Help with plague bearer build please!

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Bonobo
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Help with plague bearer build please!

Post by Bonobo » Wed Aug 14, 2019 7:47 am

:help:

Just finished the plague bearer which is my first SMC build, connected it all up and got a bit of a burning smell and some other modules in the rack went a bit nuts.

Plugged it into a little empty case and it seems to be working, however the 'high' and 'gain' knobs don't seem to work correctly. 'High' does nothing and gain does almost nothing, when I connect the CV inputs for those functions and use a +10v offset from frames they work clearly as I'd expect.

The variable resister for 'high' looks to have burned and has a black burn mark on it, nothing visible on the gain one.

So.. any suggestions on what to check before I replace the high VR since I don't want to just burn through another one.

Any help appreciated!

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Bonobo
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Post by Bonobo » Wed Aug 14, 2019 8:08 am

Any pointers on where to source a replacement potentiometer would be helpful as well, mouser don't seem to do one that'll work, only a horizontal version :bang:

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flight
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Post by flight » Thu Aug 15, 2019 11:25 pm

Bonobo wrote::help:

Just finished the plague bearer which is my first SMC build, connected it all up and got a bit of a burning smell and some other modules in the rack went a bit nuts.

Plugged it into a little empty case and it seems to be working, however the 'high' and 'gain' knobs don't seem to work correctly. 'High' does nothing and gain does almost nothing, when I connect the CV inputs for those functions and use a +10v offset from frames they work clearly as I'd expect.

The variable resister for 'high' looks to have burned and has a black burn mark on it, nothing visible on the gain one.

So.. any suggestions on what to check before I replace the high VR since I don't want to just burn through another one.

Any help appreciated!
Happy to help! Please post some high-resolution pictures of the front & back of your PCB and we can go from there.
Bonobo wrote:Any pointers on where to source a replacement potentiometer would be helpful as well, mouser don't seem to do one that'll work, only a horizontal version :bang:
I happen to have a few of those on hand and will send you one for $1.00 + shipping (PM me with your address), but lets figure the issue out before you swap it in.
~flight
flight(at)flightofharmony(dot)com
http://www.flightofharmony.com

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Bonobo
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Post by Bonobo » Fri Aug 16, 2019 1:47 pm

Thanks so much for the help :sb:

My first effort at surface mount soldering so its not pretty :hihi:

https://imgur.com/a/BwSu9dg#GX7FMx7

Cap on the back got melted somehow, I was planning to replace this just in case it was the cause and they have them in mouser.

You can see the burn mark on the high pot in the last pic.

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flight
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Post by flight » Sat Aug 17, 2019 1:26 am

Don't worry about the cap, I've melted a few and they work fine as long as you haven't melted through the yellow plastic casing.

I can't see what might have caused the problem yet, but it does look like Q1 popped as well.

Could you take some more pictures that show angle or side views of the components (like in the one that shows the burn on the high pot) please? It looks like there might be a few cold solder joints.
~flight
flight(at)flightofharmony(dot)com
http://www.flightofharmony.com

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Bonobo
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Post by Bonobo » Sat Aug 17, 2019 2:05 am

Good news about the cap, its surprisingly hard to get the photos to focus so I took a good few.

https://imgur.com/a/JYpEX76

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flight
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Post by flight » Sat Aug 17, 2019 4:42 pm

:eek: Um, I think I found the problem.

There's a potentiometer nut under the high pot. It looks like it probably shorted the wiper to ground - which is why it affected your other modules - and is offsetting the CV input. I'm not sure about the gain pot yet, I'll puzzle on that a bit more, but try taking the nut out and see if that fixes it.
~flight
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Bonobo
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Post by Bonobo » Sun Aug 18, 2019 1:32 am

flight wrote::eek: Um, I think I found the problem.

There's a potentiometer nut under the high pot. It looks like it probably shorted the wiper to ground - which is why it affected your other modules - and is offsetting the CV input. I'm not sure about the gain pot yet, I'll puzzle on that a bit more, but try taking the nut out and see if that fixes it.
:lol: ohh man, I did wonder why I was missing one. No idea how the hell that happened; thanks!

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Bonobo
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Post by Bonobo » Sun Aug 18, 2019 2:01 am

Found some pots, https://www.thonk.co.uk/shop/alpha-9mm-pots-dshaft/


Is the one I want the 'A100K - D shaft - Alpha Vertical 9mm Potentiometer'

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flight
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Post by flight » Sun Aug 18, 2019 11:02 pm

Bonobo wrote:Found some pots, https://www.thonk.co.uk/shop/alpha-9mm-pots-dshaft/


Is the one I want the 'A100K - D shaft - Alpha Vertical 9mm Potentiometer'
That is the correct one.

If it is damaged, Q1 is an MMBT3904.
~flight
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Bonobo
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Post by Bonobo » Wed Aug 21, 2019 9:32 am

Pot replaced and it seems to be sorted :nana: :banana:

Gain knob still doesn't give me as much range as the CV gain (CV gain will turn a signal to 0 in some cases whereas the knob won't go close) but it does have an effect on the sound thats noticeable. The high is working perfectly and totally killer.

Thanks so much for the help :tu:

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flight
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Post by flight » Wed Aug 21, 2019 10:59 am

Glad it's working!

The Gain knob isn't supposed to go all the way down to zero output - it's biased to prevent that - so it sounds like it's working right. Enjoy!
~flight
flight(at)flightofharmony(dot)com
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