So my kid managed to drop my synth destroying the lfo rate pot. I've replaced the pot which fixes some things (the lfo seems to run normally again).
However I'm left with something that doesn't make a clean sound anymore and also doesn't seem to track properly. Does anyone have a circuit diagram so I can check things to see if I can fix it before I scrap it for parts?
Posted Oct 2, 2022, 8:24 AM UTC
Define "clean sound" - maybe post video of it? Is the oscillator clean coming out of the OSC Pulse and Saw ports? Is the mix knob potentially broken? Maybe actual noise is the cause? Does plugging in an ext audio source automatically switch off the noise in the mix?
Is external audio also affected? Would make me think its in the filter or the VCA. I'd check all modes of each and see if anything changes.
Could also be related to the power supply. Not sure the setup, but any filtering cap issues could cause a problem. Could also be filter caps at a specific IC.
The tracking issue makes me think that there may be a bad or out of adjustment trim pot at the vco. Being behringer, I'm guessing its a 3340. There are hopefully 4 trim pots. You might look at 3340 designs like the one from Kassutronics - I'm pretty sure that one has a pretty in depth guide for calibration. I'd look for a broken trimpot first though.
Thanks. Great questions.
I will come back to the community with a recording if I can't figure it out. (might have to borrow an audio interface from somewhere first).
Right, finally got around to a proper test, as I just finished my Eurorack output module (i.e. I have an external known-good output mixer for tests).
So ... I think this will probably count as a bad thing:
- Direct oscilator output is super-wobbly in terms of frequency. Both pulse and saw. Seems to correlate with the LFO rate (which is the button that the thing fell on).
- When I use the filter with an external (Eurorack) VCO I also get some wobblyness and some bleedthrough (if I close the filter down in low-pass it bleeds the original signal through).
- VCA seems to work ok on its own (with open filter).
To me this feels like there is something going very wrong internally. To me this doesn't feel like it is a power-supply issue (especially with the bleed), but what do you think?
Worst case I have a LOT of components to salvage.
I agree that the LFO showing up where it shouldn't is probably bad. But it also makes sense, given the breaking of the LFO pot. I'm not entirely sure that its the case, but is the LFO signal perhaps bridged to ground or otherwise affecting it? Particularly the grounds around the 3340.
I would be curious if this still happened with the LFO pot removed. It could also be a crack in the board causing the issue. A strong light and a 10x loupe would be good if you haven't already given it a visual inspection.
There may also be some filtration meant to keep this signal out that is out of place. I would look for loose caps or broken solder joints at caps. I'm just spit-balling on all of these. Would be curious to see some pictures of the board if you have any.
The filter issue may or may not be related. The wobbliness sounds like the same issue as the VCO, but the bleed through of the original signal, is confusing. If it were me, I'd focus on the wobbly, LFO based issue first and perhaps fixing that will clear the filter bleed through issue. If not, then tackle that problem.
Definitely worth salvaging if they are 9mm pots and thonki style jacks (or the other style, I never remember the name of). If you don't have a hot air gun, I'd get one - it will make a lot of things easier in life, but especially pulling lots of pots and jacks. Prior to having hot air, I usually had to remove both by snipping leads, which isn't helpful in reusing them.
You also might consider selling as is for $50 or something. I'm just guessing at a fair price. People like projects. Even at $30 that buys a lot of spare parts for your future projects. I'm too deep in my own pile of projects, or I might be interested.
Sorry I'm not more help. The craves are really interesting though. I'd keep trying to fix it just to have it around.
ETA - and not sure if you've tried or if they'd be any help, but you might reach out to Behringer. I don't have great expectations from them, but it sure wouldn't hurt to ask.
Inspect it for any further damage, cracks etc.
I did, and it looks fine. But obviously it isn't. (The knock it got was pretty hard. Completely bend a decent pot)
I am mostly hoping for a diagram or something so I can look at subsections.
Behringer doesn’t make their stuff open source unfortunately. There’s no service manual or anything like that available to the public. All the information that’s out there is on this page, at the bottom under Documentation.
Thanks. I'll just muddle through