Gunk all over Minibrute PCB + non-oscillating LFO

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badboy10000000
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Gunk all over Minibrute PCB + non-oscillating LFO

Post by badboy10000000 » Sun Jun 28, 2020 11:28 am

Hi all, very new to DIY and looking at PCBs and soldering and whatnot. I have an old Minibrute (my first synth!) that's been sitting with a burst fuse for years and finally got around to replacing it. Everything works as fine as i remember except for the LFO. It seems to be acting as a sort of constant offest instead of oscillating? Clock toggle and rate knob and wave shape all seem to have no effect. Turning amp knob up gives an increase in volume if a note is played and vice versa. Filter knob offsets the cutoff.

I'm wondering if anyone has had this issue and solved it, and if anyone could help me try to diagnose it that would be really nice. I have a digital multimeter and a soldering kit, and a handful of components from a solder practice board i can reclaim if something needs replacing and i happen to have the right piece.

Also, the entire analog board has this crud all over it. Corrosion? Glue drippins? All the connecters between the two boards had huge globs of hot glue and I've had this synth sitting in closets in LA and Chicago for the past 4 or 5 years at least. So it's probably gotten hot despite not seeing much play time ever. How do i clean it?

Thanks for any help and sorry if this is the wrong board for my questions!

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Bartimaeus
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Re: Gunk all over Minibrute PCB + non-oscillating LFO

Post by Bartimaeus » Sun Jun 28, 2020 11:33 am

please post photos, it's pretty hard to say what the crud is on your board without seeing it.

have you tried probing the lfo section to see if you can get the correct waveform at any point? from what you've said, it's hard to say if the lfo isn't working, or if the lfo isn't reaching its destination.

badboy10000000
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Re: Gunk all over Minibrute PCB + non-oscillating LFO

Post by badboy10000000 » Sun Jun 28, 2020 12:15 pm

My bad! Posting from my phone and it wouldn't attach
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Trashcan
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Re: Gunk all over Minibrute PCB + non-oscillating LFO

Post by Trashcan » Sun Jun 28, 2020 12:20 pm

Isopropyl alcohol is the best thing to use in my experience, whether it will shift whatever that is though is any anyone's guess!
Where you come from is gone, where you thought you were going to, weren't never there! And where you are, ain't no good unless you can get away from it!

badboy10000000
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Re: Gunk all over Minibrute PCB + non-oscillating LFO

Post by badboy10000000 » Sun Jun 28, 2020 1:57 pm

the capacitor (?) closest to camera in my pic has a big gob of the putty gunk on it, is it battery acid?? none of them look warped or burst or otherwise abnormal to me.

@bartimaeus:: I unfortunately don't have an oscilloscope available to me, if I'm understanding your suggestion. Is there any useful probing I can do with a cheap multimeter? Probably not is my thinking, I think power must be reaching the LFO section because there is an audible effect from the amp and filter LFO knobs, just no oscillation.

Also, if I assign LFO amount to the mod wheel, it functions as expected in its current situation i.e. LFO amp knob all the way up gives a constant slight volume boost to any notes played, boosts a little more if I push the modwheel up. Turning LFO rate knob changes the rate of the blinking LED as it normally would. It's almost like maybe the LFO is running but the depth of the oscillation is stuck at 0. Maybe a trimpot I could fiddle with somewhere? Not sure how useful this information is and sorry if I'm screwing up any lingo or asking the dumb questions.

Really appreciate any help! I'm planning to copy the modifications from megabrute.com and really want to get this LFO figured out first
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Starspawn
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Re: Gunk all over Minibrute PCB + non-oscillating LFO

Post by Starspawn » Sun Jun 28, 2020 3:23 pm

If the LFO doesnt affect any paramater as an LFO lll bet the rotary switch for choosing LFO waveform is broken. I had mine on the arp octaves die on me. Easy enough to test though, just turn and measure continuity on the pins under.

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BugBrand
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Re: Gunk all over Minibrute PCB + non-oscillating LFO

Post by BugBrand » Sun Jun 28, 2020 3:40 pm

I would suggest that the residue is 'no-clean' flux residue from soldering the through-hole panel components - you see that it is always near(ish) to the solder pins, but not around SMD parts where there aren't solder pins. It should be entirely inert & unproblematic, despite looking like that.

As far as the tech goes, Starspawn's suggestion seems a good & easy starting check.

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Picard
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Re: Gunk all over Minibrute PCB + non-oscillating LFO

Post by Picard » Sun Jun 28, 2020 6:10 pm

From time to time I get frightened about the LFO is not working until I realize I unnoticed assigned the Modwheel to LFO Amt and dialed it all the way down...
Just be aware of it :hihi:

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Bartimaeus
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Re: Gunk all over Minibrute PCB + non-oscillating LFO

Post by Bartimaeus » Mon Jun 29, 2020 1:15 am

i agree that it looks like no-clean flux on the tht parts, so nothing to worry about.

you can check an lfo with a multimeter by finding the right pins and seeing if there's a cycling voltage in the correct range.

checking for continuity is also a good call, especially since the led suggests that at least part of the lfo circuit is working!

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