So there’s lots of noise 
I have stripped bare ( no not me ) , to just a PSU, DCO, Mixer and AMP. everything else is off.
Before the 1st note it’s silent, then after the first note you get the random noise.
Unplugging the MIDI does not affect the issue.
it’s a bit random really… off to blast all the solder joints.
Rob
Right… So when I looked at the board , the 100nf ( yeah 50nf actual on these cheep discs ) power filter caps for he 074’s were standing well proud of the board with long legs.
I presumed that was not going to be helping cutting down on noise of any kind. So I tried to heat the joints up and push them through but with no luck.
Instead I just cut them out as a test.
In the video there’s a bit of noise up until all notes are initialised, after that it seems ok. I am assuming that once the ADSR is in the way that “bug” will be un-noticable.
Bypass caps surrounded by resistors? Better to have them as close to the IC power pin as you can.
actually on review only one is a bypass cap, normally I do have them right as close to the power pins as possible.
It seems unusual to me, tying +12 to -12V , not sure it matters…
Normally use/see both power rails tied to ground
Like this
Anyway I replaced all the caps with better ones. and the noise returned.
took a punt and removed these from the input
And it was sounding ok again. but who know next time it powers up…
There are some designers who like to use a single cap from +12 V to -12 V for bypass. I don’t know enough to have an opinion but have heard it asserted one cap on each rail to ground is better. On at least one module with a commercial PCB I’ve kludged two caps where one was designed. Probably makes no difference at all for most analog circuits, for digital, who knows.
Sometimes you see 100 nF in addition to 10 µF near the power header and more 10 nF near the chips. From what I understood of Dave Jones’s video on the subject, that seems like it doesn’t make much sense. The small capacitance far away from the chip won’t really do any good, sounds like.
I will revise it for the next ittertaion.
So,
Ramp is sounding fine after each note initialises. I did track down a dead TLO74 that was causing a missing voice.
Pulse however is just a bunch of garbage. I will have to check all the component values for the Pulse stage…
I didn’t care so much about noise. I only used it with a VCA and ADSR and it was desired that the oscillators are on all the time. Maybe try some pull-down resistors on the inputs of the op-amps or transistors.
Pulse signal is just created by a comparator. It compares the ramp signal with a fixed voltage. Pulse width can be changed by changing this voltage. Check what voltage is going into the op-amp and compare it with the amplitude of the ramp signal.
Well this is really strange.
Pulse now outputs, but only the top 5-6 notes on the keyboard, if you go lower they stop sounding? This is very odd. if you move the output to ramp it play fine. Been on this all day , my brain is beginning to hurt. I honestly can’t understand why on earth it would stop playing notes full stop.
The only deviation I made from @jkb schematic for PWM was to go from 20K pot - 10K fixed on the PWM pot, to 10K Pot 5.1K fixed…
Success…
@jkb “Check what voltage is going into the op-amp and compare it with the amplitude of the ramp signal.”
Far too much… 9.7-9.8V on the full swing on the Pot.
Traced through and found the fixed value resistor on the pot circuit had it’s ground leg floating as the track on the PCB to ground was not there… I can’t remember if I found and fixed that missing ground after the PCB went to print. I can’t think fo any other explanation, and I can’t be bothered trawling the GIT files to see if I can confirm.
So it works 
just got to get the ADSR underway and the VCA designed!!
Thanks @jkb
Rob
Having almost got the Mixer working, the Pulse output has failed. All I can think it could be is the resistor on the control pot that had a bodged ground. Out all day so won’t find out.