See that 1.8k resistor? You could cut it out and replace it with a variable resistor. Do this by soldering a pot pin 1 to the ground and pins 2 and 3 to pin 2 of the Tlo74. This should allow you to adjust until you no longer hear the self oscillation with the resonance. ON THE OTHER HAND… it could be 2 bad resonance pots, but I doubt they would both happen to be bad in the same spot on both circuits. Not sure about your 6ch attenuators. A couple 10n caps should work for decoupling. Also, if you are using polarized caps for decoupling you need the stripe toward ground from positive and the same for negative with the stripe toward the ground.
I’ll try that if flipping the polystyrene doesn’t work! I’m just not used to these vintage ones…pretty sure the unmarked polystyrene ones still have an inner foil.i pit the scope on them and they went nuts one way and were stable the other. I’m going to flip the ones that appear backwards on the scope and see if that’s what’s acting like an antennae. I did some reading and apparently picking up nearby signals is something they do when installed wrong
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Ill install at least a trim pot there and test it that seems like a good idea. One is oscillating really high the other low it’s odd
Right on. I was just looking at your strip board pics and I noticed your decoupling caps. They look correct. The round clay ones at the bottom. Anyway, if the self oscillation won’t go away even with a dedicated power supply then I would see what happens when you change the value of that 1.8k resistor. Like I said I have not had your issue with the ms-20s, and I have used the 1.8k resistor in both of my builds without the need for a variable resistor. However, I have had self oscillation issues with other filter builds the case being there was no self oscillation when I wanted it to be there, as well as in your case where there is always self oscillation when I do not want there to be. In both cases adjusting the resistor value worked. The resonant feedback loop in these filter circuits are all very similar.
Btw, the self oscillation frequency will depend on the cutoff. Also, you will definitely need to remove the 1.8k resistor. Don’t wire the potentiometer in parallel to it or you likely will not get the resistance you are looking for.
I added 10uf caps to the main power input of one board and they both stopped picking up the frequency of the osc…but are still self oscillating so I’m going to try the other thing you suggested now
Thank you so much for the help by the way
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Added caps to the other board now they both just have a very low steady pulse like a heartbeat…getting somewhere now
Still changing pitch with attenuation though
It’s only oscillating one phase now only half the Leds are going
Have you tried the resistor?
The oscillating didn’t go away I just. Had a solder whisker when I put caps on the other board I was hearing the cap lol. But it sure did fix the stray signals it was picking up. I will try the resistor after my kids mom gets home I also think one of my input trim pots is bad it’s going up in the center and gently reducing on both extremes of the turn. Id assume that was the whole issue if both filters weren’t doing the same thing
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I know the pots wired right as the other one works fine
You definitely don’t need a scope to build the VCO-1. The only thing that it would be useful to check is the sine symmetry trim, but I’ve built 4 of the sine shaper and haven’t needed to adjust it once (I have a scope, and have checked it). I have matched Q3 and Q4 – if you get them close you should get a reasonably symmetrical output. Highly recommended mod is to use a panel pot instead of a trim pot for the sine round trim. This lets you vary the output from a triangle through sine to a distorted squarish waveform. The sine shaper is a tanh waveshaper, it does what this does:
Tanh
You could also have an input jack, with the triangle wave normalled to it, so that you could use the waveshaper independently of the oscillator.
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So the tangential hitch (tanh…I watched a bit of divkids video) is already a part of the vco1 circuit?
Yes. The sine shaper works by applying the tanh function to a triangle wave. Here’s a simulation.
Tanh
If you play around with the shape slider you’ll see what I mean about how useful it is to bring that pot out to the panel. If you change the triangle wave to a sawtooth, you’ll see the tanh curve very clearly.
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throwing in the towel on the filters for now ive tried everything and if i fix one thing i loose functionality of something else. sounded best when it was just dirty and distored pre fixs. now they barely work on multiple settings and sound quiet. tl084 chips made them tame but quiet…the caps erased 6db for one and 12db functionality for another…they only like lowpass now and the res totally just stopped having an effect for filter 1. im so freqking confused.,.ive built much more complicated stuff than these and had no issue sorting it out eventually. even like for like swaps of mouser chips make it act dif. both of my TH vcos fired up firt time and ive had zero issues with it
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It is maddening. I’ve had my share of situations like this. One reason I always do a breadboard first time for any new circuit.
also my RD8 mkii came today so i kinda just want to play with that. its nice having something that just works haha i could never build an 808 and so far it sounds fantastic. none of the trademark hot outputs that scared me away from their mixers
I was inspired to buy a volca bass for the same reasons. Always works and it helped to teach me about synthesis while working to get my diy modular building up to usable stuff.
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