A little research into the startup problems

TL;DR: The DAC isn't starting up correctly, causing the CVs to either be the wrong value or to fluctuate.

There are two weird things that sometimes happen during startup. 

1) We get massive noise (but with a fixed frequency on the output

2) We get some kind of slew on the square wave, e.g. it is not flat on top.

Not sure if 2 happens at the same time as 1, but 2 can at least happen on its own

 

After a little measuring, I found that

1 comes from the waveshaper. It is present on all inputs but the DCO input is super clean.

2 comes from something between the square wave output of the waveshaper and the input of the bus mixer.

So what's between them? The waveform mixer and the oscillator output switch, but for the moment that's replaced with a jumper - so it's only the waveform mixer. It is implemented directly on the mainboard, except for the VCA chip.

Output on top, output from waveshaper on the bottom. WS looks good 

 

Input at the source mixer on the bottom. Here it's slanted.

The mixer PULA is the square wave input, the square wave amp sits before the pulse output on the waveshaper

This is interesting. The only thing between the waveshaper and the source mix board is that op amp. But the mixer also feeds into it, so maybe some of the other waveforms are leaking through?

Also note that the output wave has some strange spikes on it (the same ones for each cycle) that are not found on the mixer input. Approximately 12? 

 

Here is something more.

When square is slanted, triangle is not aligned:

The triangle does not match up

 

Triangle with a good signal

Trims when slew: 2.0, 2.8V (center, symmetry)

Trims when clean: 2.3, 3.0V (these are also what is expected).

Let's look at the DAC

With slew

DAC ref voltage: 2.1 to 2.386

Output on DAC ref voltage pin, too low and fluctuates

Output on one of the channels, is too low

When clean

DAC ref voltage 2.5V 

DAC ref voltage is perfect

Output on the same channel as above. Looks good.

 

 

DAC +5V and GND inputs are clean even with slew

 

As for resetting the issue - turning off and waiting a short time had no effect. But quickly turning off and on made it go away.

The errors appear without turning off +5V, only +/-12V. 

On some rare occations the output flatlines on startup. On a single one, DAC output latched at something else than 0 (?) and a mix of two sources (?) could be seen on the output.

With the new seventyPWR, I have only been able to reproduce case 1 two times. It may be caused by the DAC too?

Edit: I managed t reproduce the extremely noisy case after reconnecting the old PSU and running two voice cards.

GND on the DAC is stable, the +5V very slightly wobbles, but nothing extreme.

 

The Vref however, is 4.23V, way too high:

 

DAC ref voltage should be 2.5V but is 4.23! Also note that the noise on the voice output is following the same pattern for each cycle, it's not random noise

The DAC output looks like it follows the correct "pattern" but the amplitude is of course way too high too.

 

DAC output is higher than it should

Now, as for the waveshaper - DCO input looks as it should, but everything else is garbage. Interestingly, the symmetry and center trim inputs are not stable voltages, but jump up and down. Moving further down the signal path, the mixer input channel 2 CV is also not stable. 

Mixer input channel 2 CV

Waveshaper 2 center CV

Waveshaper 2 symmetry CV

Waveshaper 1 center CV

Waveshaper 1 symmetry CV

It simply looks like the CV mux is not able to generate stable CVs, either because the clocking is wrong or because the DAC output is too high, making it settle at the wrong voltage. In other words, the massive noise is ALSO caused by the DAC. That's nice to know!