myself if it convinces me.

I'll give this signal tracer a shot. I looooove the concept, looks easier to carry around than an osciloscope, cheaper, maybe not as functional, but hey, it might teach me something. Thanks for that.

A few questions, It says there to plug it in a guitar amp (I have had cheap guitar amps that are like tanks in my early days, loaded them with 2 and 3 speaker boxes, exceeding by far their load range, and anything really bad ever happened to them), could I instead of putting a jack, say put a RCA connector and plug it into my hi-fi? or shall I get an other battle speaker for that?
I also need to get a signal generator don't I?

[Khron]

Re: soundcraft spirit m4 mixer troubleshooting

This signal tracer thing works for audio-range signals ("obviously" ).

Since you have a gain control on each input channel of the mixer, you should be able to plug the signal tracer into just about anything that has a volume control, with minimal fear of stuff going wrong.

Re: signal generator, if you can get / make an adapter from your phone's headphone jack to a 1/4" jack, i'm sure there must be plenty of free signal generator apps, regardless of platform.

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

yay, I managed to build it. I also downloaded a generator app.

I never used an osciloscope, so I don't really know the procedure, is it like checking voltages? by clipping the earth aligator to the chassy and using the other probe at different points to check the signal?
Thanks

[Khron]

Re: soundcraft spirit m4 mixer troubleshooting

Exactly

https://www.youtube.com/watch?v=Iq4QlfH-oqk

[Agent24]

Re: soundcraft spirit m4 mixer troubleshooting

yay, I managed to build it. I also downloaded a generator app.

I never used an osciloscope, so I don't really know the procedure, is it like checking voltages? by clipping the earth aligator to the chassy and using the other probe at different points to check the signal?
Thanks

Basically, yes. But there is a lot more to it than that, mostly in terms of knowing what NOT to probe and how to set the scope controls so that what you see on the display makes sense.

Also you need to learn what what you're looking at means.

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

thanks guys, taking that into consideration I'm looking into tutorials about osciloscopes before I even get close to one. And I bumepd into this one that said something relative to this case.

At minute 3 he talks about AC coming out of the DC power supply, he says something like, if there's 100mv ac it's ok, but if you have several volts you are in trouble.
https://www.youtube.com/watch?v=hUIg...2AAA430351B1F2

As I said before, I did find AC voltage on the pins going to phantom, op amps, and to the mc14053. It was 90v, 30v and 4v respectivelly, and that is way beyond the DC voltage delivered.

So do I have a bad power supply?

If this signal tracer is for audio range, then I can't use it in the power supply right?

[petehall347]

Re: soundcraft spirit m4 mixer troubleshooting

always start at power supply ...
to be sure your dmm is not fooling with you when checking for AC on a DC supply simply add a small DC blocking cap in series to one of your test probes .. something like .01uf should do it .
if problem is in all channels its a supply fault or ground fault ...or user error .

[Agent24]

Re: soundcraft spirit m4 mixer troubleshooting

If this signal tracer is for audio range, then I can't use it in the power supply right?

Indeed.

For power supply checking you need multimeter \ oscilloscope.

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

if problem is in all channels its a supply fault or ground fault ...or user error .

It could be user error, but I belive it's probably the supply then, it happens at all channels on all outputs.
BUT, direct outs, in each channel work just fine.

I'll do that cap in series thing, I saw tons of 104's in my harvesting box when looking for the 103 to build the signal tracer. Thanks Petehall

And Agent, by indeed you mean I can NOT use it in the psu? just to be sure

I apologize if my questions sound dumb sometimes. I studied carpentry, not electronics. I'm into this for fun, and lots of info I find online can not respond to questions that come up at a given time.

Thanks to you all. Really appreciated.

[Agent24]

Re: soundcraft spirit m4 mixer troubleshooting

You can NOT use an audio-signal tracer in a PSU.

Well you can, but you'll either hear nothing, or a loud bang (if you start poking around the mains side!)

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

Ok!
I used a 104G I found that read ok on my ESRmeter, soldered it to the positive tip of the dmm, and read again those voltages, and hurray, no AC voltage, well, something like a 0.02v only on phantom.

So the PSU chapter ends here? Now I have an excuse to get busy with the signal tracer.
I'll search for some more info before doing something stupid. Does anyone know any MUST references?
Thanks

[Agent24]

Re: soundcraft spirit m4 mixer troubleshooting

I've found http://www.repairfaq.org/REPAIR/F_Repair.html is a good read for electronics repair and other general information - and it's all free!

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

I just found something, where the two boards join there is a wire marked LK58 cut off, I saw a few other like LK3 or LK6. But looking into the diagram, LK3 and LK6 are supposed to be like that.
My question is, knowing this has been serviced before, is it wise to try and re-wire that LK58 connection straight away? There is a resistor beside it wich has been replaced by a chunky 1/2w, I don't know if they might have done other significant changes.
Dunno if it's a good idea to connect directly.

[petehall347]

Re: soundcraft spirit m4 mixer troubleshooting

where is link 58 in the manual ?

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

Thanks Agent, this is gold, and will keep me busy for a while.

[5inc]

Re: soundcraft spirit m4 mixer troubleshooting

where is link 58 in the manual ?

I looked at the bottom part, where the print for the boards. scroll right to the bottom and you'l see it around the center of the screen.