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Befaco Hexmix VCA issues
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Befaco Hexmix VCA issues
Ulleks
Hi everyone
I probably aimed too high, with a befaco hexmix vca as a third build.
Surprisingly everything works as it should, as long as the little switch is OFF (so that channel 6 behaves like any other channel).
If I switch it on, so that channel 6 would output the sum of channel 1-5, I get just a tiny bit of the signals feeding input 1to5 into output 6, but a huge DC OFFSET. (output 1to5 keep on working just fine)

In the last picture...what are those "SJ201" "SJ202" etc with nothing on there?

HELP!


[/img]
guest
those jumpers (SJ...) are for de-normaling the outputs so the 1-6 mixer always has all the outputs, regardless of whether something is in the jacks or not. as it stands, plugging a jack into an output takes it away from the mixer.

if the 6 outputs are all fine on their own, then the problem is in the final mixer stage. but, it only takes one of them to be bad to mess it all up. check the output of IC10B and resistors R103-111. its probably saturating due to a wrong resistor, or a short some place.
Ulleks
Thanks. I measured the resistance of those resistors, checking they would have contact where they should, everything looks ok.
Now, about the IC10...both A and B output -10.50 V

One more thing...since everything works, reading the schematics...do I understand correctly that it HAS to be related with that final stage, right? so the issue has to be in between those resistors and the miniswitch nad nowhere else?
guest
yeah, thats where the problem lies. you can try swapping opamps, its probably not the issue, but an easy thing to try.

what are the voltages on all the pins of IC10? and be sure to measure on the actual IC, and not the socket. ive seen issues where the socket was faulty.
Ulleks
Well, thing is...this is a Befaco kit, all the SMT came already in place. IC10 is SMT, there's no socket and I suppose (hope) a 200 euros kit doesn't come with a faulty opamp!
guest
sorry, didnt know if it was one of the socketed ics pictured above. if you give me the voltages on all the pins of the IC, i can tell you where the problem is.
Ulleks
guest wrote:
sorry, didnt know if it was one of the socketed ics pictured above. if you give me the voltages on all the pins of the IC, i can tell you where the problem is.


Thanks!

both input and both output are -10.90 V
guest
no, i need ALL pins. there are 8 pins on that chip, what are the voltages on all of them.
Ulleks
guest wrote:
no, i need ALL pins. there are 8 pins on that chip, what are the voltages on all of them.



1=-10.90
2=-10.89
4=-11.85
6=-10.89
7=-10.94
8=-11.28

for what I can understand from the schematics (which isn't that much! just started with no electronic knowledge! Sorry!) pin 3 and 5 are ground.
guest
just to verify, pins 3 and 5 are reading 0V? and the pins are labelled going CCW?

1-|U|-8
2-|_|-7
3-|_|-6
4-|_|-5

if so, pin8 is wrong, it should be +12V. look for a bad solder joint or broken trace to pin8.
Ulleks
Correct. pin 3 and 5 are 0. Pin 8 is -11V
guest
follow the trace that goes to pin8, and measure the voltage at every point you can. find where it stops being +12V and turns to -12V.
Ulleks
Oh my god. Thank you so much. You're the best.

The -12V switched to +12 in between the first and the second capacitor after the +12 rail of the power socket.

On the PCB the path in between those two connector is interrupted. (the blue circle on the first picture)

I made a bridge and it works.

Is there some classier way to do that?

Now...what do you advise? Should I just keep the module like that (which sucks) or should I complain with Befaco? I'm pretty damn sure I couldn't have caused that.


guest
glad you got it working. its hard to tell from the picture, but typically a break in a trace is a manufacturing issue. it might be good to let the manufacturer know there was a problem, although with all the work you did soldering parts on, getting a replacement pcb isnt much help.
Ulleks
Yeah, you're right. Thank you so much anyway. In 24 hours you basically taught me how to read schematics and measure voltages.
guest
necessity is the mother of invention, or something like that.
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