VCO2 and VCO1 behavior?

[Mesir1]

Troubleshooting my SL Plus, I just discovered that when I hook up VCO2, that starts to affect the frequency of VCO1. This is with nothing else connected, eccept for the VCA.

As a matter of fact, they start to behave as one oscillator, both affecting each other. Is it supposed to behave like this, and if not, what is wrong?
This happens even when the sync is turned off.

Help is welcome, and will be MUCH appreciated.

[jordroid]

Hello,

we may need a bit more information to diagnose the issue, it's unclear exactly what is going on.

Do you mean that VCO2 frequency modulates VCO1? It will do that if the potentiometer R209 (schematic page 4) is turned up. Could that be it?

I'm not sure what "they start to behave as one oscillator, both affecting each other" means.

-Do the coarse and fine tuners effect both VCOs?

-Or do you mean the VCOs are frequency modulating each other?

-Or that the VCOs seem to be synced together, even though the sync switch is not activated?

I'd be happy to help, any further clarification is appreciated, and sorry if i am missing something obvious.

regards,

jordan

[Mesir1]

Hello, and thank you.

I`ll try to answer your questions as presicely as my knowledge allows.

-Do the coarse and fine tuners effect both VCOs?
Yes.

-Or do you mean the VCOs are frequency modulating each other?
No, it`s the frequency that changes in both.

-Or that the VCOs seem to be synced together, even though the sync switch is not activated?
Maybe, not shure what`s supposed to happen when they are sync`d, but this happens even when sync is off.

By "they start to behave as one oscillator, both affecting each other" I mean that the controllers of both VCO`s control the frequency (not modulating) of both oscillators. Even with the sync switch off and R209 turned all the way down. And yes, both the coarse and fine adjusters affect the frequency of both.

Further investigations show that by turning the output level on the AR-gen and Sample/ Hold does the same thing, affects the frequency of OSC1. And I`ve lost all sound in OSC2 at the moment

I`ve checked all component values, wiring, bad soldering, and they are ok.

Done some readouts on the power, and that was confusing. On the pcb I have 16 v between GND and +12, 24 v between +12 and -12, and 7.2 v between -12 and GND. From the pwrsupply all readouts are correct, GND and +9/ -9 = 12v, and 24 v between +9/-9. That`s when all the ic`s are removed.

Power to the ic`s ok, all but ic1, I`ve 16v between pin14 and 7...

And there`s no output-signal either...

Hmmmm...help is needed

[Mesir1]

I have now got the oscillators working, I guess, but when I put ic`s 3, 4 and 7 in place the input voltage changes from +12 and -12 to +4 and -20 when ic 7 is added, and I get different readouts whith ic 4 and 3 as well. The LFO doesn`t oscillate, I have no output, and feel a bit lost.

But I have noise. Sound is from the signal mixer.

Help still needed...

[jordroid]

Hi again,

the readings you are getting from the power rails seem very strange, somewhere it seems like "ground" is not ground. Your readings pretty consistently show a total of about 24 volts between +12 and -12, which is good, but zero volts seems to float around.

With the power off, do you read continuity between the power supply ground and ground on the PCB when they are hooked up?

Any significant resistance (more than a few ohms) between each power supply rail and +/-12 and ground on the PCB?

When i first built my soundlab i accidentally left out a jumper on the DIY power supply i made and there was no connection between "ground" on the power supply and "ground" on the circuit. I mention it because my symptoms were similar to what you are describing, and it's consistent with your voltage readings.

Let us know how it's going

[Mesir1]

Hi again...feeling a bit lost at the moment, and confused. I had this thing running off of 2 9v batteries, working ok, but as they got old, I decided to replace them with the wallwart powersupply (MFOS.) And beeing in work-mode, I replaced Q1,2,3 and 4 with the SSM2210 as well.

I can`t find anywhere in the schem on the wall-wart saying anything about a bus. Either I`ve gone blind or I`m missing something.

I`ll spend some time tonight trying to make some readouts on your suggestions.

Your help is really keeping me going, thank you a very much a lot.

[jordroid]

Hi Mesir, apologies if i have been confusing, i am fairly new to this and don't always know the best way to describe things.

From your descriptions it seems possible that ground is floating (not connected to any reference) on your minisynth PCB.

You should get a beep when probing between ground on the power supply PCB and ground on the minisynth PCB when your meter is set to beep mode.

If you find an open circuit (no beep) between ground on the power supply PCB and ground on the minisynth PCB then either there is no ground wire there or the ground wire is not completing the circuit for some reason, like a cold solder joint, lifted pad or something.

You want one sturdy wire running directly from the power supply ground to the minisynth ground, and you want to read a very low resistance (less than a few ohms) between all grounded points, including any panel mounted components that are grounded.

You will get it working, i'll try to help out further if i can.

[Mesir1]

FOLLOWED YOUR SUGGESTIONS, AND FOUND A COUPLE OF BAD GROUND-CONNECTIONS! HOPE IS RISING!

I`ll resolder the wires, hook everything up again, and let you know how things are working out.

And agin, thank you.

[Mesir1]

Sound is back, need to replace some of the chips, but everything seems to be working now. Fixed the bad ground-connection, and voilà; SOUND!

I`ll start to calibrate soon, spend some time with the scope, and then start putting the Baby10 togheter.

And hopefyllu for the last time, thank you for your patience and clever advices. Feeling very relieved at the moment, hours spent building and troubleshooting all seems worth the wait.

[jordroid]